On 22 Aug 2018, at 05:58, Wei Mi <w...@google.com> wrote: > > On Fri, Aug 17, 2018 at 1:27 PM, Dimitry Andric <dimi...@andric.com> wrote: > On 16 Aug 2018, at 00:51, Joerg Sonnenberger via llvm-dev > <llvm-...@lists.llvm.org> wrote: > > On Tue, Aug 07, 2018 at 09:49:16PM +0200, Dimitry Andric via llvm-dev wrote: > >> This is a regression caused by https://reviews.llvm.org/rL323281: > >> > >> ------------------------------------------------------------------------ > >> r323281 | wmi | 2018-01-23 23:27:57 +0000 (Tue, 23 Jan 2018) | 12 lines > >> > >> Adjust MaxAtomicInlineWidth for i386/i486 targets. > >> > >> This is to fix the bug reported in > >> https://bugs.llvm.org/show_bug.cgi?id=34347#c6. > >> Currently, all MaxAtomicInlineWidth of x86-32 targets are set to 64. > >> However, > >> i386 doesn't support any cmpxchg related instructions. i486 only supports > >> cmpxchg. > >> So in this patch MaxAtomicInlineWidth is reset as follows: > >> For i386, the MaxAtomicInlineWidth should be 0 because no cmpxchg is > >> supported. > >> For i486, the MaxAtomicInlineWidth should be 32 because it supports > >> cmpxchg. > >> For others 32 bits x86 cpu, the MaxAtomicInlineWidth should be 64 because > >> of cmpxchg8b. > > > > This seems to be somewhat undesirable. Does *anyone* care about real > > i386 support at this point? NetBSD certainly doesn't and I think we are > > already the odd man for a number of cases like this. > > > Yes, and since this causes quite a number of regressions for us, I would > really prefer this revision to be reverted, at least in the 7.0.0 > branch. I have already reverted it locally in our FreeBSD project > branch for importing llvm/clang 7.0.0. Hans, what is your opinion about > this? > > -Dimitry > > > Sorry I missed the thread for quite a while. Dimitry, I am very confused > because you reported the issue in > https://bugs.llvm.org/show_bug.cgi?id=34347#c6, so you want r323281 to be > reverted and let llvm to generate cmxchg8b instruction for i486?
Since it's been doing this for a number of years now, I don't think it would be bad at all, at least not for FreeBSD. At least, a lot more effort is needed to supply properly working atomic libcalls for 64 bit values on i386. (They can't be implemented without at least a bit of kernel assistance.) -Dimitry
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