[InstCombine] use nsw negation for abs libcalls

Also, produce the canonical IR abs (s<0) to be more efficient. 

This is the libcall equivalent of the clang builtin change from:
rL333038

Pasting from that commit message:
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.

llvm-svn: 333042

1 files changed, 7 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index 78864ea479c..32e47b652aa 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1682,12 +1682,12 @@ Value *LibCallSimplifier::optimizeFls(CallInst *CI, IRBuilder<> &B) {
 }
 
 Value *LibCallSimplifier::optimizeAbs(CallInst *CI, IRBuilder<> &B) {
-  // abs(x) -> x >s -1 ? x : -x
-  Value *Op = CI->getArgOperand(0);
-  Value *Pos =
-      B.CreateICmpSGT(Op, Constant::getAllOnesValue(Op->getType()), "ispos");
-  Value *Neg = B.CreateNeg(Op, "neg");
-  return B.CreateSelect(Pos, Op, Neg);
+  // abs(x) -> x <s 0 ? -x : x
+  // The negation has 'nsw' because abs of INT_MIN is undefined.
+  Value *X = CI->getArgOperand(0);
+  Value *IsNeg = B.CreateICmpSLT(X, Constant::getNullValue(X->getType()));
+  Value *NegX = B.CreateNSWNeg(X, "neg");
+  return B.CreateSelect(IsNeg, NegX, X);
 }
 
 Value *LibCallSimplifier::optimizeIsDigit(CallInst *CI, IRBuilder<> &B) {
@@ -2716,4 +2716,4 @@ Value *FortifiedLibCallSimplifier::optimizeCall(CallInst *CI) {
 
 FortifiedLibCallSimplifier::FortifiedLibCallSimplifier(
     const TargetLibraryInfo *TLI, bool OnlyLowerUnknownSize)
-    : TLI(TLI), OnlyLowerUnknownSize(OnlyLowerUnknownSize) {}
\ No newline at end of file
+    : TLI(TLI), OnlyLowerUnknownSize(OnlyLowerUnknownSize) {}