Fix a logic bug when copying fast-math flags.

"Setting" does not equal "copying". This bug has sat dormant for 2 reasons:
1. The unit test was not adequate.
2. Every current user of the "copyFastMathFlags" API is operating on a new instruction.
   (ie, all existing fast-math flags are off). If you copy flags to an existing
   instruction that has some flags on already, you will not necessarily turn them off
   as expected.

I uncovered this bug while trying to implement a fix for PR20802.

llvm-svn: 216939

2 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index 86421c4ae9f..50b1f04d41e 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -143,6 +143,11 @@ void Instruction::setFastMathFlags(FastMathFlags FMF) {
   cast<FPMathOperator>(this)->setFastMathFlags(FMF);
 }
 
+void Instruction::copyFastMathFlags(FastMathFlags FMF) {
+  assert(isa<FPMathOperator>(this) && "copying fast-math flag on invalid op");
+  cast<FPMathOperator>(this)->copyFastMathFlags(FMF);
+}
+
 /// Determine whether the unsafe-algebra flag is set.
 bool Instruction::hasUnsafeAlgebra() const {
   assert(isa<FPMathOperator>(this) && "getting fast-math flag on invalid op");
@@ -183,7 +188,7 @@ FastMathFlags Instruction::getFastMathFlags() const {
 
 /// Copy I's fast-math flags
 void Instruction::copyFastMathFlags(const Instruction *I) {
-  setFastMathFlags(I->getFastMathFlags());
+  copyFastMathFlags(I->getFastMathFlags());
 }
 
 
diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index 16993f1bba5..b113d51d416 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -2043,7 +2043,7 @@ void BinaryOperator::copyFlags(const Value *V) {
   
   // Copy the fast-math flags.
   if (auto *FP = dyn_cast<FPMathOperator>(V))
-    setFastMathFlags(FP->getFastMathFlags());
+    copyFastMathFlags(FP->getFastMathFlags());
 }
 
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