[LLVMdev] LLVM 2.8 and MMX

[Nicolas Capens]

Hi all,

I think I figured it out:
112804 causes 64-bit UNPCKLBW to no longer be selected for certain cases.
112805 is benign.
112806 causes 64-bit UNPCKHBW to no longer be selected for certain cases.

I've attached a potential fix for the 2.8 branch.

The real problem is that the code above it which checks for
isUNPCK[L|H]_v_undef_Mask cases is only for when OptForSize is true. It
assumes that otherwise things can get lowered to PSHUFD (which is true for
v4i32 and v4f32 but nothing else - in particular MMX operations).

I'll file a bug now...

Nicolas


-----Original Message-----
From: Dale Johannesen [mailto:dalej at apple.com] 
Sent: Wednesday, September 22, 2010 2:37
To: Bill Wendling
Cc: Dale Johannesen; Reid Kleckner; Nicolas Capens; LLVM Developers Mailing
List
Subject: Re: [LLVMdev] LLVM 2.8 and MMX


On Sep 21, 2010, at 5:30 PMPDT, Bill Wendling wrote:

> LLVM isn't going to stop generating MMX instructions all together. We
can't do that. :-) If the user specifically wants MMX (by, say, using the
builtins), we have to support that still. The plan to cease generating MMX
for generic vectors is a work-in-progress right now. It's not in 2.8.
> 
> -bw

Right, early on there was speculation that the early phases of this work
were causing the problem Nicolas is seeing, but it now appears that that
problem is unrelated.

> On Sep 21, 2010, at 4:24 PM, Reid Kleckner wrote:
> 
>> 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.
>>> 
>>> 
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>> 
>> 
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> 
> 
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