[llvm] [X86] Invert (and X, ~(and ~Y, Z)) back into (and X, (or Y, ~Z)) (PR #109215)

[via llvm-commits]

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@@ -50034,6 +50034,32 @@ static bool hasBZHI(const X86Subtarget &Subtarget, MVT VT) {
          (VT == MVT::i32 || (VT == MVT::i64 && Subtarget.is64Bit()));
 }
 
+/// InstCombine converts:
+///    `(and X, ~(and ~Y, Z))`
+/// to
+///    `(and X, (or Y, ~Z))`
+///
+/// But we should undo this transformation if the `andn` instruction is
+/// available to us.
+static SDValue combineAndNotOrIntoAndNotAnd(SDNode *N, SelectionDAG &DAG,
+                                            const X86Subtarget &Subtarget) {
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goldsteinn wrote:

Think there is a missing check for whether andn is supported...

https://github.com/llvm/llvm-project/pull/109215