[CodeGen] use nsw negation for abs

[spatel]

Summary

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.

[lebedev.ri]

That is what happens for "hand-rolled" abs, https://godbolt.org/g/6dbgXD
I *think* this makes sense, since IIRC LLVM only supports twos-complement platforms.
But all these attributes are currently beyond me, so i won't click the accept button :)

[spatel]

In D47202#1108016, @lebedev.ri wrote:

That is what happens for "hand-rolled" abs, https://godbolt.org/g/6dbgXD
I *think* this makes sense, since IIRC LLVM only supports twos-complement platforms.
But all these attributes are currently beyond me, so i won't click the accept button :)

Thanks for taking a look! I should also note that we don't translate the standard libcall to the builtin in clang (not sure if we should), so this patch doesn't currently affect the common case. We'd probably want to do something similar in LibCallSimplifier if this is right.

[craig.topper]

This seems right to me. GCC believes believes that __bultin_abs always returns a positive number.

lib/CodeGen/CGBuiltin.cpp

case Builtin::BI__builtin_va_copy: {
     SrcPtr = Builder.CreateBitCast(SrcPtr, Type);
     return RValue::get(Builder.CreateCall(CGM.getIntrinsic(Intrinsic::vacopy),
                                           {DstPtr, SrcPtr}));
   }
   case Builtin::BI__builtin_abs:
   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");
     return RValue::get(Result);
   }
   case Builtin::BI__builtin_conj:
   case Builtin::BI__builtin_conjf:
   case Builtin::BI__builtin_conjl: {

test/CodeGen/builtin-abs.c

 // RUN: %clang_cc1 -triple x86_64-unknown-unknown -emit-llvm %s -o - | FileCheck %s
 
 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]]
 //
   return __builtin_llabs(x);
 }