[LoopStrengthReduce, x86] don't add cost for a cmp that will be macro-fused (PR35681)

In the motivating case from PR35681 and represented by the macro-fuse-cmp test:
https://bugs.llvm.org/show_bug.cgi?id=35681
...there's a 37 -> 31 byte size win for the loop because we eliminate the big base 
address offsets.

SPEC2017 on Ryzen shows no significant perf difference.

Differential Revision: https://reviews.llvm.org/D42607

llvm-svn: 324289

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index 4b8e2286ed9..b7d9a258913 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -1343,14 +1343,15 @@ void Cost::RateFormula(const TargetTransformInfo &TTI,
 
   // If ICmpZero formula ends with not 0, it could not be replaced by
   // just add or sub. We'll need to compare final result of AddRec.
-  // That means we'll need an additional instruction.
+  // That means we'll need an additional instruction. But if the target can
+  // macro-fuse a compare with a branch, don't count this extra instruction.
   // For -10 + {0, +, 1}:
   // i = i + 1;
   // cmp i, 10
   //
   // For {-10, +, 1}:
   // i = i + 1;
-  if (LU.Kind == LSRUse::ICmpZero && !F.hasZeroEnd())
+  if (LU.Kind == LSRUse::ICmpZero && !F.hasZeroEnd() && !TTI.canMacroFuseCmp())
     C.Insns++;
   // Each new AddRec adds 1 instruction to calculation.
   C.Insns += (C.AddRecCost - PrevAddRecCost);