[llvm] [InstCombine] Add reverse of ((X << nuw Z) sub nuw Y) >>u exact Z --> X sub nuw (Y >>u exact Z) (PR #91386)

[Yingwei Zheng via llvm-commits]

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@@ -1271,6 +1271,17 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
     return NewSub;
   }
 
+  // (sub nuw X, (Y << nuw Z)) >>u exact Z --> (X >>u exact Z) sub nuw Y
+  if (I.isExact() &&
+      match(Op0, m_OneUse(m_NUWSub(m_Value(X),
+                                   m_NUWShl(m_Value(Y), m_Specific(Op1)))))) {
+    Value *NewLshr = Builder.CreateLShr(X, Op1, "", /*isExact=*/true);
+    auto *NewSub = BinaryOperator::CreateNUWSub(NewLshr, Y);
+    NewSub->setHasNoSignedWrap(
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dtcxzyw wrote:

Please provide alive2 proof for flag propagation next time.


https://github.com/llvm/llvm-project/pull/91386