[llvm] [InstCombine] Infer exact for lshr by cttz (PR #136696)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: 黃國庭 (houngkoungting)

<details>
<summary>Changes</summary>

Fix #<!-- -->131444 
This patch adds a  pattern . It match to infer the 'exact' flag on an 'lshr' instruction when the shift amount is computed via a 'cttz' intrinsic on the same operand.

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Full diff: https://github.com/llvm/llvm-project/pull/136696.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp (+12) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 90cd279e8a457..c39b7dae434e0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -994,6 +994,18 @@ static bool setShiftFlags(BinaryOperator &I, const SimplifyQuery &Q) {
       I.setIsExact();
       return true;
     }
+    //Fix #131444
+    if (auto *Cttz = dyn_cast<IntrinsicInst>(I.getOperand(1))) {
+      if (Cttz->getIntrinsicID() == Intrinsic::cttz &&
+          Cttz->getOperand(0) == I.getOperand(0)) {
+        if (auto *Const = dyn_cast<ConstantInt>(Cttz->getOperand(1))) {
+          if (Const->isOne()) {  
+            I.setIsExact();
+            return true;
+          }
+        }
+      }
+    }
   }
 
   // Compute what we know about shift count.

``````````

</details>


https://github.com/llvm/llvm-project/pull/136696