maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

[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
This commit is contained in:
Sanjay Patel 2018-05-22 23:02:13 +00:00
parent aa6d13e501
commit 74c7fb002f
2 changed files with 5 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
clang/lib/CodeGen/CGBuiltin.cpp
View File

@ -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");

6
clang/test/CodeGen/builtin-abs.c
View File

@ -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]]
//