[llvm] [InstCombine] Canonicalize (a + 1 == 0) ? -1 : a + 1 -> uadd.sat(a, 1) (PR #144566)

[Craig Topper via llvm-commits]

topperc wrote:

This patch gets what you want with only 1 test update

```
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 084e7fbaa268..e6542223d695 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3160,7 +3160,7 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
       return replaceInstUsesWith(Cmp, Cond);
   }
   const APInt *C2;
-  if (Cmp.isEquality() || !match(Y, m_APInt(C2)))
+  if (!match(Y, m_APInt(C2)))
     return nullptr;
 
   // Fold icmp pred (add X, C2), C.
@@ -3184,7 +3184,7 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
       return new ICmpInst(Pred, X, ConstantInt::get(Ty, NewC));
   }
 
-  if (ICmpInst::isUnsigned(Pred) && Add->hasNoSignedWrap() &&
+  if (!Cmp.isEquality() && ICmpInst::isUnsigned(Pred) && Add->hasNoSignedWrap() &&
       C.isNonNegative() && (C - *C2).isNonNegative() &&
       computeConstantRange(X, /*ForSigned=*/true).add(*C2).isAllNonNegative())
     return new ICmpInst(ICmpInst::getSignedPredicate(Pred), X,
@@ -3205,6 +3205,9 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
       return new ICmpInst(ICmpInst::ICMP_UGE, X, ConstantInt::get(Ty, Lower));
   }
 
+  if (Cmp.isEquality())
+    return nullptr;
+
   // This set of folds is intentionally placed after folds that use no-wrapping
   // flags because those folds are likely better for later analysis/codegen.
   const APInt SMax = APInt::getSignedMaxValue(Ty->getScalarSizeInBits());
diff --git a/llvm/test/Transforms/InstCombine/uaddo.ll b/llvm/test/Transforms/InstCombine/uaddo.ll
index ae7a07ec8000..89a9569bdb7c 100644
--- a/llvm/test/Transforms/InstCombine/uaddo.ll
+++ b/llvm/test/Transforms/InstCombine/uaddo.ll
@@ -158,7 +158,7 @@ define i1 @uaddo_1(i8 %x, ptr %p) {
 ; CHECK-LABEL: @uaddo_1(
 ; CHECK-NEXT:    [[A:%.*]] = add i8 [[X:%.*]], 1
 ; CHECK-NEXT:    store i8 [[A]], ptr [[P:%.*]], align 1
-; CHECK-NEXT:    [[C:%.*]] = icmp eq i8 [[A]], 0
+; CHECK-NEXT:    [[C:%.*]] = icmp eq i8 [[X]], -1
 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %a = add i8 %x, 1
```

https://github.com/llvm/llvm-project/pull/144566