r333038 - [CodeGen] use nsw negation for builtin abs

[Sanjay Patel via cfe-commits]

Author: spatel
Date: Tue May 22 16:02:13 2018
New Revision: 333038

URL: http://llvm.org/viewvc/llvm-project?rev=333038&view=rev
Log:
[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

Modified:
    cfe/trunk/lib/CodeGen/CGBuiltin.cpp
    cfe/trunk/test/CodeGen/builtin-abs.c

Modified: cfe/trunk/lib/CodeGen/CGBuiltin.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGBuiltin.cpp?rev=333038&r1=333037&r2=333038&view=diff
==============================================================================
--- cfe/trunk/lib/CodeGen/CGBuiltin.cpp (original)
+++ cfe/trunk/lib/CodeGen/CGBuiltin.cpp Tue May 22 16:02:13 2018
@@ -1252,8 +1252,9 @@ RValue CodeGenFunction::EmitBuiltinExpr(
   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");

Modified: cfe/trunk/test/CodeGen/builtin-abs.c
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGen/builtin-abs.c?rev=333038&r1=333037&r2=333038&view=diff
==============================================================================
--- cfe/trunk/test/CodeGen/builtin-abs.c (original)
+++ cfe/trunk/test/CodeGen/builtin-abs.c Tue May 22 16:02:13 2018
@@ -2,7 +2,7 @@
 
 int absi(int x) {
 // CHECK-LABEL: @absi(
-// CHECK:   [[NEG:%.*]] = sub i32 0, [[X:%.*]]
+// CHECK:   [[NEG:%.*]] = sub nsw i32 0, [[X:%.*]]
 // CHECK:   [[CMP:%.*]] = icmp slt i32 [[X]], 0
 // CHECK:   [[SEL:%.*]] = select i1 [[CMP]], i32 [[NEG]], i32 [[X]]
 //
@@ -11,7 +11,7 @@ int absi(int x) {
 
 long absl(long x) {
 // CHECK-LABEL: @absl(
-// CHECK:   [[NEG:%.*]] = sub i64 0, [[X:%.*]]
+// CHECK:   [[NEG:%.*]] = sub nsw i64 0, [[X:%.*]]
 // CHECK:   [[CMP:%.*]] = icmp slt i64 [[X]], 0
 // CHECK:   [[SEL:%.*]] = select i1 [[CMP]], i64 [[NEG]], i64 [[X]]
 //
@@ -20,7 +20,7 @@ long absl(long x) {
 
 long long absll(long long x) {
 // CHECK-LABEL: @absll(
-// CHECK:   [[NEG:%.*]] = sub i64 0, [[X:%.*]]
+// CHECK:   [[NEG:%.*]] = sub nsw i64 0, [[X:%.*]]
 // CHECK:   [[CMP:%.*]] = icmp slt i64 [[X]], 0
 // CHECK:   [[SEL:%.*]] = select i1 [[CMP]], i64 [[NEG]], i64 [[X]]
 //