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a6a9ac5aa1
This pass is needed to break false dependencies. Without it, unlucky register assignment can result in wild (5x) swings in performance. This pass was trying to handle AVX but not getting it right. AVX doesn't have partial register defs, it has unused register reads in which the high bits of a source operand are copied into the unused bits of the dest. Fixing this requires conservative liveness analysis. This is awkard because the pass already has its own pseudo-liveness. However, proper liveness is expensive, and we would like to use a generic utility to compute it. The fix only invokes liveness on-demand. It is rare to detect a case that needs undef-read dependence breaking, but when it happens, it can be needed many times within a very large block. I think the existing heuristic which uses a register window of 16 is too conservative for loop-carried false dependencies. If the loop is a reduction. The out-of-order engine may be able to execute several loop iterations in parallel. However, I'll leave this tuning exercise for next time. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192635 91177308-0d34-0410-b5e6-96231b3b80d8
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