[PATCH] D15559: [LoopUnrollRuntime] Do unroll when the unroll factor is 2

[Junmo Park via llvm-commits]

flyingforyou created this revision.
flyingforyou added reviewers: jmolloy, zzheng.
flyingforyou added a subscriber: llvm-commits.

When the unroll factor is 2, there is no division for computing loop's runtime trip-count.
This patch gives 0.3~0.5% performance improvements on benchmarks(test-suite, spec2000, commercial benchmark)

http://reviews.llvm.org/D15559

Files:
  lib/Transforms/Utils/LoopUnrollRuntime.cpp

Index: lib/Transforms/Utils/LoopUnrollRuntime.cpp
===================================================================
--- lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -313,7 +313,10 @@
   BasicBlock *Header = L->getHeader();
   const DataLayout &DL = Header->getModule()->getDataLayout();
   SCEVExpander Expander(*SE, DL, "loop-unroll");
-  if (!AllowExpensiveTripCount && Expander.isHighCostExpansion(TripCountSC, L))
+
+  // When the unroll factor is 2, we don't need to check cost expansion.
+  if (Count != 2 && !AllowExpensiveTripCount &&
+      Expander.isHighCostExpansion(TripCountSC, L))
     return false;
 
   // We only handle cases when the unroll factor is a power of 2.


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