[llvm] [InstCombine] Use `m_NotForbidPoison` when folding `(X u< Y) ? -1 : (~X + Y) --> uadd.sat(~X, Y)` (PR #114345)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Alive2: https://alive2.llvm.org/ce/z/mTGCo-
We cannot reuse `~X` if `m_AllOnes` matches a vector constant with some poison elts. An alternative solution is to create a new not instead of reusing `~X`. But it doesn't worth the effort because we need to add a one-use check.

Fixes https://github.com/llvm/llvm-project/issues/113869.


---
Full diff: https://github.com/llvm/llvm-project/pull/114345.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+1-1) 
- (modified) llvm/test/Transforms/InstCombine/saturating-add-sub.ll (+15) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index c5f39a4c381ed1..b442e79377740d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1051,7 +1051,7 @@ static Value *canonicalizeSaturatedAdd(ICmpInst *Cmp, Value *TVal, Value *FVal,
   // Strictness of the comparison is irrelevant.
   X = Cmp0;
   Y = Cmp1;
-  if (match(FVal, m_c_Add(m_Not(m_Specific(X)), m_Specific(Y)))) {
+  if (match(FVal, m_c_Add(m_NotForbidPoison(m_Specific(X)), m_Specific(Y)))) {
     // (X u< Y) ? -1 : (~X + Y) --> uadd.sat(~X, Y)
     // (X u< Y) ? -1 : (Y + ~X) --> uadd.sat(Y, ~X)
     BinaryOperator *BO = cast<BinaryOperator>(FVal);
diff --git a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
index a88fd3cc21f1bc..af8a9314a08049 100644
--- a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
+++ b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
@@ -1890,6 +1890,21 @@ define <2 x i32> @uadd_sat_not_ugt_commute_add(<2 x i32> %x, <2 x i32> %yp) {
   ret <2 x i32> %r
 }
 
+define <2 x i32> @uadd_sat_not_ugt_commute_add_partial_poison(<2 x i32> %x, <2 x i32> %yp) {
+; CHECK-LABEL: @uadd_sat_not_ugt_commute_add_partial_poison(
+; CHECK-NEXT:    [[NOTX:%.*]] = xor <2 x i32> [[X:%.*]], <i32 -1, i32 poison>
+; CHECK-NEXT:    [[A:%.*]] = add nuw <2 x i32> [[YP:%.*]], [[NOTX]]
+; CHECK-NEXT:    [[C:%.*]] = icmp ugt <2 x i32> [[YP]], [[X]]
+; CHECK-NEXT:    [[R:%.*]] = select <2 x i1> [[C]], <2 x i32> <i32 -1, i32 -1>, <2 x i32> [[A]]
+; CHECK-NEXT:    ret <2 x i32> [[R]]
+;
+  %notx = xor <2 x i32> %x, <i32 -1, i32 poison>
+  %a = add nuw <2 x i32> %yp, %notx
+  %c = icmp ugt <2 x i32> %yp, %x
+  %r = select <2 x i1> %c, <2 x i32> <i32 -1, i32 -1>, <2 x i32> %a
+  ret <2 x i32> %r
+}
+
 define i32 @uadd_sat_not_commute_select(i32 %x, i32 %y) {
 ; CHECK-LABEL: @uadd_sat_not_commute_select(
 ; CHECK-NEXT:    [[NOTX:%.*]] = xor i32 [[X:%.*]], -1

``````````

</details>


https://github.com/llvm/llvm-project/pull/114345