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

[Dimitry Andric via llvm-dev]

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.

It's also affecting i486, which is still the default -march value on
FreeBSD.  Note that clang has been silently generating cmpxchg8b's for
years now, even for the i486 target, but I have never seen any complaint
about crashes due to this.

So basically everybody is running i586 or higher, in practice...

-Dimitry

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