[llvm] [InstCombine] Simplify a complex OR to XOR (PR #75129)

[via llvm-commits]

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@@ -3343,6 +3343,25 @@ Instruction *InstCombinerImpl::visitOr(BinaryOperator &I) {
   if (Instruction *X = foldComplexAndOrPatterns(I, Builder))
     return X;
 
+  // Change (or (and (add x, half_c1), c3), (and x, c2)) to (xor x, half_c1),
+  // iff c1, c2, c3 is constant, and half_c1 = (lshr c1, 1),  and c1 is pow2,
+  // and c2 < c1, and c3 == (c1 - 1) ^ c2, and (c1 >> 1) & c3 == (c1 >> 1) and x
+  // is known to be less than c1.
+  Type *Ty = I.getType();
+  {
+    Value *X = nullptr;
+    const APInt *HalfC1 = nullptr, *C2 = nullptr, *C3 = nullptr;
+    if (match(&I, m_c_Or(m_And(m_Add(m_Value(X), m_APInt(HalfC1)), m_APInt(C3)),
+                         m_And(m_Value(X), m_APInt(C2))))) {
+      const APInt C1 = HalfC1->shl(1);
+      KnownBits KnownX = computeKnownBits(X, 0, &I);
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bcl5980 wrote:

Check constant first, computeKnownBits is a heavy function call.

https://github.com/llvm/llvm-project/pull/75129