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diff --git a/open_issues/boehm_gc.mdwn b/open_issues/boehm_gc.mdwn new file mode 100644 index 00000000..540eed3b --- /dev/null +++ b/open_issues/boehm_gc.mdwn @@ -0,0 +1,276 @@ +[[!meta copyright="Copyright © 2010 Free Software Foundation, Inc."]] + +[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable +id="license" text="Permission is granted to copy, distribute and/or modify this +document under the terms of the GNU Free Documentation License, Version 1.2 or +any later version published by the Free Software Foundation; with no Invariant +Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license +is included in the section entitled [[GNU Free Documentation +License|/fdl]]."]]"""]] + +Here's what's to be done for maintaining Boehm GC. + +This one does need Hurd-specific configuration. + +It is, for example, used by [[/GCC]] (which has its own fork), so any changes +committed upstream should very like also be made there. + + +# [[General information|/boehm_gc]] + + +# [[Sources|source_repositories/boehm_gc]] + + +# Configuration + +[[tschwinge]] reviewed its GNU/Hurd port's configuration on 2010-11-10, based +on CVS HEAD sources from 2010-11-16, converted to Git: +9abb37b2e581b415bb1f482085891a289c2c0be1. + + * `configure.ac` + + * `PARALLEL_MARK` is not enabled; doesn't make sense so far. + + * `*-*-kfreebsd*-gnu` defines `USE_COMPILER_TLS`. What's this, and + why does not other config? + + * TODO + + [ if test "$enable_gc_debug" = "yes"; then + AC_MSG_WARN("Should define GC_DEBUG and use debug alloc. in clients.") + AC_DEFINE([KEEP_BACK_PTRS], 1, + [Define to save back-pointers in debugging headers.]) + keep_back_ptrs=true + AC_DEFINE([DBG_HDRS_ALL], 1, + [Define to force debug headers on all objects.]) + case $host in + x86-*-linux* | i586-*-linux* | i686-*-linux* | x86_64-*-linux* ) + AC_DEFINE(MAKE_BACK_GRAPH) + AC_MSG_WARN("Client must not use -fomit-frame-pointer.") + AC_DEFINE(SAVE_CALL_COUNT, 8) + ;; + AM_CONDITIONAL([KEEP_BACK_PTRS], [test x"$keep_back_ptrs" = xtrue]) + + * `configure.host` + + Nothing. + + * `Makefile.am`, `include/include.am`, `cord/cord.am`, `doc/doc.am`, + `tests/tests.am` + + Nothing. + + * `include/gc_config_macros.h` + + Should be OK. + + * `include/private/gcconfig.h` + + Hairy. But should be OK. Search for *HURD*, compare to *LINUX*, + *I386* case. + + See `doc/porting.html` and `doc/README.macros` (and others) for + documentation. + + *LINUX* has: + + * `#define LINUX_STACKBOTTOM` + + Defined instead of `STACKBOTTOM` to have the value read from `/proc/`. + + * `#define HEAP_START (ptr_t)0x1000` + + May want to define it for us, too? + + * `#ifdef USE_I686_PREFETCH`, `USE_3DNOW_PREFETCH` --- [...] + + Apparently these are optimization that we also could use. Have a + look at *LINUX* for *X86_64*, which uses `__builtin_prefetch` + (which Linux x86 could use, too?). + + * TODO + + #if defined(LINUX) && defined(USE_MMAP) + /* The kernel may do a somewhat better job merging mappings etc. */ + /* with anonymous mappings. */ + # define USE_MMAP_ANON + #endif + + * TODO + + #if defined(GC_LINUX_THREADS) && defined(REDIRECT_MALLOC) + /* Nptl allocates thread stacks with mmap, which is fine. But it */ + /* keeps a cache of thread stacks. Thread stacks contain the */ + /* thread control blocks. These in turn contain a pointer to */ + /* (sizeof (void *) from the beginning of) the dtv for thread-local */ + /* storage, which is calloc allocated. If we don't scan the cached */ + /* thread stacks, we appear to lose the dtv. This tends to */ + /* result in something that looks like a bogus dtv count, which */ + /* tends to result in a memset call on a block that is way too */ + /* large. Sometimes we're lucky and the process just dies ... */ + /* There seems to be a similar issue with some other memory */ + /* allocated by the dynamic loader. */ + /* This should be avoidable by either: */ + /* - Defining USE_PROC_FOR_LIBRARIES here. */ + /* That performs very poorly, precisely because we end up */ + /* scanning cached stacks. */ + /* - Have calloc look at its callers. */ + /* In spite of the fact that it is gross and disgusting. */ + /* In fact neither seems to suffice, probably in part because */ + /* even with USE_PROC_FOR_LIBRARIES, we don't scan parts of stack */ + /* segments that appear to be out of bounds. Thus we actually */ + /* do both, which seems to yield the best results. */ + + # define USE_PROC_FOR_LIBRARIES + #endif + + * TODO + + # if defined(GC_LINUX_THREADS) && defined(REDIRECT_MALLOC) \ + && !defined(INCLUDE_LINUX_THREAD_DESCR) + /* Will not work, since libc and the dynamic loader use thread */ + /* locals, sometimes as the only reference. */ + # define INCLUDE_LINUX_THREAD_DESCR + # endif + + * TODO + + # if defined(UNIX_LIKE) && defined(THREADS) && !defined(NO_CANCEL_SAFE) \ + && !defined(PLATFORM_ANDROID) + /* Make the code cancellation-safe. This basically means that we */ + /* ensure that cancellation requests are ignored while we are in */ + /* the collector. This applies only to Posix deferred cancellation;*/ + /* we don't handle Posix asynchronous cancellation. */ + /* Note that this only works if pthread_setcancelstate is */ + /* async-signal-safe, at least in the absence of asynchronous */ + /* cancellation. This appears to be true for the glibc version, */ + /* though it is not documented. Without that assumption, there */ + /* seems to be no way to safely wait in a signal handler, which */ + /* we need to do for thread suspension. */ + /* Also note that little other code appears to be cancellation-safe.*/ + /* Hence it may make sense to turn this off for performance. */ + # define CANCEL_SAFE + # endif + + * `CAN_SAVE_CALL_ARGS` vs. -fomit-frame-pointer now being on by + default for Linux x86 IIRC? (Which is an [[!taglink + open_issue_gcc]] for not including us.) + + * TODO + + # if defined(REDIRECT_MALLOC) && defined(THREADS) && !defined(LINUX) + # error "REDIRECT_MALLOC with THREADS works at most on Linux." + # endif + + + *HURD* has: + + * `#define STACK_GROWS_DOWN` + + * `#define HEURISTIC2` + + Defined instead of `STACKBOTTOM` to have the value probed. + + Linux also has this: + + #if defined(LINUX_STACKBOTTOM) && defined(NO_PROC_STAT) \ + && !defined(USE_LIBC_PRIVATES) + /* This combination will fail, since we have no way to get */ + /* the stack base. Use HEURISTIC2 instead. */ + # undef LINUX_STACKBOTTOM + # define HEURISTIC2 + /* This may still fail on some architectures like IA64. */ + /* We tried ... */ + #endif + + Being on [[glibc]], we could perhaps do similar as `USE_LIBC_PRIVATES` + instead of `HEURISTIC2`. Pro: avoid `SIGSEGV` (and general fragility) + during probing at startup (if I'm understanding this correctly). Con: + rely on glibc internals. Or we instead add support to parse + [[`/proc/`|hurd/translator/procfs]] (can even use the same as Linux?), + or use some other interface. [[!tag open_issue_glibc]] + + * `#define SIG_SUSPEND SIGUSR1`, `#define SIG_THR_RESTART SIGUSR2` + + * We don't `#define MPROTECT_VDB` (WIP comment); but Linux neither. + + * Where does our `GETPAGESIZE` come from? Should we `#include + <unistd.h>` like it is done for *LINUX*? + + * `include/gc_pthread_redirects.h` + + * TODO + + Cancellation stuff is Linux-only. In other places, too. + + * `mach_dep.c` + + * `#define NO_GETCONTEXT` + + [[!taglink open_issue_glibc]], but this is not a real problem here, + because we can use the following GCC internal function without much + overhead: + + * `GC_with_callee_saves_pushed` + + The `HAVE_BUILTIN_UNWIND_INIT` case is ours. + + * `os_dep.c` + + * `read` + + Sure that it doesn't internally (in [[glibc]]) use `malloc`. Probably + only / mostly (?) a problem for `--enable-redirect-malloc` + configurations? Linux with threads uses `readv`. + + * TODO. + + * `dyn_load.c` + + For `DYNAMIC_LOADING`. TODO. + + * `pthread_support.c`, `pthread_stop_world.c` + + TODO. + + * TODO. + + Other files also contain *LINUX* and other conditionals. + + * `libatomic_ops/` + + * `configure.ac` + + Nothing. + + * `Makefile`, `src/Makefile`, `src/atomic_ops/Makefile`, + `src/atomic_ops/sysdeps/Makefile`, `doc/Makefile`, `tests/Makefile` + + Nothing. + + * `src/atomic_ops/sysdeps/gcc/x86.h` + + Nothing. + + +# Testsuite + +There are different configurations possible, but in general, the testsuite +restults of GNU/Linux and GNU/Hurd look very similar. + +## `--enable-cplusplus --enable-gc-debug` + + * GNU/Hurd is missing *Call chain at allocation: [...] output*. + + `os_dep.c`:`GC_print_callers` + + +# TODO + + * Port stuff to [[/GCC]] / [[test it there|gcc/testsuite]]. + + * What are other applications to test Boehm GC? Also especially in + combination with [[/libpthread]] and dynamic loading of shared libraries? + + * <http://www.hpl.hp.com/personal/Hans_Boehm/gc/#users> |