[llvm-commits] [llvm] r171471 - /llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp

[Shuxin Yang]

I spotted this problem in some FP benchmarks (perhaps Algebra?) when I 
investigated a fast-math related problem two weeks ago.
FP applications usually have small code and big data. So the code-size 
is not a big problem. I don't know if this defect
impact other applications which care code size, and how pervasive the 
problem is.  Just let you folks aware of this issue.


On 1/4/13 3:23 PM, Michael Gottesman wrote:
> Question. Which tests were run I.e. what is comprehensive? Also in order to know what are the "hot loops" we could use pgo.
>
> On Jan 4, 2013, at 6:03 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>
>> I spot a defect that contribute the code size problem. I don't know to what ext it impacts the code sz.
>> However, it is certainly easy to fix.
>>
>> The problem is that if a scalar constant, say s=2.0f,  is broadcasted into a vector v=<2.0, 2.0, 2.0, 2.0>,  and
>> if the scalar is still referenced in the code, we will see the compiler creates read-only storage both for s and v.
>> Actually the storage for the scalar is not necessary.
>>
>> On 1/4/13 10:09 AM, Nadav Rotem wrote:
>>> Hi Hal,
>>>
>>> I spoke with chandler last night and he said that he ran comprehensive testing and he found two interesting things:
>>>
>>> 1. When vectorizing everything with O2, the code gets 3% bigger, and as a result slower.  When it works it works really well, but most vectorizable loops are in the cold path.
>>> 2. When disabling the pre-loop for loops that require: runtime checks, odd number of iterations, etc. Then the loop vectorizer does not catch much.
>>>
>>> At the moment it looks like it is better to mimic the gcc behavior and enable the loop vectorizer only on O3.
>>>
>>> Nadav
>>>
>>>
>>> On Jan 4, 2013, at 10:04 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>>>
>>>> ----- Original Message -----
>>>>> From: "Nadav Rotem" <nrotem at apple.com>
>>>>> To: llvm-commits at cs.uiuc.edu
>>>>> Sent: Friday, January 4, 2013 11:57:44 AM
>>>>> Subject: [llvm-commits] [llvm] r171471 -    /llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp
>>>>>
>>>>> Author: nadav
>>>>> Date: Fri Jan  4 11:57:44 2013
>>>>> New Revision: 171471
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=171471&view=rev
>>>>> Log:
>>>>> Move the loop vectorizer from O2 to O3. It looks like the increase in
>>>>> code size actually hurts the performance on many programs.
>>>> Can you be specific about what code size increase? (runtime checks, if conversion, unrolling)? And what does this have to do with moving the vectorizer from -02 to -O3? To be clear, I don't object to the change (enabling autovectorization at -O3 seems pretty standard these days).
>>>>
>>>> Thanks again,
>>>> Hal
>>>>
>>>>> Modified:
>>>>>     llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp
>>>>>
>>>>> Modified: llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp
>>>>> URL:
>>>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp?rev=171471&r1=171470&r2=171471&view=diff
>>>>> ==============================================================================
>>>>> --- llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp (original)
>>>>> +++ llvm/trunk/lib/Transforms/IPO/PassManagerBuilder.cpp Fri Jan  4
>>>>> 11:57:44 2013
>>>>> @@ -187,7 +187,7 @@
>>>>>    MPM.add(createLoopIdiomPass());             // Recognize idioms
>>>>>    like memset.
>>>>>    MPM.add(createLoopDeletionPass());          // Delete dead loops
>>>>>
>>>>> -  if (LoopVectorize && OptLevel > 1)
>>>>> +  if (LoopVectorize && OptLevel > 2)
>>>>>      MPM.add(createLoopVectorizePass());
>>>>>
>>>>>    if (!DisableUnrollLoops)
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>> -- 
>>>> Hal Finkel
>>>> Postdoctoral Appointee
>>>> Leadership Computing Facility
>>>> Argonne National Laboratory
>>> _______________________________________________
>>> llvm-commits mailing list
>>> llvm-commits at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
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