[LLVMdev] LLVM 2.8 and MMX

[Dale Johannesen]

On Sep 22, 2010, at 8:49 AMPDT, Bruno Cardoso Lopes wrote:

> Assign the bug to me and I'll fix it in TOT next week! Thanks for
> narrowing it down!

Thanks Bruno, it's PR 8200.

> On Wednesday, September 22, 2010, Nicolas Capens
> <nicolas.capens at gmail.com> wrote:
>> 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.c <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
> 
> -- 
> Bruno Cardoso Lopes
> http://www.brunocardoso.cc