Index: llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp
+++ llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp
@@ -1778,6 +1778,12 @@
     return nullptr; // All distinct factors, so nothing left for us to do.
 
   IRBuilder<> Builder(I);
+  // The reassociate transformation for FP operations is performed only
+  // if unsafe algebra is permitted by FastMathFlags. Propagate those flags
+  // to the newly generated operations.
+  if (auto FPI = dyn_cast<FPMathOperator>(I))
+    Builder.setFastMathFlags(FPI->getFastMathFlags());
+
   Value *V = buildMinimalMultiplyDAG(Builder, Factors);
   if (Ops.empty())
     return V;
Index: llvm/trunk/test/Transforms/Reassociate/propagate-flags.ll
===================================================================
--- llvm/trunk/test/Transforms/Reassociate/propagate-flags.ll
+++ llvm/trunk/test/Transforms/Reassociate/propagate-flags.ll
@@ -0,0 +1,14 @@
+; RUN: opt < %s -reassociate -S | FileCheck %s
+ 
+; CHECK-LABEL: func
+; CHECK:       fmul fast double
+; CHECK-NEXT:  fmul fast double
+; CHECK-NEXT:  ret
+
+define double @func(double %a, double %b) {
+entry:
+  %mul1 = fmul fast double %a, %a
+  %mul2 = fmul fast double %b, %b
+  %mul3 = fmul fast double %mul1, %mul2
+  ret double %mul3
+}