[llvm] [InstCombine] Fold icmp eq/ne X, select(icmp pred X, P, C1, C2) to set membership (PR #226756)

[Andreas Jonson via llvm-commits]

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@@ -8105,6 +8105,69 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
     }
   }
 
+  // Fold icmp eq/ne X, select(icmp pred X, P, C1, C2)
+  // When the select condition is based on X and C1/C2 are constants,
+  // we can fold this to a simple set membership test.
+  // Example: X == select(X >s 0, 2, 0) -> (X == 2) | (X == 0)
+  // This is valid when C1 satisfies the select condition (C1 >s 0) and
+  // C2 does not satisfy it (C2 <=s 0).
+  if (I.isEquality()) {
+    // Try both operand orderings for the commutative icmp eq/ne
+    for (int Swap = 0; Swap < 2; ++Swap) {
+      Value *X = Swap ? Op1 : Op0;
+      Value *Sel = Swap ? Op0 : Op1;
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andjo403 wrote:

the for loop can be removed by moving this fold in to foldSelectICmp that is already called with commuted operands via foldICmpCommutative

https://github.com/llvm/llvm-project/pull/226756