Re: [PATCH] libstdc++: Configurable exception emergency pool size

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 12 Oct 2022 at 20:30, Alexey Lapshin via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> From 8b27091310d2857880d8733a05813ca92a5a3e6f Mon Sep 17 00:00:00 2001
> From: Alexey Gerenkov <alexey@espressif.com>
> Date: Thu, 16 Nov 2017 15:06:35 +0300
> Subject: [PATCH] libstdc++: Configurable exception emergency pool size

This patch won't apply to trunk because of
https://gcc.gnu.org/g:637e3668fdc17c4e226538fb14f9fab225433d01

Please take a look at that (and the follow up in
23c3cbaed36f6d2f3a7a64f6ebda69329723514b) and decide if you still want
to change anything.

In any case, I think some targets don't support __attribute((weak)) so
any change like that would have to make it conditional (possibly on
__GXX_WEAK__ but I'm not sure if that's the right check).


> ---
>  libstdc++-v3/libsupc++/eh_alloc.cc | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/libstdc++-v3/libsupc++/eh_alloc.cc b/libstdc++-
> v3/libsupc++/eh_alloc.cc
> index 68f319869f9..6503041c20a 100644
> --- a/libstdc++-v3/libsupc++/eh_alloc.cc
> +++ b/libstdc++-v3/libsupc++/eh_alloc.cc
> @@ -50,12 +50,13 @@ extern "C" void *memset (void *, int, std::size_t);
>
>  using namespace __cxxabiv1;
>
> -// ??? How to control these parameters.
> -
>  // Guess from the size of basic types how large a buffer is
> reasonable.
>  // Note that the basic c++ exception header has 13 pointers and 2
> ints,
>  // so on a system with PSImode pointers we're talking about 56 bytes
>  // just for overhead.
> +// To fix this:
> +// Implement C function __cxx_eh_arena_size_get() in your codebase
> which
> +// returns size (size_t) for buffer to allocate.
>
>  #if INT_MAX == 32767
>  # define EMERGENCY_OBJ_SIZE    128
> @@ -73,6 +74,12 @@ using namespace __cxxabiv1;
>  # define EMERGENCY_OBJ_COUNT   4
>  #endif
>
> +extern "C" __attribute__((weak)) size_t __cxx_eh_arena_size_get(void)
> +{
> +  return EMERGENCY_OBJ_SIZE * EMERGENCY_OBJ_COUNT
> +         + EMERGENCY_OBJ_COUNT * sizeof (__cxa_dependent_exception);
> +}
> +
>  namespace __gnu_cxx
>  {
>    void __freeres();
> @@ -116,11 +123,8 @@ namespace
>
>    pool::pool()
>      {
> -      // 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);
> +      arena_size = __cxx_eh_arena_size_get();
> +      arena = arena_size ? (char *)malloc (arena_size) : NULL;
>        if (!arena)
>         {
>           // If the allocation failed go without an emergency pool.
> --
> 2.34.1
>