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

[Nikita Popov via llvm-commits]

================
@@ -4761,27 +4763,51 @@ 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);
+
+  // Canonicalize CmpLHS to be T, and CmpRHS to be F, if they're swapped.
+  if (CmpLHS == F && CmpRHS == T)
+    std::swap(CmpLHS, CmpRHS);
+
+  // This transforms is safe if at least one operand is known to not be zero.
+  // Otherwise, the select can change the sign of a zero operand.
+  if (IsEquiv || IsInverseEquiv) {
+    if (Value *V =
+            simplifySelectWithEquivalence(CmpLHS, CmpRHS, T, F, Q, MaxRecurse))
+      return V;
+    if (Value *V =
+            simplifySelectWithEquivalence(CmpRHS, CmpLHS, T, F, Q, MaxRecurse))
+      return V;
+
+    // (T == F) ? T : F --> F
+    // (T != F) ? F : T --> F
+    return CmpLHS == T && CmpRHS == F ? F : nullptr;
----------------
nikic wrote:

But shouldn't this case at least be covered by the preceding simplify code?

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