https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94649
Niall Douglas changed:
What|Removed |Added
CC||s_gccbugzilla at nedprod dot
com
--- Comment #4 from Niall Douglas ---
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::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.
