[PATCH v1] libstdc++: Default to mutex-based atomics on RISC-V

[Jonathan Wakely]

On 07/04/22 11:46 -0700, Palmer Dabbelt wrote:
>The RISC-V port requires libatomic to be linked in order to resolve
>various atomic functions, which results in builds that have
>"--with-libstdcxx-lock-policy=auto" defaulting to mutex-based locks.
>Changing this to direct atomics breaks the ABI, this forces the auto
>detection mutex-based atomics on RISC-V in order to avoid a silent ABI
>break for users.
>
>See Bug 84568 for more discussion.  In the long run there may be a way
>to get the higher-performance atomics without an ABI flag day, but
>that's going to be a much more complicated operation.  We don't even
>have support for the inline atomics yet, but given that some folks have
>been discussing hacks to make these libatomic routines appear implicitly
>it seems prudent to just turn off the automatic detection for RISC-V.
>
>libstdc++-v3/ChangeLog
>
>	* acinclude.md (GLIBCXX_ENABLE_LOCK_POLICY): Force auto to mutex
>	  for RISC-V.

As documented at https://gcc.gnu.org/lists.html all patches for
libstdc++ need to go to the libstdc++ list as well as gcc-patches
(otherwise I won't see them).

We'd usually do something like:

case "${host}" in
   *-*-riscv) libstdcxx_atomic_lock_policy=mutex ;;
   *-*-*) AC_TRY_COMPILE([ ... ],,[],[])
esac

but this way is simpler. If we add more customization for other
targets we can reconsider using the 'case "${host}"' form.

So this is OK for trunk, modulo regenerating libstdc++-v3/configure
with this change. Let me know if you want me to do that regen for you
(or commit the whole thing for you).


>---
>
>I haven't even built this one, as I'm sure there's a better way to do it
>then sticking some more C code in there.
>---
> libstdc++-v3/acinclude.m4 | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
>index f53461c85a5..945c0c66f8d 100644
>--- a/libstdc++-v3/acinclude.m4
>+++ b/libstdc++-v3/acinclude.m4
>@@ -3612,6 +3612,9 @@ AC_DEFUN([GLIBCXX_ENABLE_LOCK_POLICY], [
>     dnl Why don't we check 8-byte CAS for sparc64, where _Atomic_word is long?!
>     dnl New targets should only check for CAS for the _Atomic_word type.
>     AC_TRY_COMPILE([
>+    #if defined __riscv
>+    # error "Defaulting to mutex-based locks for ABI compatibility"
>+    #endif
>     #if ! defined __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2
>     # error "No 2-byte compare-and-swap"
>     #elif ! defined __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4