InstCombine: If the divisor of an fdiv has an exact inverse, turn it into an fmul.

Fixes PR9587.

llvm-svn: 128546

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index d1a1fd6ddfa..66513874815 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -452,6 +452,18 @@ Instruction *InstCombiner::visitFDiv(BinaryOperator &I) {
   if (Value *V = SimplifyFDivInst(Op0, Op1, TD))
     return ReplaceInstUsesWith(I, V);
 
+  if (ConstantFP *Op1C = dyn_cast<ConstantFP>(Op1)) {
+    const APFloat &Op1F = Op1C->getValueAPF();
+
+    // If the divisor has an exact multiplicative inverse we can turn the fdiv
+    // into a cheaper fmul.
+    APFloat Reciprocal(Op1F.getSemantics());
+    if (Op1F.getExactInverse(&Reciprocal)) {
+      ConstantFP *RFP = ConstantFP::get(Builder->getContext(), Reciprocal);
+      return BinaryOperator::CreateFMul(Op0, RFP);
+    }
+  }
+
   return 0;
 }