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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: AZero13 (AZero13)

<details>
<summary>Changes</summary>

This is because this actually is just:

// ((X + Y) u< X) ? -1 : (X + Y) --> uadd.sat(X, Y)
// ((X + Y) u< Y) ? -1 : (X + Y) --> uadd.sat(X, Y)

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


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+9-2) 
- (modified) llvm/test/Transforms/InstCombine/saturating-add-sub.ll (+11) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 73ba0f78e8053..d733a37efd68e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1013,12 +1013,19 @@ static Value *canonicalizeSaturatedAdd(ICmpInst *Cmp, Value *TVal, Value *FVal,
 
   // uge -1 is canonicalized to eq -1 and requires special handling
   // (a == -1) ? -1 : a + 1 -> uadd.sat(a, 1)
+  // ult 1 is canonicalized to eq 0
+  // (a + 1 == 0) ? -1 : a + 1 -> uadd.sat(a, 1)
   if (Pred == ICmpInst::ICMP_EQ) {
     if (match(FVal, m_Add(m_Specific(Cmp0), m_One())) &&
-        match(Cmp1, m_AllOnes())) {
+        match(Cmp1, m_AllOnes()))
       return Builder.CreateBinaryIntrinsic(
           Intrinsic::uadd_sat, Cmp0, ConstantInt::get(Cmp0->getType(), 1));
-    }
+
+    if (match(Cmp1, m_Zero()) && match(Cmp0, m_Add(m_Value(X), m_One())) &&
+        match(FVal, m_Add(m_Specific(X), m_One())))
+      return Builder.CreateBinaryIntrinsic(Intrinsic::uadd_sat, X,
+                                           ConstantInt::get(X->getType(), 1));
+
     return nullptr;
   }
 
diff --git a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
index cfd679c0cc592..392551defb7ef 100644
--- a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
+++ b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
@@ -2350,4 +2350,15 @@ define i8 @fold_add_umax_to_usub_multiuse(i8 %a) {
   ret i8 %sel
 }
 
+define i32 @add_check_zero(i32 %num) {
+; CHECK-LABEL: @add_check_zero(
+; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.uadd.sat.i32(i32 [[ADD:%.*]], i32 1)
+; CHECK-NEXT:    ret i32 [[COND]]
+;
+  %add = add i32 %num, 1
+  %cmp = icmp eq i32 %add, 0
+  %cond = select i1 %cmp, i32 -1, i32 %add
+  ret i32 %cond
+}
+
 declare void @usei8(i8)

``````````

</details>


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