Fold icmp samesign u{gt/ge/lt/le} (add nsw X, C2), C -> icmp s{gt/ge/lt/le} X, (C - C2)

Author: wermos

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -3189,6 +3189,13 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
     return new ICmpInst(ICmpInst::getSignedPredicate(Pred), X,
                         ConstantInt::get(Ty, C - *C2));
 
+  // Fold icmp samesign u{gt/ge/lt/le} (add nsw X, C2), C
+  //      -> icmp s{gt/ge/lt/le} X, (C - C2)
+  CmpPredicate CP(Pred, Cmp.hasSameSign());
+  if (CP.hasSameSign() && Add->hasNoSignedWrap())
+    return new ICmpInst(CP.getPreferredSignedPredicate(), X,
+                        ConstantInt::get(Ty, C - *C2));
+
   auto CR = ConstantRange::makeExactICmpRegion(Pred, C).subtract(*C2);
   const APInt &Upper = CR.getUpper();
   const APInt &Lower = CR.getLower();

llvm/test/Transforms/InstCombine/icmp-add.ll

@@ -3444,8 +3444,7 @@ define i1 @val_is_aligend_pred_mismatch(i32 %num) {
 define i1 @icmp_samesign_with_nsw_add(i32 %arg0) {
 ; CHECK-LABEL: @icmp_samesign_with_nsw_add(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[ARG0:%.*]], -26
-; CHECK-NEXT:    [[V1:%.*]] = icmp ult i32 [[TMP0]], -8
+; CHECK-NEXT:    [[V1:%.*]] = icmp sgt i32 [[ARG0:%.*]], 25
 ; CHECK-NEXT:    ret i1 [[V1]]
 ;
 entry: