[llvm] InstSimplify: support floating-point equivalences (PR #115152)

[Nikita Popov via llvm-commits]

================
@@ -4761,27 +4763,47 @@ static Value *simplifySelectWithICmpCond(Value *CondVal, Value *TrueVal,
 /// Try to simplify a select instruction when its condition operand is a
 /// floating-point comparison.
 static Value *simplifySelectWithFCmp(Value *Cond, Value *T, Value *F,
-                                     const SimplifyQuery &Q) {
+                                     const SimplifyQuery &Q,
+                                     unsigned MaxRecurse) {
   FCmpInst::Predicate Pred;
-  if (!match(Cond, m_FCmp(Pred, m_Specific(T), m_Specific(F))) &&
-      !match(Cond, m_FCmp(Pred, m_Specific(F), m_Specific(T))))
+  Value *CmpLHS, *CmpRHS;
+  if (!match(Cond, m_FCmp(Pred, m_Value(CmpLHS), m_Value(CmpRHS))))
+    return nullptr;
+  FCmpInst *I = cast<FCmpInst>(Cond);
+
+  bool IsEquiv = I->isEquivalence(),
+       IsInverseEquiv = I->isEquivalence(/*Invert=*/true);
+
+  if (IsInverseEquiv)
+    std::swap(T, F);
----------------
nikic wrote:

This is leaving behind a big footgun. I think the code here should be:

```
if (I->isEquivalence(/*Invert=*/true)) {
  std::swap(T, F);
  Pred = FCmpInst::getInversePredicate(Pred);
  IsEquiv = true;
}
```
so the values are correct after the swap.

https://github.com/llvm/llvm-project/pull/115152