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-[[!meta copyright="Copyright © 2010, 2012, 2013, 2014 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.
-
-[[!toc levels=2]]
-
-
-# [[General information|/boehm_gc]]
-
-
-# Configuration
-
-<!--
-
-git checkout reviewed
-git log --reverse --pretty=fuller --stat=$COLUMNS,$COLUMNS -w -p -C --cc ..upstream/master
--i
-/^commit |^---$|hurd|linux|glibc
-
--->
-
-Last reviewed up to the 5f492b98dd131bdd6c67eb56c31024420c1e7dab (2012-06-08)
-sources, and for `libatomic_ops` to the
-6a0afde033f105c6320f1409162e3765a1395bfd (2012-05-15) sources.
-
-  * `configure.ac`
-
-      * `PARALLEL_MARK` is not enabled; doesn't make sense so far.
-
-      * `*-*-kfreebsd*-gnu` defines `USE_COMPILER_TLS`.  What's this, and
-        why does not other config?
-
-      * TODO
-
-            [ if test "$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]]
-        This is also likely the issue causing the GDB [[!tag open_issue_gdb]]
-        `GC_find_limit_with_bound` SIGSEGV startup confusion described in
-        [[binutils]].
-
-      * `#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.
-
-  * b8b65e8a5c2c4896728cd00d008168a6293f55b1 configure.ac probably not all
-    correct.
-
-  * `mmap`, b64dd3bc1e5a23e677c96b478d55648a0730ab75
-
-  * `parallel mark`, 07c2b8e455c9e70d1f173475bbf1196320812154, pass
-    `--disable-parallel-mark` or enable for us, too?
-
-  * `HANDLE_FORK`, e9b11b6655c45ad3ab3326707aa31567a767134b,
-    806d656802a1e3c2b55cd9e4530c6420340886c9,
-    1e882b98c2cf9479a9cd08a67439dab7f9622924
-
-  * Check `include/private/thread_local_alloc.h` re
-    `USE_COMPILER_TLS`/`USE_PTHREAD_SPECIFIC`.
-
-
-# Build
-
-Here's a log of a binutils build run; this is from the
-5f492b98dd131bdd6c67eb56c31024420c1e7dab (2012-06-08) sources, and for
-`libatomic_ops` for the 6a0afde033f105c6320f1409162e3765a1395bfd (2012-05-15)
-sources, run on kepler.SCHWINGE and coulomb.SCHWINGE.
-
-    $ export LC_ALL=C
-    $ (cd ../master/ && ln -sfn ../libatomic_ops/master libatomic_ops)
-    $ (cd ../master/ && autoreconf -vfi)
-    $ ../master/configure --prefix="$PWD".install SHELL=/bin/bash CC=gcc-4.6 CXX=g++-4.6 --enable-cplusplus --enable-gc-debug --enable-gc-assertions --enable-assertions 2>&1 | tee log_build
-    [...]
-    $ make 2>&1 | tee log_build_
-    [...]
-
-Different hosts may default to different shells and compiler versions; thus
-harmonized.  Using bash instead of dash as otherwise libtool explodes.
-
-This takes up around X MiB, and needs roughly X min on kepler.SCHWINGE and
-X min on coulomb.SCHWINGE.
-
-<!--
-
-    $ (make && touch .go-install) 2>&1 | tee log_build_ && test -f .go-install && (make install && touch .go-check) 2>&1 | tee log_install && test -f .go-check && { make -k check 2>&1 | tee log_check; (cd libatomic_ops/ && make -k check) 2>&1 | tee log_check_; }
-
--->
-
-## Analysis
-
-    $ ssh kepler.SCHWINGE 'cd tmp/source/boehm-gc/ && cat master.build/log_build* | sed -e "s%\(/media/data\)\?${PWD}%[...]%g"' > toolchain/logs/boehm-gc/linux/log_build
-    $ ssh coulomb.SCHWINGE 'cd tmp/boehm-gc/ && cat master.build/log_build* | sed -e "s%\(/media/erich\)\?${PWD}%[...]%g"' > toolchain/logs/boehm-gc/hurd/log_build
-    $ diff -wu <(sed -f toolchain/logs/boehm-gc/linux/log_build.sed < toolchain/logs/boehm-gc/linux/log_build) <(sed -f toolchain/logs/boehm-gc/hurd/log_build.sed < toolchain/logs/boehm-gc/hurd/log_build) > toolchain/logs/boehm-gc/log_build.diff
-
-  * only GNU/Linux: `configure: WARNING: "Explicit GC_INIT() calls may be
-    required."`
-
-  * only GNU/Linux: `configure: WARNING: "Client must not use
-    -fomit-frame-pointer."`
-
-
-# Install
-
-    $ make install 2>&1 | tee log_install
-    [...]
-
-This takes up around X MiB, and needs roughly X min on kepler.SCHWINGE and X
-min on coulomb.SCHWINGE.
-
-
-## Analysis
-
-    $ ssh kepler.SCHWINGE 'cd tmp/source/boehm-gc/ && cat master.build/log_install | sed -e "s%\(/media/data\)\?${PWD}%[...]%g"' > toolchain/logs/boehm-gc/linux/log_install
-    $ ssh coulomb.SCHWINGE 'cd tmp/boehm-gc/ && cat master.build/log_install | sed -e "s%\(/media/erich\)\?${PWD}%[...]%g"' > toolchain/logs/boehm-gc/hurd/log_install
-    $ diff -wu toolchain/logs/boehm-gc/linux/log_install toolchain/logs/boehm-gc/hurd/log_install > toolchain/logs/boehm-gc/log_install.diff
-
-
-# Testsuite
-
-    $ make -k check
-    [...]
-    $ (cd libatomic_ops/ && make -k check)
-    [...]
-
-This needs roughly X min on kepler.SCHWINGE and X min on coulomb.SCHWINGE.
-
-
-## Analysis
-
-    $ ssh kepler.SCHWINGE 'cd tmp/source/boehm-gc/ && cat master.build/log_check* | sed -e "s%\(/media/data\)\?${PWD}%[...]%g"' > toolchain/logs/boehm-gc/linux/log_check
-    $ ssh coulomb.SCHWINGE 'cd tmp/boehm-gc/ && cat master.build/log_check* | sed -e "s%\(/media/erich\)\?${PWD}%[...]%g"' > toolchain/logs/boehm-gc/hurd/log_check
-    $ diff -wu <(sed -f toolchain/logs/boehm-gc/linux/log_check.sed < toolchain/logs/boehm-gc/linux/log_check) <(sed -f toolchain/logs/boehm-gc/hurd/log_check.sed < toolchain/logs/boehm-gc/hurd/log_check) > toolchain/logs/boehm-gc/log_check.diff
-
-There are different configurations possible, but in general, the testsuite
-restults of GNU/Linux and GNU/Hurd look very similar.
-
-  * GNU/Hurd is missing `Call chain at allocation: [...]` output.
-
-    `os_dep.c`:`GC_print_callers`
-
-
-# TODO
-
-  * What are other applications to test Boehm GC?  Also especially in
-    combination with [[/libpthread]] and dynamic loading of shared libraries?
-
-      * There are patches (apparently not committed) that GCC itself can use
-        it, too: <http://gcc.gnu.org/wiki/Garbage_collection_tuning>.
-
-      * There's been some talking about it on GNU guile mailing lists, and two
-        Git branches (2010-12-15: last change 2009-09).
-
-      * <http://www.hpl.hp.com/personal/Hans_Boehm/gc/#users>
-
-
-## IRC, OFTC, #debian-hurd, 2012-02-05
-
-[[!tag open_issue_porting]]
-
-    <pinotree> youpi: i think i found out the possible cause of the ecl and
-      mono issuess
-    <pinotree> -s
-    <youpi> oh
-    <pinotree> basically, we don't have the realtime signals (so no
-      SIGRTMIN/SIGRTMAX defined), hence things use either SIGUSR1 or
-      SIGUSR2... which are used in libgc to resp. stop/resume threads when
-      "collecting"
-    <pinotree> i just patched ecl to use SIGINFO instead of SIGUSR1 (used when
-      no SIGRTMIN+2 is available), and it seems going on for a while
-    <youpi> uh, why would SIGINFO work better than SIGUSR1?
-    <pinotree> it was a test, i tried the first "not common" signal i saw
-    <pinotree> my test was, use any signal different than USR1/2
-    <youpi> ah, sorry, I hadn't understood
-    <youpi> you mean there's a conflict between ecl and mono using SIGUSR1, as
-      well as libgc?
-    <pinotree> yes
-    <pinotree> for example, in ecl sources see src/c/unixint.d,
-      install_process_interrupt_handler()
-    <youpi> SIGINFO seems a sane choice
-    <youpi> SIGPWR could have been a better choice if it was available :)
-    <pinotree> i would have chose an "unassigned" number, say SIGLOST (the
-      bigger one) + 10, but it would be greater than _NSIG (and thus discarded)
-    <youpi> not a good idea indeed
-    <pinotree> it seems that linux, beside the range for rt signals, has some
-      "free space"
-    <pinotree> i'll start now another ecl build, from scratch this time, with
-      s/SIGUSR1/SIGINFO/ (making sure ctags won't bother), and if it works i'll
-      update svante's bug
-
-    <pinotree> mmap(...PROT_NONE...) failed
-    <pinotree> hmm...
-    <pinotree> apparently enabling MMAP_ANON in mono's libgc copy was a good
-      step, let's see
-
-
-### IRC, OFTC, #debian-hurd, 2012-03-18
-
-    <pinotree> youpi: mono is afflicted by the SIGUSR1/2 conflict with libgc
-    <youpi> pinotree: didn't we have a solution for that?
-    <pinotree> well, it works just for one signal
-    <pinotree> the ideal solution would be having a range for RT signals, and
-      make libgc use RTMIN+5/6, like done on most of other OSes
-    <youpi> but we don't have RT signals, do we?
-    <pinotree> right :(
-
-
-### IRC, freenode, #hurd, 2012-03-21
-
-    <pinotree> civodul: given we have to realtime signals (so no range of
-      signals for them), libgc uses SIGUSR1/2 instead of using SIGRTMIN+5/6 for
-      its thread synchronization stuff
-    <pinotree> civodul: which means that if an application using libgc then
-      sets its own handlers for either of SIGUSR1/2, hell breaks
-    <civodul> pinotree: ok
-    <civodul> pinotree: is it a Debian-specific change, or included upstream?
-    <pinotree> libgc using SIGUSR1/2? upstream
-    <civodul> ok
-
-
-### IRC, freenode, #hurd, 2013-09-03
-
-    <congzhang> braunr: when will libc malloc say memory corruption?
-    <braunr> congzhang: usually on free
-    <braunr> sometimes on alloc
-    <congzhang> and after one thread be created
-    <congzhang> I want to know why and how to find the source
-    <congzhang> does libgc work well on hurd?
-    <braunr> i don't think it does
-    <congzhang> so , why it can't?
-    <braunr> congzhang: what ?
-    <congzhang> libgc was not work on hurd
-    <pinotree> why?
-    <congzhang> I try porting dotgnu
-    <braunr> ah
-    <braunr> nested signal handling
-    <congzhang> one program always receive Abort signal
-    <pinotree> and why it should be a problem in libgc?
-    <congzhang> for malloc memory corruption
-    <braunr> libgc relies on this
-    <congzhang> yes
-    <congzhang> so, is there a workaround to make it work? 
-    <braunr> show the error please
-    <congzhang> http://paste.debian.net/34416/
-    <pinotree> where's libgc?
-    <congzhang> i compile dotgnu with enable-gc
-    <pinotree> so?
-    <congzhang> I am not sure about it
-    <pinotree> so why did you say earlier that libgc doesn't work?
-    <congzhang> because after I see one thread was created notice by gdb, it
-      memory corruption
-    <pinotree> so what?
-    <congzhang> maybe gabage collection happen, and gc thread start
-    <pinotree> that's speculation
-    <pinotree> you cannot debug things speculating on code you don't know
-    <pinotree> less speculation and more in-deep debugging, please
-    * congzhang I try again, to check weather thread list changing
-    <congzhang> sorry for this
-    <braunr> it simply looks like a real memory corruption (an overflow)
-    <congzhang> maybe PATH related problem
-    <pinotree> PATH?
-    <congzhang> yes
-    <braunr> PATH_MAX
-    <braunr> but unlikely
-    <congzhang> csant do path traverse
-    <congzhang> I fond the macro
-    <congzhang> found
-    <congzhang> #if defined(__sun__) || defined(__BEOS__)
-    <congzhang> #define     BROKEN_DIRENT   1
-    <congzhang> #endif
-    <congzhang> and so for hurd?
-    <pinotree> BROKEN_DIRENT doesn't say much about what it does
-    <WhiteKIBA> nope
-    <WhiteKIBA> whoops
-    <congzhang> it seems other port meet the trouble too
-    <pinotree> which trouble?
-    <congzhang> http://comments.gmane.org/gmane.comp.gnu.dotgnu.developer/3642
-    <congzhang> (gdb) ptype struct dirent
-    <congzhang> type = struct dirent {
-    <congzhang>     __ino_t d_ino;
-    <congzhang>     unsigned short d_reclen;
-    <congzhang>     unsigned char d_type;
-    <congzhang>     unsigned char d_namlen;
-    <congzhang>     char d_name[1];
-    <congzhang> }
-    <congzhang>  
-    <congzhang> d_name should be char[PATH_MAX]?
-    <congzhang> and
-      http://libjit-linear-scan-register-allocator.googlecode.com/svn/trunk/pnet/support/dir.c
-    <pinotree> no
-    <braunr> stop pasting that much
-    <_d3f> uhm PATH_MAX on the hurd?
-    <braunr> and stop saying nonsense
-    <congzhang> sorry, i think four line was not worth to pastbin 
-    <pinotree> they are 8
-    <congzhang> never again
-    <braunr> just try by defining BROKEN_DIRENT to 1 in all cases and see how
-      it goes
-    * congzhang read dir.c again
-    <congzhang> braunr: it does not crash this time, I do more test
-
-
-#### IRC, freenode, #hurd, 2013-09-04
-
-    <congzhang> hi, I am dotgnu work on hurd, and even winforms app 
-    <congzhang> s/am/make
-    <congzhang> and maybe c# hello world translate another day :)
-
-
-### IRC, freenode, #hurd, 2013-12-16
-
-    <braunr> gnu_srs: ah, libgc
-    <braunr> there are signal-related problems with libgc
-
-
-## Leak Detection
-
-### IRC, freenode, #hurd, 2013-10-17
-
-    <teythoon> I spent the last two days integrating libgc - the boehm
-      conservative garbage collector - into hurd
-    <teythoon> it can be used in leak detection mode
-    <azeem> whoa, cool
-    <teythoon> and it actually kind of works, finds malloc leaks in translators
-    <braunr> i think there were problems with signal handling in libgc
-    <braunr> i'm not sure we support nested signal handling well
-    <teythoon> yes, I read about them
-    <teythoon> libgc uses SIGUSR1/2, so any program installing handlers on them
-      will break
-    <azeem> (which is not a problem on Linux, cause there some RT-signals or so
-      are used)
-    <teythoon> yes