On Wed, 4 Aug 2021 at 16:47, Maged Michael <maged.mich...@gmail.com> wrote: > > Thanks, Jonathan! > > On Wed, Aug 4, 2021 at 11:32 AM Jonathan Wakely <jwakely....@gmail.com> wrote: >> >> On Tue, 3 Aug 2021 at 21:59, Jonathan Wakely wrote: >> > >> > On Mon, 2 Aug 2021 at 14:29, Maged Michael wrote: >> > > >> > > This is the right patch. The previous one is missing noexcept. Sorry. >> > > >> > > >> > > On Mon, Aug 2, 2021 at 9:23 AM Maged Michael <maged.mich...@gmail.com> >> > > wrote: >> > >> >> > >> Please find attached an updated patch after incorporating Jonathan's >> > >> suggestions. >> > >> >> > >> Changes from the last patch include: >> > >> - Add a TSAN macro to bits/c++config. >> > >> - Use separate constexpr bool-s for the conditions for lock-freedom, >> > >> double-width and alignment. >> > >> - Move the code in the optimized path to a separate function >> > >> _M_release_double_width_cas. >> > >> > Thanks for the updated patch. At a quick glance it looks great. I'll >> > apply it locally and test it tomorrow. >> >> >> It occurs to me now that _M_release_double_width_cas is the wrong >> name, because we're not doing a DWCAS, just a double-width load. What >> do you think about renaming it to _M_release_combined instead? Since >> it does a combined load of the two counts. > > > Yes definitely _M_release_combined makes sense. Will do.
See the patch I attached to my previous mail, which has a refactored version that gets rid of that function entirely. > >> >> I'm no longer confident in the alignof suggestion I gave you. >> >> + constexpr bool __double_word >> + = sizeof(long long) == 2 * sizeof(_Atomic_word); >> + // The ref-count members follow the vptr, so are aligned to >> + // alignof(void*). >> + constexpr bool __aligned = alignof(long long) <= alignof(void*); >> >> For IA32 (i.e. 32-bit x86) this constant will be true, because >> alignof(long long) and alignof(void*) are both 4, even though >> sizeof(long long) is 8. So in theory the _M_use_count and >> _M_weak_count members could be in different cache lines, and the >> atomic load will not be atomic. I think we want to use __alignof(long >> long) here to get the "natural" alignment, not the smaller 4B >> alignment allowed by SysV ABI. That means that we won't do this >> optimization on IA32, but I think that's acceptable. >> >> Alternatively, we could keep using alignof, but with an additional >> run-time check something like (uintptr_t)&_M_use_count / 64 == >> (uintptr_t)&_M_weak_count / 64 to check they're on the same cache >> line. I think for now we should just use __alignof and live without >> the optimization on IA32. >> > I'd rather avoid any runtime checks because they may negate the speed > rationale for doing this optimization. > I'd be OK with not doing this optimization for any 32-bit architecture. > > Is it OK to change the __align condition to the following? > constexpr bool __aligned = > (alignof(long long) <= alignof(void*)) > && (sizeof(long long) == sizeof(void*)); Yes, that will work fine.
