Replace assert with early-out in tryEmitFMulAdd

[scanon]

Summary

r253269 exposed a pre-existing issue in tryEmitFMulAdd, where we would assert if a fusable operation had operands with multiple uses (we didn't see it previously because FP_CONTRACT was off by default). Fix this issue by changing the assert into an early-out where we simply won't try to fuse.

Once this is done, we can try again at enabling FP_CONTRACT ON as a default.

[scanon]

Friendly ping.

[majnemer]

Is it possible to have a test which covers this change?

lib/CodeGen/CGExprScalar.cpp

2568

I'd use auto *LHSBinOp here, the type is obvious from the right hand side.

2570–2575

I think use_empty() is more canonical than hasNUses(0) in this situation.

2573

Ditto here.

[scanon]

Abandoned for D15165.

lib/CodeGen/CGExprScalar.cpp

static Value* tryEmitFMulAdd(const BinOpInfo &op,
   if (!op.FPContractable)
     return nullptr;
 
   // Check whether -ffp-contract=on. (If -ffp-contract=off/fast, fusing is
   // either disabled, or handled entirely by the LLVM backend).
   if (CGF.CGM.getCodeGenOpts().getFPContractMode() != CodeGenOptions::FPC_On)
     return nullptr;
 
-  // We have a potentially fusable op. Look for a mul on one of the operands.
+  // We have a potentially fusable op. Look for a mul on one of the operands
+  // that does not have any other uses.
   if (llvm::BinaryOperator* LHSBinOp = dyn_cast<llvm::BinaryOperator>(op.LHS)) {
-    if (LHSBinOp->getOpcode() == llvm::Instruction::FMul) {
-      assert(LHSBinOp->getNumUses() == 0 &&
-             "Operations with multiple uses shouldn't be contracted.");
+    if (LHSBinOp->getOpcode() == llvm::Instruction::FMul &&
+        LHSBinOp->hasNUses(0))
       return buildFMulAdd(LHSBinOp, op.RHS, CGF, Builder, false, isSub);
-    }
-  } else if (llvm::BinaryOperator* RHSBinOp =
-               dyn_cast<llvm::BinaryOperator>(op.RHS)) {
-    if (RHSBinOp->getOpcode() == llvm::Instruction::FMul) {
-      assert(RHSBinOp->getNumUses() == 0 &&
-             "Operations with multiple uses shouldn't be contracted.");
+  if (llvm::BinaryOperator* RHSBinOp = dyn_cast<llvm::BinaryOperator>(op.RHS)) {
+    if (RHSBinOp->getOpcode() == llvm::Instruction::FMul &&
+        RHSBinOp->hasNUses(0))
       return buildFMulAdd(RHSBinOp, op.LHS, CGF, Builder, isSub, false);
-    }
-  }
   
   return nullptr;
 }
 
 Value *ScalarExprEmitter::EmitAdd(const BinOpInfo &op) {
   if (op.LHS->getType()->isPointerTy() ||
       op.RHS->getType()->isPointerTy())
     return emitPointerArithmetic(CGF, op, /*subtraction*/ false);