[llvm] [InstCombine] Improve coverage of `foldSelectValueEquivalence` (PR #88298)

[Nikita Popov via llvm-commits]

================
@@ -1288,40 +1288,94 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
     Swapped = true;
   }
 
-  // In X == Y ? f(X) : Z, try to evaluate f(Y) and replace the operand.
-  // Make sure Y cannot be undef though, as we might pick different values for
-  // undef in the icmp and in f(Y). Additionally, take care to avoid replacing
-  // X == Y ? X : Z with X == Y ? Y : Z, as that would lead to an infinite
-  // replacement cycle.
   Value *CmpLHS = Cmp.getOperand(0), *CmpRHS = Cmp.getOperand(1);
-  if (TrueVal != CmpLHS &&
-      isGuaranteedNotToBeUndefOrPoison(CmpRHS, SQ.AC, &Sel, &DT)) {
-    if (Value *V = simplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, SQ,
-                                          /* AllowRefinement */ true))
-      // Require either the replacement or the simplification result to be a
-      // constant to avoid infinite loops.
-      // FIXME: Make this check more precise.
-      if (isa<Constant>(CmpRHS) || isa<Constant>(V))
+  auto ReplaceOldOpWithNewOp = [&](Value *OldOp,
+                                   Value *NewOp) -> Instruction * {
+    // In X == Y ? f(X) : Z, try to evaluate f(Y) and replace the operand.
+    // Take care to avoid replacing X == Y ? X : Z with X == Y ? Y : Z, as that
+    // would lead to an infinite replacement cycle.
+    // If we will be able to evaluate f(Y) to a constant, we can allow undef,
+    // otherwise Y cannot be undef as we might pick different values for undef
+    // in the icmp and in f(Y).
+    if (TrueVal == OldOp)
+      return nullptr;
+
+    bool NewOpNeverUndef = false;
+
+    if (Value *V = simplifyWithOpReplaced(TrueVal, OldOp, NewOp, SQ,
+                                          /* AllowRefinement= */ true)) {
----------------
nikic wrote:

```suggestion
                                          /* AllowRefinement=*/true)) {
```

https://github.com/llvm/llvm-project/pull/88298