+++ /dev/null
-Copyright (c) 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.
-Copyright (c) 1999-2001 by Hewlett-Packard Company. All rights reserved.
-
-The file linux_threads.c is also
-Copyright (c) 1998 by Fergus Henderson. All rights reserved.
-
-The files Makefile.am, and configure.in are
-Copyright (c) 2001 by Red Hat Inc. All rights reserved.
-
-The files config.guess and a few others are copyrighted by the Free
-Software Foundation.
-
-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.
-
-A few of the files needed to use the GNU-style build procedure come with
-slightly different licenses, though they are all similar in spirit. A few
-are GPL'ed, but with an exception that should cover all uses in the
-collector. (If you are concerned about such things, I recommend you look
-at the notice in config.guess or ltmain.sh.)
-
-This is version 6.1alpha3 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
-
-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.
-
-Boehm H., "Reducing Garbage Collector Cache Misses", Proceedings of the
-2000 International Symposium on Memory Management.
-
- 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.
-
-(Some of these are also available from
-http://www.hpl.hp.com/personal/Hans_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.) On the most common platforms, it can be built
-with or without thread support. On a few platforms, it can take advantage
-of a multiprocessor to speed up garbage collection.
-
- Many of the ideas underlying the collector have previously been explored
-by others. Notably, some of the run-time systems developed at Xerox PARC
-in the early 1980s conservatively scanned thread stacks to locate possible
-pointers (cf. Paul Rovner, "On Adding Garbage Collection and Runtime Types
-to a Strongly-Typed Statically Checked, Concurrent Language" Xerox PARC
-CSL 84-7). Doug McIlroy wrote a simpler fully conservative collector that
-was part of version 8 UNIX (tm), but appears to not have received
-widespread use.
-
- Rudimentary tools for use of the collector as a leak detector are included
-(see http://www.hpl.hp.com/personal/Hans_Boehm/gc/leak.html),
-as is a fairly sophisticated string package "cord" that makes use of the
-collector. (See doc/README.cords and H.-J. Boehm, R. Atkinson, and M. Plass,
-"Ropes: An Alternative to Strings", Software Practice and Experience 25, 12
-(December 1995), pp. 1315-1330. This is very similar to the "rope" package
-in Xerox Cedar, or the "rope" package in the SGI STL or the g++ distribution.)
-
-Further collector documantation can be found at
-
-http://www.hpl.hp.com/personal/Hans_Boehm/gc
-
-
-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.
-
- WARNING: 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.)
-
- WARNING: 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.
-If you're writing a program that depends on the collector scanning
-dynamic library data areas, it may be a good idea to include at least
-one call to GC_is_visible() to ensure that those areas are visible
-to 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.
-Like standard ANSI C mallocs, by default it is unsafe to invoke
-malloc (and other GC routines) from a signal handler while another
-malloc call may be in progress. Removing -DNO_SIGNALS from Makefile
-attempts to remedy that. But that may not be reliable with a compiler that
-substantially reorders memory operations inside GC_malloc.
-
- 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.)
-WARNING: the collector does not guarantee to scan thread-local storage
-(e.g. of the kind accessed with pthread_getspecific()). The collector
-does scan thread stacks, though, so generally the best solution is to
-ensure that any pointers stored in thread-local storage are also
-stored on the thread's stack for the duration of their lifetime.
-(This is arguably a longstanding bug, but it hasn't been fixed yet.)
-
-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. It may use up to 8 MB of memory. (The
-multi-threaded version will use more. 64-bit versions may 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, but
-the garbage collector itself should still be usable.
-
- 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
-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),
-Linux, 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. Such objects will
-also be automatically reclaimed.
- 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).
-
projects / msp430-gcc.git / blobdiff