Set FMF for -ffp-contract=fast

With this, FMF(contract) becomes an alternative way to express the request to
contract.

These are currently only propagated for FMul, FAdd and FSub.  The rest will be
added as more FMFs are hooked up for this.

This is toward fixing PR25721.

Differential Revision: https://reviews.llvm.org/D31168

llvm-svn: 299469

2 files changed, 53 insertions, 4 deletions

diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 8ee4d3f7be1..a6430383117 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -113,6 +113,22 @@ static bool CanElideOverflowCheck(const ASTContext &Ctx, const BinOpInfo &Op) {
          (2 * Ctx.getTypeSize(RHSTy)) < PromotedSize;
 }
 
+/// Update the FastMathFlags of LLVM IR from the FPOptions in LangOptions.
+static void updateFastMathFlags(llvm::FastMathFlags &FMF,
+                                FPOptions FPFeatures) {
+  FMF.setAllowContract(FPFeatures.allowFPContractAcrossStatement());
+}
+
+/// Propagate fast-math flags from \p Op to the instruction in \p V.
+static Value *propagateFMFlags(Value *V, const BinOpInfo &Op) {
+  if (auto *I = dyn_cast<llvm::Instruction>(V)) {
+    llvm::FastMathFlags FMF = I->getFastMathFlags();
+    updateFastMathFlags(FMF, Op.FPFeatures);
+    I->setFastMathFlags(FMF);
+  }
+  return V;
+}
+
 class ScalarExprEmitter
   : public StmtVisitor<ScalarExprEmitter, Value*> {
   CodeGenFunction &CGF;
@@ -553,8 +569,10 @@ public:
         !CanElideOverflowCheck(CGF.getContext(), Ops))
       return EmitOverflowCheckedBinOp(Ops);
 
-    if (Ops.LHS->getType()->isFPOrFPVectorTy())
-      return Builder.CreateFMul(Ops.LHS, Ops.RHS, "mul");
+    if (Ops.LHS->getType()->isFPOrFPVectorTy()) {
+      Value *V = Builder.CreateFMul(Ops.LHS, Ops.RHS, "mul");
+      return propagateFMFlags(V, Ops);
+    }
     return Builder.CreateMul(Ops.LHS, Ops.RHS, "mul");
   }
   /// Create a binary op that checks for overflow.
@@ -2722,7 +2740,8 @@ Value *ScalarExprEmitter::EmitAdd(const BinOpInfo &op) {
     if (Value *FMulAdd = tryEmitFMulAdd(op, CGF, Builder))
       return FMulAdd;
 
-    return Builder.CreateFAdd(op.LHS, op.RHS, "add");
+    Value *V = Builder.CreateFAdd(op.LHS, op.RHS, "add");
+    return propagateFMFlags(V, op);
   }
 
   return Builder.CreateAdd(op.LHS, op.RHS, "add");
@@ -2755,7 +2774,8 @@ Value *ScalarExprEmitter::EmitSub(const BinOpInfo &op) {
       // Try to form an fmuladd.
       if (Value *FMulAdd = tryEmitFMulAdd(op, CGF, Builder, true))
         return FMulAdd;
-      return Builder.CreateFSub(op.LHS, op.RHS, "sub");
+      Value *V = Builder.CreateFSub(op.LHS, op.RHS, "sub");
+      return propagateFMFlags(V, op);
     }
 
     return Builder.CreateSub(op.LHS, op.RHS, "sub");
diff --git a/clang/test/CodeGen/ffp-contract-fast-option.cpp b/clang/test/CodeGen/ffp-contract-fast-option.cpp
new file mode 100644
index 00000000000..3db93de107c
--- /dev/null
+++ b/clang/test/CodeGen/ffp-contract-fast-option.cpp
@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -O3 -ffp-contract=fast -triple %itanium_abi_triple -emit-llvm -o - %s | FileCheck %s
+
+float fp_contract_1(float a, float b, float c) {
+  // CHECK-LABEL: fp_contract_1fff(
+  // CHECK: fmul contract float
+  // CHECK: fadd contract float
+  return a * b + c;
+}
+
+float fp_contract_2(float a, float b, float c) {
+  // CHECK-LABEL: fp_contract_2fff(
+  // CHECK: fmul contract float
+  // CHECK: fsub contract float
+  return a * b - c;
+}
+
+void fp_contract_3(float *a, float b, float c) {
+  // CHECK-LABEL: fp_contract_3Pfff(
+  // CHECK: fmul contract float
+  // CHECK: fadd contract float
+  a[0] += b * c;
+}
+
+void fp_contract_4(float *a, float b, float c) {
+  // CHECK-LABEL: fp_contract_4Pfff(
+  // CHECK: fmul contract float
+  // CHECK: fsub contract float
+  a[0] -= b * c;
+}