[PATCH] D47202: [CodeGen] use nsw negation for abs

[Sanjay Patel via Phabricator via cfe-commits]

spatel created this revision.
spatel added reviewers: craig.topper, efriedma, lebedev.ri.
Herald added a subscriber: mcrosier.

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."

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


https://reviews.llvm.org/D47202

Files:
  lib/CodeGen/CGBuiltin.cpp
  test/CodeGen/builtin-abs.c


Index: test/CodeGen/builtin-abs.c
===================================================================
--- test/CodeGen/builtin-abs.c
+++ test/CodeGen/builtin-abs.c
@@ -2,25 +2,25 @@
 
 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]]
 //
   return __builtin_abs(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]]
 //
   return __builtin_labs(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]]
 //
Index: lib/CodeGen/CGBuiltin.cpp
===================================================================
--- lib/CodeGen/CGBuiltin.cpp
+++ lib/CodeGen/CGBuiltin.cpp
@@ -1252,8 +1252,9 @@
   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");


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