[LLVMdev] LLVM 2.8 and MMX

Reid Kleckner
Tue, 21 Sep 2010 23:24:45 -0000

This thread confuses me.  I thought Chris said that LLVM 2.8 will not
lower generic vectors to MMX because it breaks x87 code, and I didn't
see an answer to your question about a switch to tell the code
generator otherwise.  However, you're complaining that MMX performance
is subpar, even though LLVM 2.8 isn't supposed to generate MMX
instructions.

Can someone clarify the situation for me?

Thanks,
Reid

On Tue, Sep 21, 2010 at 6:13 PM, Nicolas Capens
<nicolas.capens at gmail.com> wrote:
> Hi Dale,
>
> I suspect that these patches were intended to improve 128-bit vector
> performance but caused certain 64-bit vector operations to no longer lower
> to MMX instructions. Anyway, now that I've narrowed it down to these
patches
> I think I can narrow it down further to a specific case so I can file a
> bug...
>
> Will Bruno be back soon or is he no longer working on the project for good?
>
> Cheers,
>
> Nicolas
>
>
> -----Original Message-----
> From: Dale Johannesen [mailto:dalej at apple.com]
> Sent: Tuesday, September 21, 2010 20:12
> To: Nicolas Capens
> Cc: Dale Johannesen; 'Chris Lattner'; 'Eli Friedman';
llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] LLVM 2.8 and MMX
>
>
> On Sep 21, 2010, at 10:23 AMPDT, Nicolas Capens wrote:
>
>> Hi all,
>>
>> Sorry for the late reply. I got sidetracked by other fun projects. ;-)
>>
>> I found that the performance regression is caused by revisions 112804,
>> 112805 and 112806. Those changes were made 2 days prior to the 2.8
>> branching, so it may have not been the intention to include them there?
>> Either way they make my vector-intensive code two times slower so it
would
>> be much appreciated to revert these changes for the 2.8 release.
>>
>> Thanks,
>>
>> Nicolas
>
> Interesting.  These are all Bruno's patches, and I'm pretty sure
they
> weren't intended to affect MMX.   I doubt reverting them is right since
the
> effect on SSE is presumably positive.  Unfortunately Bruno is not here any
> more.
>
>> -----Original Message-----
>> From: Chris Lattner [mailto:clattner at apple.com]
>>>
>>> I think some changes related to MMX landed before 2.8 branched
which
>>> shouldn't have... please file a bug.
>
> So please file a bug, with example.
>
>
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