[llvm-dev] [Release-testers] [7.0.0 Release] rc1 has been tagged

[Dimitry Andric via llvm-dev]

On 22 Aug 2018, at 05:58, Wei Mi <wmi at google.com> wrote:
> 
> On Fri, Aug 17, 2018 at 1:27 PM, Dimitry Andric <dimitry at andric.com> wrote:
> On 16 Aug 2018, at 00:51, Joerg Sonnenberger via llvm-dev <llvm-dev at 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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