[llvm] [InstCombine] Set zero_is_poison for ctlz/cttz if they are only used as shift amounts (PR #85035)

[Yingwei Zheng via llvm-commits]

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@@ -1948,11 +1948,20 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     break;
 
   case Intrinsic::cttz:
-  case Intrinsic::ctlz:
+  case Intrinsic::ctlz: {
+    // If ctlz/cttz is only used as a shift amount, set is_zero_poison to true.
+    bool Changed = false;
+    if (II->hasOneUse() && match(II->getArgOperand(1), m_Zero()) &&
+        match(II->user_back(), m_Shift(m_Value(), m_Specific(II)))) {
+      replaceOperand(*II, 1, Builder.getTrue());
+      Changed = true;
+    }
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dtcxzyw wrote:

Done.

https://github.com/llvm/llvm-project/pull/85035