[Bug libstdc++/88264] Support glibc-style tunables for <memory_resource>

[Bug libstdc++/88264] Support glibc-style tunables for <memory_resource>

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88264

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:637e3668fdc17c4e226538fb14f9fab225433d01

commit r13-3234-g637e3668fdc17c4e226538fb14f9fab225433d01
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Oct 5 21:21:54 2022 +0100

    libstdc++: Allow emergency EH alloc pool size to be tuned [PR68606]

    Implement a long-standing request to support tuning the size of the
    emergency buffer for allocating exceptions after malloc fails, or to
    disable that buffer entirely.

    It's now possible to disable the dynamic allocation of the buffer and
    use a fixed-size static buffer, via --enable-libstdcxx-static-eh-pool.
    This is a built-time choice that is baked into libstdc++ and so affects
    all code linked against that build of libstdc++.

    The size of the pool can be set by --with-libstdcxx-eh-pool-obj-count=N
    which is measured in units of sizeof(void*) not bytes. A given exception
    type such as std::system_error depends on the target, so giving a size
    in bytes wouldn't be portable across 16/32/64-bit targets.

    When libstdc++ is configured to use a dynamic buffer, the size of that
    buffer can now be tuned at runtime by setting the GLIBCXX_TUNABLES
    environment variable (c.f. PR libstdc++/88264). The number of exceptions
    to reserve space for is controlled by the "glibcxx.eh_pool.obj_count"
    and "glibcxx.eh_pool.obj_size" tunables. The pool will be sized to be
    able to allocate obj_count exceptions of size obj_size*sizeof(void*) and
    obj_count "dependent" exceptions rethrown by std::rethrow_exception.

    With the ability to tune the buffer size, we can reduce the default pool
    size on 32-bit and 16-bit targets. Most users never need to throw 1kB
    exceptions in parallel from hundreds of threads after malloc is OOM. The
    users who do need that can use the tunables to select larger sizes.

    The old defaults can be chosen at runtime by setting GLIBCXX_TUNABLES
    to:
    64-bit: glibcxx.eh_pool.obj_count=64:glibcxx.eh_pool.obj_size=112
    32-bit: glibcxx.eh_pool.obj_count=32:glibcxx.eh_pool.obj_size=104

    Or approximated by configuring with:
    64-bit: --with-libstdcxx-eh-pool-obj-count=252
    32-bit: --with-libstdcxx-eh-pool-obj-count=94

    libstdc++-v3/ChangeLog:

            PR libstdc++/68606
            * Makefile.in: Regenerate.
            * acinclude.m4 (GLIBCXX_EMERGENCY_EH_ALLOC): New macro.
            * configure: Regenerate.
            * configure.ac: Use GLIBCXX_EMERGENCY_EH_ALLOC.
            * crossconfig.m4: Check for secure_getenv.
            * doc/Makefile.in: Regenerate.
            * doc/xml/manual/configure.xml: Document new configure options.
            * doc/xml/manual/evolution.xml: Document addition of tunables.
            * doc/xml/manual/using_exceptions.xml: Document emergency
            buffer and tunables.
            * doc/html/*: Regenerate.
            * include/Makefile.in: Regenerate.
            * libsupc++/Makefile.am: Use EH_POOL_FLAGS.
            * libsupc++/Makefile.in: Regenerate.
            * libsupc++/eh_alloc.cc (EMERGENCY_OBJ_SIZE): Define in units
            of sizeof(void*) not including the ABI's exception header.
            (EMERGENCY_OBJ_COUNT): Define as target-independent calculation
            based on word size.
            (MAX_OBJ_COUNT): Define macro for upper limit on pool size.
            (pool) [_GLIBCXX_EH_POOL_STATIC]: Use fixed-size buffer.
            (pool::buffer_size_in_bytes): New static member function.
            (pool::pool): Parse GLIBCXX_TUNABLES environment variable to set
            pool size at runtime.
            (pool::in_pool): Use std::less<void*> for total order.
            (__freeres) [_GLIBCXX_EH_POOL_STATIC]: Do nothing.
            (__cxa_free_exception, __cxa_free_dependent_exception): Add
            [[unlikely]] attributes.
            * po/Makefile.in: Regenerate.
            * python/Makefile.in: Regenerate.
            * src/Makefile.in: Regenerate.
            * src/c++11/Makefile.in: Regenerate.
            * src/c++17/Makefile.in: Regenerate.
            * src/c++20/Makefile.in: Regenerate.
            * src/c++98/Makefile.in: Regenerate.
            * src/filesystem/Makefile.in: Regenerate.
            * src/libbacktrace/Makefile.in: Regenerate.
            * testsuite/Makefile.in: Regenerate.

[Bug libstdc++/88264] Support glibc-style tunables for <memory_resource>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88264

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> The emergency EH pool size should be tunable too.
> 
> From libsupc++/eh_alloc.cc:
> 
>       // Allocate the arena - we could add a GLIBCXX_EH_ARENA_SIZE
> environment
>       // to make this tunable.
>       arena_size = (EMERGENCY_OBJ_SIZE * EMERGENCY_OBJ_COUNT
> 		    + EMERGENCY_OBJ_COUNT * sizeof (__cxa_dependent_exception));
>       arena = (char *)malloc (arena_size);

Done for PR 68606.

[Bug libstdc++/88264] Support glibc-style tunables for <memory_resource>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88264

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This can't be done safely in libstdc++ because we can't call getenv safely. We
could be dlopened while another thread is calling setenv.

But if that could be solved, other useful tunables would be the filenames to
use for chrono::tzdb (for tzdata.zi and leapseconds), and formatting options
for the default contract violation handler (e.g. human readable vs
log-friendly).

[Bug libstdc++/88264] Support glibc-style tunables for libstdc++

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88264

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Another tunable idea: disable exceptions from the runtime.

We use _GLIBCXX_THROW_OR_ABORT so that throwing an exception will abort instead
when -fno-exceptions is defined. However, this only works when the code doing
the throw is compiled with that flag. For all the __throw_foo_error functions
declared in <bits/functexcept.h> the actual definition is
src/c++11/functexcept.cc which isn't affected by any -fno-exceptions that users
compile with. You need to recompile libstdc++ itself with -fno-exceptions. We
could add a tunable to disable all exceptions within the runtime.
(If doing this, don't forget places like throw_revursive_init_exception in
libsupc++/guard.cc which doesn't use _GLIBCXX_THROW_OR_ABORT because it wants
to trap not std::abort(), presumably for freestanding reasons.)