[LLVMdev] [llvm] r187267 - SLP Vectorier: Don't vectorize really short chains because they are already handled by the SelectionDAG store-vectorizer, which does a better job in deciding when to vectorize.

[Nadav Rotem]

Hi Daniel, 

Thanks for the review!

+  // The SelectionDAG vectorizer can only handle pairs (trees of height = 2).

r187316.

-Nadav

On Jul 27, 2013, at 10:48 AM, Daniel Berlin <dberlin at dberlin.org> wrote:

> Hi Nadav,
> 
> Okay.
> 
> 1. The comment doesn't make this clear. I would suggest, at a minimum, updating it to mention pairs specifically, to avoid the issue in #2
> 
> 2. If the day comes when the selectiondag store vectorizer handles more than pairs, and does so better, is anyone really going to remember this random 3 exists in the other vectorizer?
> 
> I would posit, based on experience, the answer is "no" :)
> 
> 
> 
> On Fri, Jul 26, 2013 at 7:56 PM, Nadav Rotem <nrotem at apple.com> wrote:
> Hi Daniel,
> 
> Maybe my commit message was not clear. The idea is that the SelectionDAG store vectorizer can only handle pairs. So, the number three means "more than a pair".  
> 
> Thanks,
> Nadav
> 
> Sent from my iPhone
> 
> On Jul 26, 2013, at 17:48, Daniel Berlin <dberlin at dberlin.org> wrote:
> 
>> Hey Nadav,
>> I'd humbly suggest that rather than use 3 directly, you should add a shared constant between these two passes, so when one changes, the other doesn't need to be updated. It would also ensure this bit of info about what needs to be updated isn't only contained in the comments..  
>> 
>> On Fri, Jul 26, 2013 at 4:07 PM, Nadav Rotem <nrotem at apple.com> wrote:
>> Author: nadav
>> Date: Fri Jul 26 18:07:55 2013
>> New Revision: 187267
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=187267&view=rev
>> Log:
>> SLP Vectorier:  Don't vectorize really short chains because they are already handled by the SelectionDAG store-vectorizer, which does a better job in deciding when to vectorize.
>>  
>> 
>> Modified: llvm/trunk/lib/Transforms/Vectorize/SLPVectorizer.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Vectorize/SLPVectorizer.cpp?rev=187267&r1=187266&r2=187267&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Transforms/Vectorize/SLPVectorizer.cpp (original)
>> +++ llvm/trunk/lib/Transforms/Vectorize/SLPVectorizer.cpp Fri Jul 26 18:07:55 2013
>> @@ -898,8 +898,12 @@ int BoUpSLP::getTreeCost() {
>>    DEBUG(dbgs() << "SLP: Calculating cost for tree of size " <<
>>          VectorizableTree.size() << ".\n");
>> 
>> -  if (!VectorizableTree.size()) {
>> -    assert(!ExternalUses.size() && "We should not have any external users");
>> +  // Don't vectorize tiny trees. Small load/store chains or consecutive stores
>> +  // of constants will be vectoried in SelectionDAG in MergeConsecutiveStores.
>> +  if (VectorizableTree.size() < 3) {
>> +    if (!VectorizableTree.size()) {
>> +      assert(!ExternalUses.size() && "We should not have any external users");
>> +    }
>>      return 0;
>>    }

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