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diff --git a/gc/README b/gc/README new file mode 100644 index 0000000..80cb26a --- /dev/null +++ b/gc/README @@ -0,0 +1,1517 @@ +Copyright 1988, 1989 Hans-J. Boehm, Alan J. Demers +Copyright (c) 1991-1996 by Xerox Corporation. All rights reserved. +Copyright (c) 1996-1999 by Silicon Graphics. All rights reserved. + +THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED +OR IMPLIED. ANY USE IS AT YOUR OWN RISK. + +Permission is hereby granted to use or copy this program +for any purpose, provided the above notices are retained on all copies. +Permission to modify the code and to distribute modified code is granted, +provided the above notices are retained, and a notice that the code was +modified is included with the above copyright notice. + +This is version 5.0alpha3 of a conservative garbage collector for C and C++. + +You might find a more recent version of this at + +http://www.hpl.hp.com/personal/Hans_Boehm/gc + +HISTORY - + + Early versions of this collector were developed as a part of research +projects supported in part by the National Science Foundation +and the Defense Advance Research Projects Agency. +Much of the code was rewritten by Hans-J. Boehm (boehm@acm.org) at Xerox PARC +and at SGI. + +Some other contributors: + +More recent contributors are mentioned in the modification history at the +end of this file. My apologies for any omissions. + +The SPARC specific code was contributed by Mark Weiser +(weiser@parc.xerox.com). The Encore Multimax modifications were supplied by +Kevin Kenny (kenny@m.cs.uiuc.edu). The adaptation to the RT is largely due +to Vernon Lee (scorpion@rice.edu), on machines made available by IBM. +Much of the HP specific code and a number of good suggestions for improving the +generic code are due to Walter Underwood (wunder@hp-ses.sde.hp.com). +Robert Brazile (brazile@diamond.bbn.com) originally supplied the ULTRIX code. +Al Dosser (dosser@src.dec.com) and Regis Cridlig (Regis.Cridlig@cl.cam.ac.uk) +subsequently provided updates and information on variation between ULTRIX +systems. Parag Patel (parag@netcom.com) supplied the A/UX code. +Jesper Peterson(jep@mtiame.mtia.oz.au), Michel Schinz, and +Martin Tauchmann (martintauchmann@bigfoot.com) supplied the Amiga port. +Thomas Funke (thf@zelator.in-berlin.de(?)) and +Brian D.Carlstrom (bdc@clark.lcs.mit.edu) supplied the NeXT ports. +Douglas Steel (doug@wg.icl.co.uk) provided ICL DRS6000 code. +Bill Janssen (janssen@parc.xerox.com) supplied the SunOS dynamic loader +specific code. Manuel Serrano (serrano@cornas.inria.fr) supplied linux and +Sony News specific code. Al Dosser provided Alpha/OSF/1 code. He and +Dave Detlefs(detlefs@src.dec.com) also provided several generic bug fixes. +Alistair G. Crooks(agc@uts.amdahl.com) supplied the NetBSD and 386BSD ports. +Jeffrey Hsu (hsu@soda.berkeley.edu) provided the FreeBSD port. +Brent Benson (brent@jade.ssd.csd.harris.com) ported the collector to +a Motorola 88K processor running CX/UX (Harris NightHawk). +Ari Huttunen (Ari.Huttunen@hut.fi) generalized the OS/2 port to +nonIBM development environments (a nontrivial task). +Patrick Beard (beard@cs.ucdavis.edu) provided the initial MacOS port. +David Chase, then at Olivetti Research, suggested several improvements. +Scott Schwartz (schwartz@groucho.cse.psu.edu) supplied some of the +code to save and print call stacks for leak detection on a SPARC. +Jesse Hull and John Ellis supplied the C++ interface code. +Zhong Shao performed much of the experimentation that led to the +current typed allocation facility. (His dynamic type inference code hasn't +made it into the released version of the collector, yet.) +(Blame for misinstallation of these modifications goes to the first author, +however.) + +OVERVIEW + + This is intended to be a general purpose, garbage collecting storage +allocator. The algorithms used are described in: + +Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Environment", +Software Practice & Experience, September 1988, pp. 807-820. + +Boehm, H., A. Demers, and S. Shenker, "Mostly Parallel Garbage Collection", +Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design +and Implementation, SIGPLAN Notices 26, 6 (June 1991), pp. 157-164. + +Boehm, H., "Space Efficient Conservative Garbage Collection", Proceedings +of the ACM SIGPLAN '91 Conference on Programming Language Design and +Implementation, SIGPLAN Notices 28, 6 (June 1993), pp. 197-206. + + Possible interactions between the collector and optimizing compilers are +discussed in + +Boehm, H., and D. Chase, "A Proposal for GC-safe C Compilation", +The Journal of C Language Translation 4, 2 (December 1992). + +and + +Boehm H., "Simple GC-safe Compilation", Proceedings +of the ACM SIGPLAN '96 Conference on Programming Language Design and +Implementation. + +(Both are also available from +http://reality.sgi.com/boehm/papers/, among other places.) + + Unlike the collector described in the second reference, this collector +operates either with the mutator stopped during the entire collection +(default) or incrementally during allocations. (The latter is supported +on only a few machines.) It does not rely on threads, but is intended +to be thread-safe. + + Some of the ideas underlying the collector have previously been explored +by others. (Doug McIlroy wrote a vaguely similar collector that is part of +version 8 UNIX (tm).) However none of this work appears to have been widely +disseminated. + + Rudimentary tools for use of the collector as a leak detector are included, as +is a fairly sophisticated string package "cord" that makes use of the collector. +(See cord/README.) + + +GENERAL DESCRIPTION + + This is a garbage collecting storage allocator that is intended to be +used as a plug-in replacement for C's malloc. + + Since the collector does not require pointers to be tagged, it does not +attempt to ensure that all inaccessible storage is reclaimed. However, +in our experience, it is typically more successful at reclaiming unused +memory than most C programs using explicit deallocation. Unlike manually +introduced leaks, the amount of unreclaimed memory typically stays +bounded. + + In the following, an "object" is defined to be a region of memory allocated +by the routines described below. + + Any objects not intended to be collected must be pointed to either +from other such accessible objects, or from the registers, +stack, data, or statically allocated bss segments. Pointers from +the stack or registers may point to anywhere inside an object. +The same is true for heap pointers if the collector is compiled with + ALL_INTERIOR_POINTERS defined, as is now the default. + +Compiling without ALL_INTERIOR_POINTERS may reduce accidental retention +of garbage objects, by requiring pointers from the heap to to the beginning +of an object. But this no longer appears to be a significant +issue for most programs. + +There are a number of routines which modify the pointer recognition +algorithm. GC_register_displacement allows certain interior pointers +to be recognized even if ALL_INTERIOR_POINTERS is nor defined. +GC_malloc_ignore_off_page allows some pointers into the middle of large objects +to be disregarded, greatly reducing the probablility of accidental +retention of large objects. For most purposes it seems best to compile +with ALL_INTERIOR_POINTERS and to use GC_malloc_ignore_off_page if +you get collector warnings from allocations of very large objects. +See README.debugging for details. + + Note that pointers inside memory allocated by the standard "malloc" are not +seen by the garbage collector. Thus objects pointed to only from such a +region may be prematurely deallocated. It is thus suggested that the +standard "malloc" be used only for memory regions, such as I/O buffers, that +are guaranteed not to contain pointers to garbage collectable memory. +Pointers in C language automatic, static, or register variables, +are correctly recognized. (Note that GC_malloc_uncollectable has semantics +similar to standard malloc, but allocates objects that are traced by the +collector.) + + The collector does not always know how to find pointers in data +areas that are associated with dynamic libraries. This is easy to +remedy IF you know how to find those data areas on your operating +system (see GC_add_roots). Code for doing this under SunOS, IRIX 5.X and 6.X, +HP/UX, Alpha OSF/1, Linux, and win32 is included and used by default. (See +README.win32 for win32 details.) On other systems pointers from dynamic +library data areas may not be considered by the collector. + + Note that the garbage collector does not need to be informed of shared +read-only data. However if the shared library mechanism can introduce +discontiguous data areas that may contain pointers, then the collector does +need to be informed. + + Signal processing for most signals may be deferred during collection, +and during uninterruptible parts of the allocation process. Unlike +standard ANSI C mallocs, it can be safe to invoke malloc +from a signal handler while another malloc is in progress, provided +the original malloc is not restarted. (Empirically, many UNIX +applications already assume this.) To obtain this level of signal +safety, remove the definition of -DNO_SIGNALS in Makefile. This incurs +a minor performance penalty, and hence is no longer the default. + + The allocator/collector can also be configured for thread-safe operation. +(Full signal safety can also be achieved, but only at the cost of two system +calls per malloc, which is usually unacceptable.) + +INSTALLATION AND PORTABILITY + + As distributed, the macro SILENT is defined in Makefile. +In the event of problems, this can be removed to obtain a moderate +amount of descriptive output for each collection. +(The given statistics exhibit a few peculiarities. +Things don't appear to add up for a variety of reasons, most notably +fragmentation losses. These are probably much more significant for the +contrived program "test.c" than for your application.) + + Note that typing "make test" will automatically build the collector +and then run setjmp_test and gctest. Setjmp_test will give you information +about configuring the collector, which is useful primarily if you have +a machine that's not already supported. Gctest is a somewhat superficial +test of collector functionality. Failure is indicated by a core dump or +a message to the effect that the collector is broken. Gctest takes about +35 seconds to run on a SPARCstation 2. On a slower machine, +expect it to take a while. It may use up to 8 MB of memory. (The +multi-threaded version will use more.) "Make test" will also, as +its last step, attempt to build and test the "cord" string library. +This will fail without an ANSI C compiler. + + The Makefile will generate a library gc.a which you should link against. +Typing "make cords" will add the cord library to gc.a. +Note that this requires an ANSI C compiler. + + It is suggested that if you need to replace a piece of the collector +(e.g. GC_mark_rts.c) you simply list your version ahead of gc.a on the + work.) +ld command line, rather than replacing the one in gc.a. (This will +generate numerous warnings under some versions of AIX, but it still +works.) + + All include files that need to be used by clients will be put in the +include subdirectory. (Normally this is just gc.h. "Make cords" adds +"cord.h" and "ec.h".) + + The collector currently is designed to run essentially unmodified on +machines that use a flat 32-bit or 64-bit address space. +That includes the vast majority of Workstations and X86 (X >= 3) PCs. +(The list here was deleted because it was getting too long and constantly +out of date.) + It does NOT run under plain 16-bit DOS or Windows 3.X. There are however +various packages (e.g. win32s, djgpp) that allow flat 32-bit address +applications to run under those systemsif the have at least an 80386 processor, +and several of those are compatible with the collector. + + In a few cases (Amiga, OS/2, Win32, MacOS) a separate makefile +or equivalent is supplied. Many of these have separate README.system +files. + + Dynamic libraries are completely supported only under SunOS +(and even that support is not functional on the last Sun 3 release), +IRIX 5&6, HP-PA, Win32 (not Win32S) and OSF/1 on DEC AXP machines. +On other machines we recommend that you do one of the following: + + 1) Add dynamic library support (and send us the code). + 2) Use static versions of the libraries. + 3) Arrange for dynamic libraries to use the standard malloc. + This is still dangerous if the library stores a pointer to a + garbage collected object. But nearly all standard interfaces + prohibit this, because they deal correctly with pointers + to stack allocated objects. (Strtok is an exception. Don't + use it.) + + In all cases we assume that pointer alignment is consistent with that +enforced by the standard C compilers. If you use a nonstandard compiler +you may have to adjust the alignment parameters defined in gc_priv.h. + + A port to a machine that is not byte addressed, or does not use 32 bit +or 64 bit addresses will require a major effort. A port to plain MSDOS +or win16 is hard. + + For machines not already mentioned, or for nonstandard compilers, the +following are likely to require change: + +1. The parameters in gcconfig.h. + The parameters that will usually require adjustment are + STACKBOTTOM, ALIGNMENT and DATASTART. Setjmp_test + prints its guesses of the first two. + DATASTART should be an expression for computing the + address of the beginning of the data segment. This can often be + &etext. But some memory management units require that there be + some unmapped space between the text and the data segment. Thus + it may be more complicated. On UNIX systems, this is rarely + documented. But the adb "$m" command may be helpful. (Note + that DATASTART will usually be a function of &etext. Thus a + single experiment is usually insufficient.) + STACKBOTTOM is used to initialize GC_stackbottom, which + should be a sufficient approximation to the coldest stack address. + On some machines, it is difficult to obtain such a value that is + valid across a variety of MMUs, OS releases, etc. A number of + alternatives exist for using the collector in spite of this. See the + discussion in gcconfig.h immediately preceding the various + definitions of STACKBOTTOM. + +2. mach_dep.c. + The most important routine here is one to mark from registers. + The distributed file includes a generic hack (based on setjmp) that + happens to work on many machines, and may work on yours. Try + compiling and running setjmp_t.c to see whether it has a chance of + working. (This is not correct C, so don't blame your compiler if it + doesn't work. Based on limited experience, register window machines + are likely to cause trouble. If your version of setjmp claims that + all accessible variables, including registers, have the value they + had at the time of the longjmp, it also will not work. Vanilla 4.2 BSD + on Vaxen makes such a claim. SunOS does not.) + If your compiler does not allow in-line assembly code, or if you prefer + not to use such a facility, mach_dep.c may be replaced by a .s file + (as we did for the MIPS machine and the PC/RT). + At this point enough architectures are supported by mach_dep.c + that you will rarely need to do more than adjust for assembler + syntax. + +3. os_dep.c (and gc_priv.h). + Several kinds of operating system dependent routines reside here. + Many are optional. Several are invoked only through corresponding + macros in gc_priv.h, which may also be redefined as appropriate. + The routine GC_register_data_segments is crucial. It registers static + data areas that must be traversed by the collector. (User calls to + GC_add_roots may sometimes be used for similar effect.) + Routines to obtain memory from the OS also reside here. + Alternatively this can be done entirely by the macro GET_MEM + defined in gc_priv.h. Routines to disable and reenable signals + also reside here if they are need by the macros DISABLE_SIGNALS + and ENABLE_SIGNALS defined in gc_priv.h. + In a multithreaded environment, the macros LOCK and UNLOCK + in gc_priv.h will need to be suitably redefined. + The incremental collector requires page dirty information, which + is acquired through routines defined in os_dep.c. Unless directed + otherwise by gcconfig.h, these are implemented as stubs that simply + treat all pages as dirty. (This of course makes the incremental + collector much less useful.) + +4. dyn_load.c + This provides a routine that allows the collector to scan data + segments associated with dynamic libraries. Often it is not + necessary to provide this routine unless user-written dynamic + libraries are used. + + For a different version of UN*X or different machines using the +Motorola 68000, Vax, SPARC, 80386, NS 32000, PC/RT, or MIPS architecture, +it should frequently suffice to change definitions in gcconfig.h. + + +THE C INTERFACE TO THE ALLOCATOR + + The following routines are intended to be directly called by the user. +Note that usually only GC_malloc is necessary. GC_clear_roots and GC_add_roots +calls may be required if the collector has to trace from nonstandard places +(e.g. from dynamic library data areas on a machine on which the +collector doesn't already understand them.) On some machines, it may +be desirable to set GC_stacktop to a good approximation of the stack base. +(This enhances code portability on HP PA machines, since there is no +good way for the collector to compute this value.) Client code may include +"gc.h", which defines all of the following, plus many others. + +1) GC_malloc(nbytes) + - allocate an object of size nbytes. Unlike malloc, the object is + cleared before being returned to the user. Gc_malloc will + invoke the garbage collector when it determines this to be appropriate. + GC_malloc may return 0 if it is unable to acquire sufficient + space from the operating system. This is the most probable + consequence of running out of space. Other possible consequences + are that a function call will fail due to lack of stack space, + or that the collector will fail in other ways because it cannot + maintain its internal data structures, or that a crucial system + process will fail and take down the machine. Most of these + possibilities are independent of the malloc implementation. + +2) GC_malloc_atomic(nbytes) + - allocate an object of size nbytes that is guaranteed not to contain any + pointers. The returned object is not guaranteed to be cleared. + (Can always be replaced by GC_malloc, but results in faster collection + times. The collector will probably run faster if large character + arrays, etc. are allocated with GC_malloc_atomic than if they are + statically allocated.) + +3) GC_realloc(object, new_size) + - change the size of object to be new_size. Returns a pointer to the + new object, which may, or may not, be the same as the pointer to + the old object. The new object is taken to be atomic iff the old one + was. If the new object is composite and larger than the original object, + then the newly added bytes are cleared (we hope). This is very likely + to allocate a new object, unless MERGE_SIZES is defined in gc_priv.h. + Even then, it is likely to recycle the old object only if the object + is grown in small additive increments (which, we claim, is generally bad + coding practice.) + +4) GC_free(object) + - explicitly deallocate an object returned by GC_malloc or + GC_malloc_atomic. Not necessary, but can be used to minimize + collections if performance is critical. Probably a performance + loss for very small objects (<= 8 bytes). + +5) GC_expand_hp(bytes) + - Explicitly increase the heap size. (This is normally done automatically + if a garbage collection failed to GC_reclaim enough memory. Explicit + calls to GC_expand_hp may prevent unnecessarily frequent collections at + program startup.) + +6) GC_malloc_ignore_off_page(bytes) + - identical to GC_malloc, but the client promises to keep a pointer to + the somewhere within the first 256 bytes of the object while it is + live. (This pointer should nortmally be declared volatile to prevent + interference from compiler optimizations.) This is the recommended + way to allocate anything that is likely to be larger than 100Kbytes + or so. (GC_malloc may result in failure to reclaim such objects.) + +7) GC_set_warn_proc(proc) + - Can be used to redirect warnings from the collector. Such warnings + should be rare, and should not be ignored during code development. + +8) GC_enable_incremental() + - Enables generational and incremental collection. Useful for large + heaps on machines that provide access to page dirty information. + Some dirty bit implementations may interfere with debugging + (by catching address faults) and place restrictions on heap arguments + to system calls (since write faults inside a system call may not be + handled well). + +9) Several routines to allow for registration of finalization code. + User supplied finalization code may be invoked when an object becomes + unreachable. To call (*f)(obj, x) when obj becomes inaccessible, use + GC_register_finalizer(obj, f, x, 0, 0); + For more sophisticated uses, and for finalization ordering issues, + see gc.h. + + The global variable GC_free_space_divisor may be adjusted up from its +default value of 4 to use less space and more collection time, or down for +the opposite effect. Setting it to 1 or 0 will effectively disable collections +and cause all allocations to simply grow the heap. + + The variable GC_non_gc_bytes, which is normally 0, may be changed to reflect +the amount of memory allocated by the above routines that should not be +considered as a candidate for collection. Careless use may, of course, result +in excessive memory consumption. + + Some additional tuning is possible through the parameters defined +near the top of gc_priv.h. + + If only GC_malloc is intended to be used, it might be appropriate to define: + +#define malloc(n) GC_malloc(n) +#define calloc(m,n) GC_malloc((m)*(n)) + + For small pieces of VERY allocation intensive code, gc_inl.h +includes some allocation macros that may be used in place of GC_malloc +and friends. + + All externally visible names in the garbage collector start with "GC_". +To avoid name conflicts, client code should avoid this prefix, except when +accessing garbage collector routines or variables. + + There are provisions for allocation with explicit type information. +This is rarely necessary. Details can be found in gc_typed.h. + +THE C++ INTERFACE TO THE ALLOCATOR: + + The Ellis-Hull C++ interface to the collector is included in +the collector distribution. If you intend to use this, type +"make c++" after the initial build of the collector is complete. +See gc_cpp.h for the definition of the interface. This interface +tries to approximate the Ellis-Detlefs C++ garbage collection +proposal without compiler changes. + +Cautions: +1. Arrays allocated without new placement syntax are +allocated as uncollectable objects. They are traced by the +collector, but will not be reclaimed. + +2. Failure to use "make c++" in combination with (1) will +result in arrays allocated using the default new operator. +This is likely to result in disaster without linker warnings. + +3. If your compiler supports an overloaded new[] operator, +then gc_cpp.cc and gc_cpp.h should be suitably modified. + +4. Many current C++ compilers have deficiencies that +break some of the functionality. See the comments in gc_cpp.h +for suggested workarounds. + +USE AS LEAK DETECTOR: + + The collector may be used to track down leaks in C programs that are +intended to run with malloc/free (e.g. code with extreme real-time or +portability constraints). To do so define FIND_LEAK in Makefile +This will cause the collector to invoke the report_leak +routine defined near the top of reclaim.c whenever an inaccessible +object is found that has not been explicitly freed. The collector will +no longer reclaim inaccessible memory; in this form it is purely a +debugging tool. + Productive use of this facility normally involves redefining report_leak +to do something more intelligent. This typically requires annotating +objects with additional information (e.g. creation time stack trace) that +identifies their origin. Such code is typically not very portable, and is +not included here, except on SPARC machines. + If all objects are allocated with GC_DEBUG_MALLOC (see next section), +then the default version of report_leak will report the source file +and line number at which the leaked object was allocated. This may +sometimes be sufficient. (On SPARC/SUNOS4 machines, it will also report +a cryptic stack trace. This can often be turned into a sympolic stack +trace by invoking program "foo" with "callprocs foo". Callprocs is +a short shell script that invokes adb to expand program counter values +to symbolic addresses. It was largely supplied by Scott Schwartz.) + Note that the debugging facilities described in the next section can +sometimes be slightly LESS effective in leak finding mode, since in +leak finding mode, GC_debug_free actually results in reuse of the object. +(Otherwise the object is simply marked invalid.) Also note that the test +program is not designed to run meaningfully in FIND_LEAK mode. +Use "make gc.a" to build the collector. + +DEBUGGING FACILITIES: + + The routines GC_debug_malloc, GC_debug_malloc_atomic, GC_debug_realloc, +and GC_debug_free provide an alternate interface to the collector, which +provides some help with memory overwrite errors, and the like. +Objects allocated in this way are annotated with additional +information. Some of this information is checked during garbage +collections, and detected inconsistencies are reported to stderr. + + Simple cases of writing past the end of an allocated object should +be caught if the object is explicitly deallocated, or if the +collector is invoked while the object is live. The first deallocation +of an object will clear the debugging info associated with an +object, so accidentally repeated calls to GC_debug_free will report the +deallocation of an object without debugging information. Out of +memory errors will be reported to stderr, in addition to returning +NIL. + + GC_debug_malloc checking during garbage collection is enabled +with the first call to GC_debug_malloc. This will result in some +slowdown during collections. If frequent heap checks are desired, +this can be achieved by explicitly invoking GC_gcollect, e.g. from +the debugger. + + GC_debug_malloc allocated objects should not be passed to GC_realloc +or GC_free, and conversely. It is however acceptable to allocate only +some objects with GC_debug_malloc, and to use GC_malloc for other objects, +provided the two pools are kept distinct. In this case, there is a very +low probablility that GC_malloc allocated objects may be misidentified as +having been overwritten. This should happen with probability at most +one in 2**32. This probability is zero if GC_debug_malloc is never called. + + GC_debug_malloc, GC_malloc_atomic, and GC_debug_realloc take two +additional trailing arguments, a string and an integer. These are not +interpreted by the allocator. They are stored in the object (the string is +not copied). If an error involving the object is detected, they are printed. + + The macros GC_MALLOC, GC_MALLOC_ATOMIC, GC_REALLOC, GC_FREE, and +GC_REGISTER_FINALIZER are also provided. These require the same arguments +as the corresponding (nondebugging) routines. If gc.h is included +with GC_DEBUG defined, they call the debugging versions of these +functions, passing the current file name and line number as the two +extra arguments, where appropriate. If gc.h is included without GC_DEBUG +defined, then all these macros will instead be defined to their nondebugging +equivalents. (GC_REGISTER_FINALIZER is necessary, since pointers to +objects with debugging information are really pointers to a displacement +of 16 bytes form the object beginning, and some translation is necessary +when finalization routines are invoked. For details, about what's stored +in the header, see the definition of the type oh in debug_malloc.c) + +INCREMENTAL/GENERATIONAL COLLECTION: + +The collector normally interrupts client code for the duration of +a garbage collection mark phase. This may be unacceptable if interactive +response is needed for programs with large heaps. The collector +can also run in a "generational" mode, in which it usually attempts to +collect only objects allocated since the last garbage collection. +Furthermore, in this mode, garbage collections run mostly incrementally, +with a small amount of work performed in response to each of a large number of +GC_malloc requests. + +This mode is enabled by a call to GC_enable_incremental(). + +Incremental and generational collection is effective in reducing +pause times only if the collector has some way to tell which objects +or pages have been recently modified. The collector uses two sources +of information: + +1. Information provided by the VM system. This may be provided in +one of several forms. Under Solaris 2.X (and potentially under other +similar systems) information on dirty pages can be read from the +/proc file system. Under other systems (currently SunOS4.X) it is +possible to write-protect the heap, and catch the resulting faults. +On these systems we require that system calls writing to the heap +(other than read) be handled specially by client code. +See os_dep.c for details. + +2. Information supplied by the programmer. We define "stubborn" +objects to be objects that are rarely changed. Such an object +can be allocated (and enabled for writing) with GC_malloc_stubborn. +Once it has been initialized, the collector should be informed with +a call to GC_end_stubborn_change. Subsequent writes that store +pointers into the object must be preceded by a call to +GC_change_stubborn. + +This mechanism performs best for objects that are written only for +initialization, and such that only one stubborn object is writable +at once. It is typically not worth using for short-lived +objects. Stubborn objects are treated less efficiently than pointerfree +(atomic) objects. + +A rough rule of thumb is that, in the absence of VM information, garbage +collection pauses are proportional to the amount of pointerful storage +plus the amount of modified "stubborn" storage that is reachable during +the collection. + +Initial allocation of stubborn objects takes longer than allocation +of other objects, since other data structures need to be maintained. + +We recommend against random use of stubborn objects in client +code, since bugs caused by inappropriate writes to stubborn objects +are likely to be very infrequently observed and hard to trace. +However, their use may be appropriate in a few carefully written +library routines that do not make the objects themselves available +for writing by client code. + + +BUGS: + + Any memory that does not have a recognizable pointer to it will be +reclaimed. Exclusive-or'ing forward and backward links in a list +doesn't cut it. + Some C optimizers may lose the last undisguised pointer to a memory +object as a consequence of clever optimizations. This has almost +never been observed in practice. Send mail to boehm@acm.org +for suggestions on how to fix your compiler. + This is not a real-time collector. In the standard configuration, +percentage of time required for collection should be constant across +heap sizes. But collection pauses will increase for larger heaps. +(On SPARCstation 2s collection times will be on the order of 300 msecs +per MB of accessible memory that needs to be scanned. Your mileage +may vary.) The incremental/generational collection facility helps, +but is portable only if "stubborn" allocation is used. + Please address bug reports to boehm@acm.org. If you are +contemplating a major addition, you might also send mail to ask whether +it's already been done (or whether we tried and discarded it). + +RECENT VERSIONS: + + Version 1.3 and immediately preceding versions contained spurious +assembly language assignments to TMP_SP. Only the assignment in the PC/RT +code is necessary. On other machines, with certain compiler options, +the assignments can lead to an unsaved register being overwritten. +Known to cause problems under SunOS 3.5 WITHOUT the -O option. (With +-O the compiler recognizes it as dead code. It probably shouldn't, +but that's another story.) + + Version 1.4 and earlier versions used compile time determined values +for the stack base. This no longer works on Sun 3s, since Sun 3/80s use +a different stack base. We now use a straightforward heuristic on all +machines on which it is known to work (incl. Sun 3s) and compile-time +determined values for the rest. There should really be library calls +to determine such values. + + Version 1.5 and earlier did not ensure 8 byte alignment for objects +allocated on a sparc based machine. + + Version 1.8 added ULTRIX support in gc_private.h. + + Version 1.9 fixed a major bug in gc_realloc. + + Version 2.0 introduced a consistent naming convention for collector +routines and added support for registering dynamic library data segments +in the standard mark_roots.c. Most of the data structures were revamped. +The treatment of interior pointers was completely changed. Finalization +was added. Support for locking was added. Object kinds were added. +We added a black listing facility to avoid allocating at addresses known +to occur as integers somewhere in the address space. Much of this +was accomplished by adapting ideas and code from the PCR collector. +The test program was changed and expanded. + + Version 2.1 was the first stable version since 1.9, and added support +for PPCR. + + Version 2.2 added debugging allocation, and fixed various bugs. Among them: +- GC_realloc could fail to extend the size of the object for certain large object sizes. +- A blatant subscript range error in GC_printf, which unfortunately + wasn't exercised on machines with sufficient stack alignment constraints. +- GC_register_displacement did the wrong thing if it was called after + any allocation had taken place. +- The leak finding code would eventually break after 2048 byte + byte objects leaked. +- interface.c didn't compile. +- The heap size remained much too small for large stacks. +- The stack clearing code behaved badly for large stacks, and perhaps + on HP/PA machines. + + Version 2.3 added ALL_INTERIOR_POINTERS and fixed the following bugs: +- Missing declaration of etext in the A/UX version. +- Some PCR root-finding problems. +- Blacklisting was not 100% effective, because the plausible future + heap bounds were being miscalculated. +- GC_realloc didn't handle out-of-memory correctly. +- GC_base could return a nonzero value for addresses inside free blocks. +- test.c wasn't really thread safe, and could erroneously report failure + in a multithreaded environment. (The locking primitives need to be + replaced for other threads packages.) +- GC_CONS was thoroughly broken. +- On a SPARC with dynamic linking, signals stayed diabled while the + client code was running. + (Thanks to Manuel Serrano at INRIA for reporting the last two.) + + Version 2.4 added GC_free_space_divisor as a tuning knob, added + support for OS/2 and linux, and fixed the following bugs: +- On machines with unaligned pointers (e.g. Sun 3), every 128th word could + fail to be considered for marking. +- Dynamic_load.c erroneously added 4 bytes to the length of the data and + bss sections of the dynamic library. This could result in a bad memory + reference if the actual length was a multiple of a page. (Observed on + Sun 3. Can probably also happen on a Sun 4.) + (Thanks to Robert Brazile for pointing out that the Sun 3 version + was broken. Dynamic library handling is still broken on Sun 3s + under 4.1.1U1, but apparently not 4.1.1. If you have such a machine, + use -Bstatic.) + + Version 2.5 fixed the following bugs: +- Removed an explicit call to exit(1) +- Fixed calls to GC_printf and GC_err_printf, so the correct number of + arguments are always supplied. The OS/2 C compiler gets confused if + the number of actuals and the number of formals differ. (ANSI C + doesn't require this to work. The ANSI sanctioned way of doing things + causes too many compatibility problems.) + + Version 3.0 added generational/incremental collection and stubborn + objects. + + Version 3.1 added the following features: +- A workaround for a SunOS 4.X SPARC C compiler + misfeature that caused problems when the collector was turned into + a dynamic library. +- A fix for a bug in GC_base that could result in a memory fault. +- A fix for a performance bug (and several other misfeatures) pointed + out by Dave Detlefs and Al Dosser. +- Use of dirty bit information for static data under Solaris 2.X. +- DEC Alpha/OSF1 support (thanks to Al Dosser). +- Incremental collection on more platforms. +- A more refined heap expansion policy. Less space usage by default. +- Various minor enhancements to reduce space usage, and to reduce + the amount of memory scanned by the collector. +- Uncollectable allocation without per object overhead. +- More conscientious handling of out-of-memory conditions. +- Fixed a bug in debugging stubborn allocation. +- Fixed a bug that resulted in occasional erroneous reporting of smashed + objects with debugging allocation. +- Fixed bogus leak reports of size 4096 blocks with FIND_LEAK. + + Version 3.2 fixed a serious and not entirely repeatable bug in + the incremental collector. It appeared only when dirty bit info + on the roots was available, which is normally only under Solaris. + It also added GC_general_register_disappearing_link, and some + testing code. Interface.c disappeared. + + Version 3.3 fixes several bugs and adds new ports: +- PCR-specific bugs. +- Missing locking in GC_free, redundant FASTUNLOCK + in GC_malloc_stubborn, and 2 bugs in + GC_unregister_disappearing_link. + All of the above were pointed out by Neil Sharman + (neil@cs.mu.oz.au). +- Common symbols allocated by the SunOS4.X dynamic loader + were not included in the root set. +- Bug in GC_finalize (reported by Brian Beuning and Al Dosser) +- Merged Amiga port from Jesper Peterson (untested) +- Merged NeXT port from Thomas Funke (significantly + modified and untested) + + Version 3.4: +- Fixed a performance bug in GC_realloc. +- Updated the amiga port. +- Added NetBSD and 386BSD ports. +- Added cord library. +- Added trivial performance enhancement for + ALL_INTERIOR_POINTERS. (Don't scan last word.) + + Version 3.5 +- Minor collections now mark from roots only once, if that + doesn't cause an excessive pause. +- The stack clearing heuristic was refined to prevent anomalies + with very heavily recursive programs and sparse stacks. +- Fixed a bug that prevented mark stack growth in some cases. + GC_objects_are_marked should be set to TRUE after a call + to GC_push_roots and as part of GC_push_marked, since + both can now set mark bits. I think this is only a performance + bug, but I wouldn't bet on it. It's certainly very hard to argue + that the old version was correct. +- Fixed an incremental collection bug that prevented it from + working at all when HBLKSIZE != getpagesize() +- Changed dynamic_loading.c to include gc_priv.h before testing + DYNAMIC_LOADING. SunOS dynamic library scanning + must have been broken in 3.4. +- Object size rounding now adapts to program behavior. +- Added a workaround (provided by Manuel Serrano and + colleagues) to a long-standing SunOS 4.X (and 3.X?) ld bug + that I had incorrectly assumed to have been squished. + The collector was broken if the text segment size was within + 32 bytes of a multiple of 8K bytes, and if the beginning of + the data segment contained interesting roots. The workaround + assumes a demand-loadable executable. The original may have + have "worked" in some other cases. +- Added dynamic library support under IRIX5. +- Added support for EMX under OS/2 (thanks to Ari Huttunen). + +Version 3.6: +- fixed a bug in the mark stack growth code that was introduced + in 3.4. +- fixed Makefile to work around DEC AXP compiler tail recursion + bug. + +Version 3.7: +- Added a workaround for an HP/UX compiler bug. +- Fixed another stack clearing performance bug. Reworked + that code once more. + +Version 4.0: +- Added support for Solaris threads (which was possible + only by reimplementing some fraction of Solaris threads, + since Sun doesn't currently make the thread debugging + interface available). +- Added non-threads win32 and win32S support. +- (Grudgingly, with suitable muttering of obscenities) renamed + files so that the collector distribution could live on a FAT + file system. Files that are guaranteed to be useless on + a PC still have long names. Gc_inline.h and gc_private.h + still exist, but now just include gc_inl.h and gc_priv.h. +- Fixed a really obscure bug in finalization that could cause + undetected mark stack overflows. (I would be surprised if + any real code ever tickled this one.) +- Changed finalization code to dynamically resize the hash + tables it maintains. (This probably does not matter for well- + -written code. It no doubt does for C++ code that overuses + destructors.) +- Added typed allocation primitives. Rewrote the marker to + accommodate them with more reasonable efficiency. This + change should also speed up marking for GC_malloc allocated + objects a little. See gc_typed.h for new primitives. +- Improved debugging facilities slightly. Allocation time + stack traces are now kept by default on SPARC/SUNOS4. + (Thanks to Scott Schwartz.) +- Added better support for small heap applications. +- Significantly extended cord package. Fixed a bug in the + implementation of lazily read files. Printf and friends now + have cord variants. Cord traversals are a bit faster. +- Made ALL_INTERIOR_POINTERS recognition the default. +- Fixed de so that it can run in constant space, independent + of file size. Added simple string searching to cords and de. +- Added the Hull-Ellis C++ interface. +- Added dynamic library support for OSF/1. + (Thanks to Al Dosser and Tim Bingham at DEC.) +- Changed argument to GC_expand_hp to be expressed + in units of bytes instead of heap blocks. (Necessary + since the heap block size now varies depending on + configuration. The old version was never very clean.) +- Added GC_get_heap_size(). The previous "equivalent" + was broken. +- Restructured the Makefile a bit. + +Since version 4.0: +- Changed finalization implementation to guarantee that + finalization procedures are called outside of the allocation + lock, making direct use of the interface a little less dangerous. + MAY BREAK EXISTING CLIENTS that assume finalizers + are protected by a lock. Since there seem to be few multithreaded + clients that use finalization, this is hopefully not much of + a problem. +- Fixed a gross bug in CORD_prev. +- Fixed a bug in blacklst.c that could result in unbounded + heap growth during startup on machines that do not clear + memory obtained from the OS (e.g. win32S). +- Ported de editor to win32/win32S. (This is now the only + version with a mouse-sensitive UI.) +- Added GC_malloc_ignore_off_page to allocate large arrays + in the presence of ALL_INTERIOR_POINTERS. +- Changed GC_call_with_alloc_lock to not disable signals in + the single-threaded case. +- Reduced retry count in GC_collect_or_expand for garbage + collecting when out of memory. +- Made uncollectable allocations bypass black-listing, as they + should. +- Fixed a bug in typed_test in test.c that could cause (legitimate) + GC crashes. +- Fixed some potential synchronization problems in finalize.c +- Fixed a real locking problem in typd_mlc.c. +- Worked around an AIX 3.2 compiler feature that results in + out of bounds memory references. +- Partially worked around an IRIX5.2 beta problem (which may + or may not persist to the final release). +- Fixed a bug in the heap integrity checking code that could + result in explicitly deallocated objects being identified as + smashed. Fixed a bug in the dbg_mlc stack saving code + that caused old argument pointers to be considered live. +- Fixed a bug in CORD_ncmp (and hence CORD_str). +- Repaired the OS2 port, which had suffered from bit rot + in 4.0. Worked around what appears to be CSet/2 V1.0 + optimizer bug. +- Fixed a Makefile bug for target "c++". + +Since version 4.1: +- Multiple bug fixes/workarounds in the Solaris threads version. + (It occasionally failed to locate some register contents for + marking. It also turns out that thr_suspend and friends are + unreliable in Solaris 2.3. Dirty bit reads appear + to be unreliable under some weird + circumstances. My stack marking code + contained a serious performance bug. The new code is + extremely defensive, and has not failed in several cpu + hours of testing. But no guarantees ...) +- Added MacOS support (thanks to Patrick Beard.) +- Fixed several syntactic bugs in gc_c++.h and friends. (These + didn't bother g++, but did bother most other compilers.) + Fixed gc_c++.h finalization interface. (It didn't.) +- 64 bit alignment for allocated objects was not guaranteed in a + few cases in which it should have been. +- Added GC_malloc_atomic_ignore_off_page. +- Added GC_collect_a_little. +- Added some prototypes to gc.h. +- Some other minor bug fixes (notably in Makefile). +- Fixed OS/2 / EMX port (thanks to Ari Huttunen). +- Fixed AmigaDOS port. (thanks to Michel Schinz). +- Fixed the DATASTART definition under Solaris. There + was a 1 in 16K chance of the collector missing the first + 64K of static data (and thus crashing). +- Fixed some blatant anachronisms in the README file. +- Fixed PCR-Makefile for upcoming PPCR release. + +Since version 4.2: +- Fixed SPARC alignment problem with GC_DEBUG. +- Fixed Solaris threads /proc workaround. The real + problem was an interaction with mprotect. +- Incorporated fix from Patrick Beard for gc_c++.h (now gc_cpp.h). +- Slightly improved allocator space utilization by + fixing the GC_size_map mechanism. +- Integrated some Sony News and MIPS RISCos 4.51 + patches. (Thanks to Nobuyuki Hikichi of + Software Research Associates, Inc. Japan) +- Fixed HP_PA alignment problem. (Thanks to + xjam@cork.cs.berkeley.edu.) +- Added GC_same_obj and friends. Changed GC_base + to return 0 for pointers past the end of large objects. + Improved GC_base performance with ALL_INTERIOR_POINTERS + on machines with a slow integer mod operation. + Added GC_PTR_ADD, GC_PTR_STORE, etc. to prepare + for preprocessor. +- changed the default on most UNIX machines to be that + signals are not disabled during critical GC operations. + This is still ANSI-conforming, though somewhat dangerous + in the presence of signal handlers. But the performance + cost of the alternative is sometimes problematic. + Can be changed back with a minor Makefile edit. +- renamed IS_STRING in gc.h, to CORD_IS_STRING, thus + following my own naming convention. Added the function + CORD_to_const_char_star. +- Fixed a gross bug in GC_finalize. Symptom: occasional + address faults in that function. (Thanks to Anselm + Baird-Smith (Anselm.BairdSmith@inria.fr) +- Added port to ICL DRS6000 running DRS/NX. Restructured + things a bit to factor out common code, and remove obsolete + code. Collector should now run under SUNOS5 with either + mprotect or /proc dirty bits. (Thanks to Douglas Steel + (doug@wg.icl.co.uk)). +- More bug fixes and workarounds for Solaris 2.X. (These were + mostly related to putting the collector in a dynamic library, + which didn't really work before. Also SOLARIS_THREADS + didn't interact well with dl_open.) Thanks to btlewis@eng.sun.com. +- Fixed a serious performance bug on the DEC Alpha. The text + segment was getting registered as part of the root set. + (Amazingly, the result was still fast enough that the bug + was not conspicuous.) The fix works on OSF/1, version 1.3. + Hopefully it also works on other versions of OSF/1 ... +- Fixed a bug in GC_clear_roots. +- Fixed a bug in GC_generic_malloc_words_small that broke + gc_inl.h. (Reported by Antoine de Maricourt. I broke it + in trying to tweak the Mac port.) +- Fixed some problems with cord/de under Linux. +- Fixed some cord problems, notably with CORD_riter4. +- Added DG/UX port. + Thanks to Ben A. Mesander (ben@piglet.cr.usgs.gov) +- Added finalization registration routines with weaker ordering + constraints. (This is necessary for C++ finalization with + multiple inheritance, since the compiler often adds self-cycles.) +- Filled the holes in the SCO port. (Thanks to Michael Arnoldus + <chime@proinf.dk>.) +- John Ellis' additions to the C++ support: From John: + +* I completely rewrote the documentation in the interface gc_c++.h +(later renamed gc_cpp.h). I've tried to make it both clearer and more +precise. + +* The definition of accessibility now ignores pointers from an +finalizable object (an object with a clean-up function) to itself. +This allows objects with virtual base classes to be finalizable by the +collector. Compilers typically implement virtual base classes using +pointers from an object to itself, which under the old definition of +accessibility prevented objects with virtual base classes from ever +being collected or finalized. + +* gc_cleanup now includes gc as a virtual base. This was enabled by +the change in the definition of accessibility. + +* I added support for operator new[]. Since most (all?) compilers +don't yet support operator new[], it is conditionalized on +-DOPERATOR_NEW_ARRAY. The code is untested, but its trivial and looks +correct. + +* The test program test_gc_c++ (later renamed test_cpp.cc) +tries to test for the C++-specific functionality not tested by the +other programs. +- Added <unistd.h> include to misc.c. (Needed for ppcr.) +- Added PowerMac port. (Thanks to Patrick Beard again.) +- Fixed "srcdir"-related Makefile problems. Changed things so + that all externally visible include files always appear in the + include subdirectory of the source. Made gc.h directly + includable from C++ code. (These were at Per + Bothner's suggestion.) +- Changed Intel code to also mark from ebp (Kevin Warne's + suggestion). +- Renamed C++ related files so they could live in a FAT + file system. (Charles Fiterman's suggestion.) +- Changed Windows NT Makefile to include C++ support in + gc.lib. Added C++ test as Makefile target. + +Since version 4.3: + - ASM_CLEAR_CODE was erroneously defined for HP + PA machines, resulting in a compile error. + - Fixed OS/2 Makefile to create a library. (Thanks to + Mark Boulter (mboulter@vnet.ibm.com)). + - Gc_cleanup objects didn't work if they were created on + the stack. Fixed. + - One copy of Gc_cpp.h in the distribution was out of + synch, and failed to document some known compiler + problems with explicit destructor invocation. Partially + fixed. There are probably other compilers on which + gc_cleanup is miscompiled. + - Fixed Makefile to pass C compiler flags to C++ compiler. + - Added Mac fixes. + - Fixed os_dep.c to work around what appears to be + a new and different VirtualQuery bug under newer + versions of win32S. + - GC_non_gc_bytes was not correctly maintained by + GC_free. Fixed. Thanks to James Clark (jjc@jclark.com). + - Added GC_set_max_heap_size. + - Changed allocation code to ignore blacklisting if it is preventing + use of a very large block of memory. This has the advantage + that naive code allocating very large objects is much more + likely to work. The downside is you might no + longer find out that such code should really use + GC_malloc_ignore_off_page. + - Changed GC_printf under win32 to close and reopen the file + between calls. FAT file systems otherwise make the log file + useless for debugging. + - Added GC_try_to_collect and GC_get_bytes_since_gc. These + allow starting an abortable collection during idle times. + This facility does not require special OS support. (Thanks to + Michael Spertus of Geodesic Systems for suggesting this. It was + actually an easy addition. Kumar Srikantan previously added a similar + facility to a now ancient version of the collector. At the time + this was much harder, and the result was less convincing.) + - Added some support for the Borland development environment. (Thanks + to John Ellis and Michael Spertus.) + - Removed a misfeature from checksums.c that caused unexpected + heap growth. (Thanks to Scott Schwartz.) + - Changed finalize.c to call WARN if it encounters a finalization cycle. + WARN is defined in gc_priv.h to write a message, usually to stdout. + In many environments, this may be inappropriate. + - Renamed NO_PARAMS in gc.h to GC_NO_PARAMS, thus adhering to my own + naming convention. + - Added GC_set_warn_proc to intercept warnings. + - Fixed Amiga port. (Thanks to Michel Schinz (schinz@alphanet.ch).) + - Fixed a bug in mark.c that could result in an access to unmapped + memory from GC_mark_from_mark_stack on machines with unaligned + pointers. + - Fixed a win32 specific performance bug that could result in scanning of + objects allocated with the system malloc. + - Added REDIRECT_MALLOC. + +Since version 4.4: + - Fixed many minor and one major README bugs. (Thanks to Franklin Chen + (chen@adi.com) for pointing out many of them.) + - Fixed ALPHA/OSF/1 dynamic library support. (Thanks to Jonathan Bachrach + (jonathan@harlequin.com)). + - Added incremental GC support (MPROTECT_VDB) for Linux (with some + help from Bruno Haible). + - Altered SPARC recognition tests in gc.h and config.h (mostly as + suggested by Fergus Henderson). + - Added basic incremental GC support for win32, as implemented by + Windows NT and Windows 95. GC_enable_incremental is a noop + under win32s, which doesn't implement enough of the VM interface. + - Added -DLARGE_CONFIG. + - Fixed GC_..._ignore_off_page to also function without + -DALL_INTERIOR_POINTERS. + - (Hopefully) fixed RS/6000 port. (Only the test was broken.) + - Fixed a performance bug in the nonincremental collector running + on machines supporting incremental collection with MPROTECT_VDB + (e.g. SunOS 4, DEC AXP). This turned into a correctness bug under + win32s with win32 incremental collection. (Not all memory protection + was disabled.) + - Fixed some ppcr related bit rot. + - Caused dynamic libraries to be unregistered before reregistering. + The old way turned out to be a performance bug on some machines. + - GC_root_size was not properly maintained under MSWIN32. + - Added -DNO_DEBUGGING and GC_dump. + - Fixed a couple of bugs arising with SOLARIS_THREADS + + REDIRECT_MALLOC. + - Added NetBSD/M68K port. (Thanks to Peter Seebach + <seebs@taniemarie.solon.com>.) + - Fixed a serious realloc bug. For certain object sizes, the collector + wouldn't scan the expanded part of the object. (Thanks to Clay Spence + (cds@peanut.sarnoff.com) for noticing the problem, and helping me to + track it down.) + +Since version 4.5: + - Added Linux ELF support. (Thanks to Arrigo Triulzi <arrigo@ic.ac.uk>.) + - GC_base crashed if it was called before any other GC_ routines. + This could happen if a gc_cleanup object was allocated outside the heap + before any heap allocation. + - The heap expansion heuristic was not stable if all objects had finalization + enabled. Fixed finalize.c to count memory in finalization queue and + avoid explicit deallocation. Changed alloc.c to also consider this count. + (This is still not recommended. It's expensive if nothing else.) Thanks + to John Ellis for pointing this out. + - GC_malloc_uncollectable(0) was broken. Thanks to Phong Vo for pointing + this out. + - The collector didn't compile under Linux 1.3.X. (Thanks to Fred Gilham for + pointing this out.) The current workaround is ugly, but expected to be + temporary. + - Fixed a formatting problem for SPARC stack traces. + - Fixed some '=='s in os_dep.c that should have been assignments. + Fortunately these were in code that should never be executed anyway. + (Thanks to Fergus Henderson.) + - Fixed the heap block allocator to only drop blacklisted blocks in small + chunks. Made BL_LIMIT self adjusting. (Both of these were in response + to heap growth observed by Paul Graham.) + - Fixed the Metrowerks/68K Mac code to also mark from a6. (Thanks + to Patrick Beard.) + - Significantly updated README.debugging. + - Fixed some problems with longjmps out of signal handlers, especially under + Solaris. Added a workaround for the fact that siglongjmp doesn't appear to + do the right thing with -lthread under Solaris. + - Added MSDOS/djgpp port. (Thanks to Mitch Harris (maharri@uiuc.edu).) + - Added "make reserved_namespace" and "make user_namespace". The + first renames ALL "GC_xxx" identifiers as "_GC_xxx". The second is the + inverse transformation. Note that doing this is guaranteed to break all + clients written for the other names. + - descriptor field for kind NORMAL in GC_obj_kinds with ADD_BYTE_AT_END + defined should be -ALIGNMENT not WORDS_TO_BYTES(-1). This is + a serious bug on machines with pointer alignment of less than a word. + - GC_ignore_self_finalize_mark_proc didn't handle pointers to very near the + end of the object correctly. Caused failures of the C++ test on a DEC Alpha + with g++. + - gc_inl.h still had problems. Partially fixed. Added warnings at the + beginning to hopefully specify the remaining dangers. + - Added DATAEND definition to config.h. + - Fixed some of the .h file organization. Fixed "make floppy". + +Since version 4.6: + - Fixed some compilation problems with -DCHECKSUMS (thanks to Ian Searle) + - Updated some Mac specific files to synchronize with Patrick Beard. + - Fixed a serious bug for machines with non-word-aligned pointers. + (Thanks to Patrick Beard for pointing out the problem. The collector + should fail almost any conceivable test immediately on such machines.) + +Since version 4.7: + - Changed a "comment" in a MacOS specific part of mach-dep.c that caused + gcc to fail on other platforms. + +Since version 4.8 + - More README.debugging fixes. + - Objects ready for finalization, but not finalized in the same GC + cycle, could be prematurely collected. This occasionally happened + in test_cpp. + - Too little memory was obtained from the system for very large + objects. That could cause a heap explosion if these objects were + not contiguous (e.g. under PCR), and too much of them was blacklisted. + - Due to an improper initialization, the collector was too hesitant to + allocate blacklisted objects immediately after system startup. + - Moved GC_arrays from the data into the bss segment by not explicitly + initializing it to zero. This significantly + reduces the size of executables, and probably avoids some disk accesses + on program startup. It's conceivable that it might break a port that I + didn't test. + - Fixed EMX_MAKEFILE to reflect the gc_c++.h to gc_cpp.h renaming which + occurred a while ago. + +Since 4.9: + - Fixed a typo around a call to GC_collect_or_expand in alloc.c. It broke + handling of out of memory. (Thanks to Patrick Beard for noticing.) + +Since 4.10: + - Rationalized (hopefully) GC_try_to_collect in an incremental collection + environment. It appeared to not handle a call while a collection was in + progress, and was otherwise too conservative. + - Merged GC_reclaim_or_delete_all into GC_reclaim_all to get rid of some + code. + - Added Patrick Beard's Mac fixes, with substantial completely untested + modifications. + - Fixed the MPROTECT_VDB code to deal with large pages and imprecise + fault addresses (as on an UltraSPARC running Solaris 2.5). Note that this + was not a problem in the default configuration, which uses PROC_VDB. + - The DEC Alpha assembly code needed to restore $gp between calls. + Thanks to Fergus Henderson for tracking this down and supplying a + patch. + - The write command for "de" was completely broken for large files. + I used the easiest portable fix, which involved changing the semantics + so that f.new is written instead of overwriting f. That's safer anyway. + - Added README.solaris2 with a discussion of the possible problems of + mixing the collector's sbrk allocation with malloc/realloc. + - Changed the data segment starting address for SGI machines. The + old code failed under IRIX6. + - Required double word alignment for MIPS. + - Various minor fixes to remove warnings. + - Attempted to fix some Solaris threads problems reported by Zhiying Chen. + In particular, the collector could try to fork a thread with the + world stopped as part of GC_thr_init. It also failed to deal with + the case in which the original thread terminated before the whole + process did. + - Added -DNO_EXECUTE_PERMISSION. This has a major performance impact + on the incremental collector under Irix, and perhaps under other + operating systems. + - Added some code to support allocating the heap with mmap. This may + be preferable under some circumstances. + - Integrated dynamic library support for HP. + (Thanks to Knut Tvedten <knuttv@ifi.uio.no>.) + - Integrated James Clark's win32 threads support, and made a number + of changes to it, many of which were suggested by Pontus Rydin. + This is still not 100% solid. + - Integrated Alistair Crooks' support for UTS4 running on an Amdahl + 370-class machine. + - Fixed a serious bug in explicitly typed allocation. Objects requiring + large descriptors where handled in a way that usually resulted in + a segmentation fault in the marker. (Thanks to Jeremy Fitzhardinge + for helping to track this down.) + - Added partial support for GNU win32 development. (Thanks to Fergus + Henderson.) + - Added optional support for Java-style finalization semantics. (Thanks + to Patrick Bridges.) This is recommended only for Java implementations. + - GC_malloc_uncollectable faulted instead of returning 0 when out of + memory. (Thanks to dan@math.uiuc.edu for noticing.) + - Calls to GC_base before the collector was initialized failed on a + DEC Alpha. (Thanks to Matthew Flatt.) + - Added base pointer checking to GC_REGISTER_FINALIZER in debugging + mode, at the suggestion of Jeremy Fitzhardinge. + - GC_debug_realloc failed for uncollectable objects. (Thanks to + Jeremy Fitzhardinge.) + - Explicitly typed allocation could crash if it ran out of memory. + (Thanks to Jeremy Fitzhardinge.) + - Added minimal support for a DEC Alpha running Linux. + - Fixed a problem with allocation of objects whose size overflowed + ptrdiff_t. (This now fails unconditionally, as it should.) + - Added the beginning of Irix pthread support. + - Integrated Xiaokun Zhu's fixes for djgpp 2.01. + - Added SGI-style STL allocator support (gc_alloc.h). + - Fixed a serious bug in README.solaris2. Multithreaded programs must include + gc.h with SOLARIS_THREADS defined. + - Changed GC_free so it actually deallocates uncollectable objects. + (Thanks to Peter Chubb for pointing out the problem.) + - Added Linux ELF support for dynamic libararies. (Thanks again to + Patrick Bridges.) + - Changed the Borland cc configuration so that the assembler is not + required. + - Fixed a bug in the C++ test that caused it to fail in 64-bit + environments. + +Since 4.11: + - Fixed ElfW definition in dyn_load.c. (Thanks to Fergus Henderson.) + This prevented the dynamic library support from compiling on some + older ELF Linux systems. + - Fixed UTS4 port (which I apparently mangled during the integration) + (Thanks to again to Alistair Crooks.) + - "Make C++" failed on Suns with SC4.0, due to a problem with "bool". + Fixed in gc_priv.h. + - Added more pieces for GNU win32. (Thanks to Timothy N. Newsham.) + The current state of things should suffice for at least some + applications. + - Changed the out of memory retry count handling as suggested by + Kenjiro Taura. (This matters only if GC_max_retries > 0, which + is no longer the default.) + - If a /proc read failed repeatedly, GC_written_pages was not updated + correctly. (Thanks to Peter Chubb for diagnosing this.) + - Under unlikely circumstances, the allocator could infinite loop in + an out of memory situation. (Thanks again to Kenjiro Taura for + identifying the problem and supplying a fix.) + - Fixed a syntactic error in the DJGPP code. (Thanks to Fergus + Henderson for finding this by inspection.) Also fixed a test program + problem with DJGPP (Thanks to Peter Monks.) + - Atomic uncollectable objects were not treated correctly by the + incremental collector. This resulted in weird log statistics and + occasional performance problems. (Thanks to Peter Chubb for pointing + this out.) + - Fixed some problems resulting from compilers that dont define + __STDC__. In this case void * and char * were used inconsistently + in some cases. (Void * should not have been used at all. If + you have an ANSI superset compiler that does not define __STDC__, + please compile with -D__STDC__=0. Thanks to Manuel Serrano and others + for pointing out the problem.) + - Fixed a compilation problem on Irix with -n32 and -DIRIX_THREADS. + Also fixed some other IRIX_THREADS problems which may or may not have + had observable symptoms. + - Fixed an HP PA compilation problem in dyn_load.c. (Thanks to + Philippe Queinnec.) + - SEGV fault handlers sometimes did not get reset correctly. (Thanks + to David Pickens.) + - Added a fix for SOLARIS_THREADS on Intel. (Thanks again to David + Pickens.) This probably needs more work to become functional. + - Fixed struct sigcontext_struct in os_dep.c for compilation under + Linux 2.1.X. (Thanks to Fergus Henderson.) + - Changed the DJGPP STACKBOTTOM and DATASTART values to those suggested + by Kristian Kristensen. These may still not be right, but it is + it is likely to work more often than what was there before. They may + even be exactly right. + - Added a #include <string.h> to test_cpp.cc. This appears to help + with HP/UX and gcc. (Thanks to assar@sics.se.) + - Version 4.11 failed to run in incremental mode on recent 64-bit Irix + kernels. This was a problem related to page unaligned heap segments. + Changed the code to page align heap sections on all platforms. + (I had mistakenly identified this as a kernel problem earlier. + It was not.) + - Version 4.11 did not make allocated storage executable, except on + one or two platforms, due to a bug in a #if test. (Thanks to Dave + Grove for pointing this out.) + - Added sparc_sunos4_mach_dep.s to support Sun's compilers under SunOS4. + - Added GC_exclude_static_roots. + - Fixed the object size mapping algorithm. This shouldn't matter, + but the old code was ugly. + - Heap checking code could die if one of the allocated objects was + larger than its base address. (Unsigned underflow problem. Thanks + to Clay Spence for isolating the problem.) + - Added RS6000 (AIX) dynamic library support and fixed STACK_BOTTOM. + (Thanks to Fred Stearns.) + - Added Fergus Henderson's patches for improved robustness with large + heaps and lots of blacklisting. + - Added Peter Chubb's changes to support Solaris Pthreads, to support + MMAP allocation in Solaris, to allow Solaris to find dynamic libraries + through /proc, to add malloc_typed_ignore_off_page, and a few other + minor features and bug fixes. + - The Solaris 2 port should not use sbrk. I received confirmation from + Sun that the use of sbrk and malloc in the same program is not + supported. The collector now defines USE_MMAP by default on Solaris. + - Replaced the djgpp makefile with Gary Leavens' version. + - Fixed MSWIN32 detection test. + - Added Fergus Henderson's patches to allow putting the collector into + a DLL under GNU win32. + - Added Ivan V. Demakov's port to Watcom C on X86. + - Added Ian Piumarta's Linux/PowerPC port. + - On Brian Burton's suggestion added PointerFreeGC to the placement + options in gc_cpp.h. This is of course unsafe, and may be controversial. + On the other hand, it seems to be needed often enough that it's worth + adding as a standard facility. + +Since 4.12: + - Fixed a crucial bug in the Watcom port. There was a redundant decl + of GC_push_one in gc_priv.h. + - Added FINALIZE_ON_DEMAND. + - Fixed some pre-ANSI cc problems in test.c. + - Removed getpagesize() use for Solaris. It seems to be missing in one + or two versions. + - Fixed bool handling for SPARCCompiler version 4.2. + - Fixed some files in include that had gotten unlinked from the main + copy. + - Some RS/6000 fixes (missing casts). Thanks to Toralf Foerster. + - Fixed several problems in GC_debug_realloc, affecting mostly the + FIND_LEAK case. + - GC_exclude_static_roots contained a buggy unsigned comparison to + terminate a loop. (Thanks to Wilson Ho.) + - CORD_str failed if the substring occurred at the last possible position. + (Only affects cord users.) + - Fixed Linux code to deal with RedHat 5.0 and integrated Peter Bigot's + os_dep.c code for dealing with various Linux versions. + - Added workaround for Irix pthreads sigaction bug and possible signal + misdirection problems. +Since alpha1: + - Changed RS6000 STACKBOTTOM. + - Integrated Patrick Beard's Mac changes. + - Alpha1 didn't compile on Irix m.n, m < 6. + - Replaced Makefile.dj with a new one from Gary Leavens. + - Added Andrew Stitcher's changes to support SCO OpenServer. + - Added PRINT_BLACK_LIST, to allow debugging of high densities of false + pointers. + - Added code to debug allocator to keep track of return address + in GC_malloc caller, thus giving a bit more context. + - Changed default behavior of large block allocator to more + aggressively avoid fragmentation. This is likely to slow down the + collector when it succeeds at reducing space cost. + - Integrated Fergus Henderson's CYGWIN32 changes. They are untested, + but needed for newer versions. + - USE_MMAP had some serious bugs. This caused the collector to fail + consistently on Solaris with -DSMALL_CONFIG. + - Added Linux threads support, thanks largely to Fergus Henderson. +Since alpha2: + - Fixed more Linux threads problems. + - Changed default GC_free_space_divisor to 3 with new large block allocation. + (Thanks to Matthew Flatt for some measurements that suggest the old + value sometimes favors space too much over time.) + - More CYGWIN32 fixes. + - Integrated Tyson-Dowd's Linux-M68K port. + - Minor HP PA and DEC UNIX fixes from Fergus Henderson. + - Integrated Christoffe Raffali's Linux-SPARC changes. + - Allowed for one more GC fixup iteration after a full GC in incremental + mode. Some quick measurements suggested that this significantly + reduces pause times even with smaller GC_RATE values. + - Moved some more GC data structures into GC_arrays. This decreases + pause times and GC overhead, but makes debugging slightly less convenient. + - Fixed namespace pollution problem ("excl_table"). + - Made GC_incremental a constant for -DSMALL_CONFIG, hopefully shrinking + that slightly. + - Added some win32 threads fixes. + - Integrated Ivan Demakov and David Stes' Watcom fixes. + - Various other minor fixes contributed by many people. + - Renamed config.h to gcconfig.h, since config.h tends to be used for + many other things. + - Integrated Matthew Flatt's support for 68K MacOS "far globals". + - Fixed up some of the dynamic library Makefile targets for consistency + across platforms. + - Fixed a USE_MMAP typo that caused out-of-memory handling to fail + on Solaris. + - Added code to test.c to test thread creation a bit more. + - Integrated GC_win32_free_heap, as suggested by Ivan Demakov. + - Fixed Solaris 2.7 stack base finding problem. (This may actually + have been done in an earlier alpha release.) +Since alpha3: + - Fixed MSWIN32 recognition test, which interfered with cygwin. + - Removed unnecessary gc_watcom.asm from distribution. Removed + some obsolete README.win32 text. + - Added Alpha Linux incremental GC support. (Thanks to Philipp Tomsich + for code for retrieving the fault address in a signal handler.) + Changed Linux signal handler context argument to be a pointer. + - Took care of some new warnings generated by the 7.3 SGI compiler. + - Integrated Phillip Musumeci's FreeBSD/ELF fixes. + - -DIRIX_THREADS was broken with the -o32 ABI (typo in gc_priv.h> + +Since 4.13: + - Fixed GC_print_source_ptr to not use a prototype. + - generalized CYGWIN test. + - gc::new did the wrong thing with PointerFreeGC placement. + (Thanks to Rauli Ruohonen.) + - In the ALL_INTERIOR_POINTERS (default) case, some callee-save register + values could fail to be scanned if the register was saved and + reused in a GC frame. This showed up in verbose mode with gctest + compiled with an unreleased SGI compiler. I vaguely recall an old + bug report that may have been related. The bug was probably quite old. + (The problem was that the stack scanning could be deferred until + after the relevant frame was overwritten, and the new save location + might be outside the scanned area. Fixed by more eager stack scanning.) + - PRINT_BLACK_LIST had some problems. A few source addresses were garbage. + - Replaced Makefile.dj and added -I flags to cord make targets. + (Thanks to Gary Leavens.) + - GC_try_to_collect was broken with the nonincremental collector. + - gc_cleanup destructors could pass the wrong address to + GC_register_finalizer_ignore_self in the presence of multiple + inheritance. (Thanks to Darrell Schiebel.) + - Changed PowerPC Linux stack finding code. + +Since 4.14alpha1 + - -DSMALL_CONFIG did not work reliably with large (> 4K) pages. + Recycling the mark stack during expansion could result in a size + zero heap segment, which confused things. (This was probably also an + issue with the normal config and huge pages.) + - Did more work to make sure that callee-save registers were scanned + completely, even with the setjmp-based code. Added USE_GENERIC_PUSH_REGS + macro to facilitate testing on machines I have access to. + - Added code to explicitly push register contents for win32 threads. + This seems to be necessary. (Thanks to Pierre de Rop.) + +Since 4.14alpha2 + - changed STACKBOTTOM for DJGPP (Thanks to Salvador Eduardo Tropea). + +Since 4.14 + - Reworked large block allocator. Now uses multiple doubly linked free + lists to approximate best fit. + - Changed heap expansion heuristic. Entirely free blocks are no longer + counted towards the heap size. This seems to have a major impact on + heap size stability; the old version could expand the heap way too + much in the presence of large block fragmentation. + - added -DGC_ASSERTIONS and some simple assertions inside the collector. + This is mainlyt for collector debugging. + - added -DUSE_MUNMAP to allow the heap to shrink. Suupported on only + a few UNIX-like platforms for now. + - added GC_dump_regions() for debugging of fragmentation issues. + - Changed PowerPC pointer alignment under Linux to 4. (This needs + checking by someone who has one. The suggestions came to me via a + rather circuitous path.) + - Changed the Linux/Alpha port to walk the data segment backwards until + it encounters a SIGSEGV. The old way to find the start of the data + segment broke with a recent release. + - cordxtra.c needed to call GC_REGISTER_FINALIZER instead of + GC_register_finalizer, so that it would continue to work with GC_DEBUG. + - allochblk sometimes cleared the wrong block for debugging purposes + when it dropped blacklisted blocks. This could result in spurious + error reports with GC_DEBUG. + - added MACOS X Server support. (Thanks to Andrew Stone.) + - Changed the Solaris threads code to ignore stack limits > 8 MB with + a warning. Empirically, it is not safe to access arbitrary pages + in such large stacks. And the dirty bit implementation does not + guarantee that none of them will be accessed. + - Integrated Martin Tauchmann's Amiga changes. + - Integrated James Dominy's OpenBSD/SPARC port. + +Since 5.0alpha1 + - Fixed bugs introduced in alpha1 (OpenBSD & large block initialization). + - Added -DKEEP_BACK_PTRS and backptr.h interface. (The implementation + idea came from Al Demers.) + +Since 5.0alpha2 + - Added some highly incomplete code to support a copied young generation. + Comments on nursery.h are appreciated. + - Changed -DFIND_LEAK, -DJAVA_FINALIZATION, and -DFINALIZE_ON_DEMAND, + so the same effect could be obtained with a runtime switch. This is + a step towards standardizing on a single dynamic GC library. + - Significantly changed the way leak detection is handled, as a consequence + of the above. + +To do: + - Very large root set sizes (> 16 MB or so) could cause the collector + to abort with an unexpected mark stack overflow. (Thanks again to + Peter Chubb.) NOT YET FIXED. Workaround is to increase the initial + size. + - The SGI version of the collector marks from mmapped pages, even + if they are not part of dynamic library static data areas. This + causes performance problems with some SGI libraries that use mmap + as a bitmap allocator. NOT YET FIXED. It may be possible to turn + off DYNAMIC_LOADING in the collector as a workaround. It may also + be possible to conditionally intercept mmap and use GC_exclude_static_roots. + The real fix is to walk rld data structures, which looks possible. + - Integrate MIT and DEC pthreads ports. + - Deal with very uneven black-listing distributions. If all the black listed + blocks reside in the newly allocated heap section, the heuristic for + temporarily ignoring black-listing fails, and the heap grows too much. + (This was observed in only one case, and could be worked around, but ...) + - Some platform specific updates are waiting for 4.15alpha1. |