[CodeGen] use nsw negation for builtin abs

The clang builtins have the same semantics as the stdlib functions.
The stdlib functions are defined in section 7.20.6.1 of the C standard with:
"If the result cannot be represented, the behavior is undefined."

That lets us mark the negation with 'nsw' because "sub i32 0, INT_MIN" would
be UB/poison.

Differential Revision: https://reviews.llvm.org/D47202

llvm-svn: 333038

1 files changed, 2 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 724b3d234a9..5d8db103210 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -1252,8 +1252,9 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
   case Builtin::BI__builtin_labs:
   case Builtin::BI__builtin_llabs: {
     // X < 0 ? -X : X
+    // The negation has 'nsw' because abs of INT_MIN is undefined.
     Value *ArgValue = EmitScalarExpr(E->getArg(0));
-    Value *NegOp = Builder.CreateNeg(ArgValue, "neg");
+    Value *NegOp = Builder.CreateNSWNeg(ArgValue, "neg");
     Constant *Zero = llvm::Constant::getNullValue(ArgValue->getType());
     Value *CmpResult = Builder.CreateICmpSLT(ArgValue, Zero, "abscond");
     Value *Result = Builder.CreateSelect(CmpResult, NegOp, ArgValue, "abs");