[llvm] [ValueTracking] Improve ananlysis of PHI nodes. (PR #71224)

[Nikita Popov via llvm-commits]

================
@@ -3020,32 +3020,41 @@ static bool isNonEqualShl(const Value *V1, const Value *V2, unsigned Depth,
   return false;
 }
 
-static bool isNonEqualPHIs(const PHINode *PN1, const PHINode *PN2,
-                           unsigned Depth, const SimplifyQuery &Q) {
-  // Check two PHIs are in same block.
-  if (PN1->getParent() != PN2->getParent())
-    return false;
-
+static bool isNonEqualPhisSameBB(const PHINode *PN1, const PHINode *PN2,
+                                 unsigned Depth, const SimplifyQuery &Q) {
   SmallPtrSet<const BasicBlock *, 8> VisitedBBs;
-  bool UsedFullRecursion = false;
   for (const BasicBlock *IncomBB : PN1->blocks()) {
     if (!VisitedBBs.insert(IncomBB).second)
       continue; // Don't reprocess blocks that we have dealt with already.
     const Value *IV1 = PN1->getIncomingValueForBlock(IncomBB);
     const Value *IV2 = PN2->getIncomingValueForBlock(IncomBB);
-    const APInt *C1, *C2;
-    if (match(IV1, m_APInt(C1)) && match(IV2, m_APInt(C2)) && *C1 != *C2)
-      continue;
 
-    // Only one pair of phi operands is allowed for full recursion.
-    if (UsedFullRecursion)
+    SimplifyQuery RecQ = Q;
+    RecQ.CxtI = IncomBB->getTerminator();
+    if (!isKnownNonEqual(IV1, IV2, Depth + 1, RecQ))
       return false;
+  }
+  return true;
+}
+
+static bool isNonEqualPhi(const Value *V1, const Value *V2, unsigned Depth,
+                          const SimplifyQuery &Q) {
+  const PHINode *PN1 = dyn_cast<PHINode>(V1);
+  if (!PN1)
+    return false;
 
+  if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) {
+    if (PN1->getParent() == PN2->getParent())
+      return isNonEqualPhisSameBB(PN1, PN2, Depth, Q);
+  }
+  for (const BasicBlock *IncomBB : PN1->blocks()) {
+    Value *V = PN1->getIncomingValueForBlock(IncomBB);
+    if (V == PN1)
+      continue;
     SimplifyQuery RecQ = Q;
     RecQ.CxtI = IncomBB->getTerminator();
-    if (!isKnownNonEqual(IV1, IV2, Depth + 1, RecQ))
+    if (!isKnownNonEqual(V, V2, Depth + 1, RecQ))
----------------
nikic wrote:

This is incorrect. V and V2 here can be values from different cycle iterations.

https://github.com/llvm/llvm-project/pull/71224