[InstCombine] X / (X * Y) --> 1.0 / Y

This is similar to the instsimplify fold added with D42385 
( rL323716 )
...but this can't be in instsimplify because we're creating/morphing
a different instruction.

llvm-svn: 324927

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index 3cd24035f3d..b1cad6ba009 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -1523,6 +1523,16 @@ Instruction *InstCombiner::visitFDiv(BinaryOperator &I) {
     return &I;
   }
 
+  // X / (X * Y) --> 1.0 / Y
+  // Reassociate to (X / X -> 1.0) is legal when NaNs are not allowed.
+  // We can ignore the possibility that X is infinity because INF/INF is NaN.
+  if (I.hasNoNaNs() && I.hasAllowReassoc() &&
+      match(Op1, m_c_FMul(m_Specific(Op0), m_Value(Y)))) {
+    I.setOperand(0, ConstantFP::get(I.getType(), 1.0));
+    I.setOperand(1, Y);
+    return &I;
+  }
+
   return nullptr;
 }