[llvm] [InstCombine] Fold zext(icmp (A, xxx)) == shr(A, BW - 1) => not(trunc(xor(zext(icmp), shl))) (PR #68244)

[via llvm-commits]

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@@ -5311,11 +5311,7 @@ Instruction *InstCombinerImpl::foldICmpEquality(ICmpInst &I) {
       return new ICmpInst(Pred, A, Builder.CreateTrunc(B, A->getType()));
   }
 
-  // Test if 2 values have different or same signbits:
-  // (X u>> BitWidth - 1) == zext (Y s> -1) --> (X ^ Y) < 0
-  // (X u>> BitWidth - 1) != zext (Y s> -1) --> (X ^ Y) > -1
-  // (X s>> BitWidth - 1) == sext (Y s> -1) --> (X ^ Y) < 0
-  // (X s>> BitWidth - 1) != sext (Y s> -1) --> (X ^ Y) > -1
+  // Signbit test
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goldsteinn wrote:

"test" -> "test folds"

https://github.com/llvm/llvm-project/pull/68244