[Bug target/94649] 16-byte aligned atomic_compare_exchange doesn not generate cmpxcg16b on x86_64

["s_gccbugzilla at nedprod dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94649

Niall Douglas <s_gccbugzilla at nedprod dot com> changed:

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                 CC|                            |s_gccbugzilla at nedprod dot com

--- Comment #4 from Niall Douglas <s_gccbugzilla at nedprod dot com> ---
Relocating my issue from PR 80878 to here:

I got bit by this GCC regression today at work. Consider
https://godbolt.org/z/M9fd7nhdh where
std::atomic<__int128>::compare_exchange_weak() is called with option
-march=sandybridge passed to the command line:

- On GCC 6.4 and earlier, this emits lock cmpxchg16b, as you would expect.

- From GCC 7 up to trunk (12?), this emits __atomic_compare_exchange_16.

- On clang, this emits lock cmpxchg16b, as you would expect.

This is clearly a regression. GCCs before 7 did the right thing. GCCs from 7
onwards do not. clangs with libstdc++ do do the right thing.

Please mark this bug as a regression affecting all versions of GCC from 7 to
trunk.

--- cut ---

NOTE that unlike the original PR above where the struct is a UDT, I am talking
here about std::atomic<__int128>::compare_exchange_weak(). It seems weird that
__int128 is treated as a UDT when the CPU is perfectly capable of hardware CAS.

Common feedback from this and other PRs:

1. Changing this would break ABI

Firstly, I told GCC -march=sandybridge, and we know that libatomic will choose
cmpxchg16b to implement __atomic_compare_exchange_16 because cpuid for
sandybridge will say cmpxchg16b is supported. So, it's the same implementation
for __atomic_compare_exchange_16, nothing breaks here.


2. static const std::atomic<__int128>::load() will segfault

std::atomic<__int128> could examine the macro environment
(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 et al) and if only 128 bit compare and
swap is available, but 128 bit atomics are not, then std::atomic<__int128>
could be conditionally marked with attribute section to prevent it being stored
into the read only code section.

That said, I don't actually consider static const std::atomic<__int128>::load()
segfaulting important enough to special case, in my opinion.


3. This was changed in GCC 7 because _Atomic is broken

_Atomic is indeed broken, but I am talking about std::atomic the C++ library
type here. As Mr. Wakely said in another PR:

> std::atomic just calls the relevant __atomic built-in for all operations.
> What the built-in does is not up to libstdc++.

... to this I would say both yes and no. __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 
is not defined if the architecture relies on software emulation (libatomic) to
implement 128 bit CAS. So std::atomic<types sizeof(16)>::compare_exchange_X()
*could* examine macros for architecture and presence of
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 and inline some assembler for certain
architectures as a QoI measure, which is not ABI breaking because if
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 is 1, then libatomic will be choosing that
same assembler in any case. Note that I refer to the CAS operation only, for
load and store it's trivial to write CAS based emulations, but you could just
leave those continue to call libatomic.

Ultimately I probably agree that because _Atomic is broken, the compiler is not
the right thing to change here. But libstdc++'s std::atomic implementation is
another matter.