[llvm] [InstCombine] Loosen one-use restriction if common operand is constant (PR #79767)

[Nikita Popov via llvm-commits]

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@@ -4851,6 +4850,16 @@ Instruction *InstCombinerImpl::visitXor(BinaryOperator &I) {
     }
   }
 
+  // (A | B) ^ (A | C) --> (B ^ C) & ~A -- There are 4 commuted variants.
+  // We do this if common operand is a constant.
+  if (match(Op0, m_c_Or(m_Value(A), m_Value(B))) &&
+      match(Op1, m_c_Or(m_Specific(A), m_Value(C)))) {
+    if (match(A, m_ImmConstant())) {
+      Value *NotA = Builder.CreateNot(A);
+      return BinaryOperator::CreateAnd(Builder.CreateXor(B, C), NotA);
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nikic wrote:

It's not possible to remove one-use entirely. This may replace one instruction with two instructions.

https://github.com/llvm/llvm-project/pull/79767