[llvm] Add flipped version of (X u< ~C) ? (X + C) : -1 --> uadd.sat(X, C) (PR #97973)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: AtariDreams (AtariDreams)

<details>
<summary>Changes</summary>

For some reason, LLVM is not considering X u > C, a, b to be the same as X <= C, b, a, so add that case here.

---
Full diff: https://github.com/llvm/llvm-project/pull/97973.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+8) 
- (modified) llvm/test/Transforms/InstCombine/saturating-add-sub.ll (+12) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 394dfca262e13..4ab8106d55761 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -985,6 +985,14 @@ static Value *canonicalizeSaturatedAdd(ICmpInst *Cmp, Value *TVal, Value *FVal,
     return Builder.CreateBinaryIntrinsic(
         Intrinsic::uadd_sat, X, ConstantInt::get(X->getType(), *C));
   }
+  if (Pred == ICmpInst::ICMP_UGT &&
+      match(FVal, m_Add(m_Value(X), m_APInt(C))) && X == Cmp0 &&
+      match(TVal, m_AllOnes()) && match(Cmp1, m_APInt(CmpC)) &&
+      *CmpC == (~*C - 1)) {
+    // (X u > ~C - 1) ? -1, (X + C) --> uadd.sat(X, C)
+    return Builder.CreateBinaryIntrinsic(Intrinsic::uadd_sat, X,
+                                         ConstantInt::get(X->getType(), *C));
+  }
 
   // Match unsigned saturated add of 2 variables with an unnecessary 'not'.
   // There are 8 commuted variants.
diff --git a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
index bf1568f1cd8c0..d220109ac17c0 100644
--- a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
+++ b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
@@ -1395,6 +1395,18 @@ define i32 @uadd_sat(i32 %x, i32 %y) {
   %r = select i1 %c, i32 -1, i32 %a
   ret i32 %r
 }
+
+define i32 @uadd_sat_flipped(i32 %x) {
+; CHECK-LABEL: @uadd_sat_flipped(
+; CHECK-NEXT:    [[COND:%.*]] = tail call i32 @llvm.uadd.sat.i32(i32 %x, i32 9)
+; CHECK-NEXT:    ret i32 [[COND]]
+;
+  %cmp = icmp uge i32 %x, -10
+  %add = add i32 %x, 9
+  %cond = select i1 %cmp, i32 -1, i32 %add
+  ret i32 %cond
+}
+
 define i32 @uadd_sat_nonstrict(i32 %x, i32 %y) {
 ; CHECK-LABEL: @uadd_sat_nonstrict(
 ; CHECK-NEXT:    [[R:%.*]] = call i32 @llvm.uadd.sat.i32(i32 [[X:%.*]], i32 [[Y:%.*]])

``````````

</details>


https://github.com/llvm/llvm-project/pull/97973