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

[Matthew Simpson via Phabricator via llvm-commits]

mssimpso added a comment.

Hi Elena,

This patch causes a crash in spec2006/povray on AArch64. I've pasted a test case over at https://reviews.llvm.org/P7951. The problem has to do with the analysis in collectLoopUniforms and the new decision to scalarize. collectLoopUniforms is very conservative about what instructions remain uniform after vectorization. If a memory access has the possibility of being scalarized (even though it may not be), it's pointer operand is not marked uniform. What's happening here is that you've introduced a new scalarization decision based on the cost model that collectLoopUniforms doesn't know about (and  likely can't know about). In the test case, collectLoopUniforms marks the pointer operands of the loads uniform even though the loads are now scalarized, which causes the pointer operands to be non-uniform.

I'm not really sure what the best way to fix this would be. Any thoughts?  One way would be to remove scalarization from your widening decision.


Repository:
  rL LLVM

https://reviews.llvm.org/D27919