[PATCH] D27919: [Loop Vectorizer] Interleave vs Gather - in some cases Gather is better.

Elena Demikhovsky via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Thu Jan 19 04:40:19 PST 2017

delena added inline comments.

Comment at: ../lib/Transforms/Vectorize/LoopVectorize.cpp:7008-7009
   // the scalar version.
   if (Legal->isUniformAfterVectorization(I))
     VF = 1;
mssimpso wrote:
> Hi Elena,
> I had been thinking about the use of isUniformAfterVectorization() here in getInstructionCost(). Wouldn't it now be possible for the set of uniforms to differ from the first collection (before VF selection) and the second collection (after VF selection)? So we would choose a VF based on costs assuming an instruction may or may not be uniform. Then we could later reverse our initial decision about the instruction's uniformity after VF selection, making the total cost on which we based our VF decision inaccurate. Or am I missing something? I haven't yet thought through the implications of this in enough detail to know whether this would matter much or not.
About the list of Uniforms. We insert and then remove only GEPs and Induction variables. We do not calculate cost for them anyway. All other Uniform values stay in place. So, the cost is accurate at the end. There is no circular dependency here.



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