[llvm] [HashRecognize] Rewrite arePHIsIntertwined (PR #144878)

[Piotr Fusik via llvm-commits]

================
@@ -497,45 +497,29 @@ CRCTable HashRecognize::genSarwateTable(const APInt &GenPoly,
   return Table;
 }
 
-/// Checks if \p Reference is reachable from \p Needle on the use-def chain, and
-/// that there are no stray PHI nodes while digging the use-def chain. \p
-/// BOToMatch is a CRC peculiarity: at least one of the Users of Needle needs to
-/// match this OpCode, which is XOR for CRC.
-static bool arePHIsIntertwined(
-    const PHINode *Needle, const PHINode *Reference, const Loop &L,
-    Instruction::BinaryOps BOToMatch = Instruction::BinaryOpsEnd) {
-  // Initialize the worklist with Users of the Needle.
-  SmallVector<const Instruction *> Worklist;
-  for (const User *U : Needle->users()) {
-    if (auto *UI = dyn_cast<Instruction>(U))
-      if (L.contains(UI))
-        Worklist.push_back(UI);
-  }
-
-  // BOToMatch is usually XOR for CRC.
-  if (BOToMatch != Instruction::BinaryOpsEnd) {
-    if (count_if(Worklist, [BOToMatch](const Instruction *I) {
-          return I->getOpcode() == BOToMatch;
-        }) != 1)
-      return false;
-  }
-
-  while (!Worklist.empty()) {
-    const Instruction *I = Worklist.pop_back_val();
-
-    // Since Needle is never pushed onto the Worklist, I must either be the
-    // Reference PHI node (in which case we're done), or a stray PHI node (in
-    // which case we abort).
-    if (isa<PHINode>(I))
-      return I == Reference;
+/// A small helper for arePHIsIntertwined that is asymmetric in \p L and \p R,
+/// and hence needs to be called twice.
+static bool isXoredWith(const Value *L, const Value *R) {
+  return count_if(L->users(), [L, R](const User *U) {
+           return match(U, m_c_Xor(m_Specific(L),
+                                   m_CombineOr(m_ZExtOrSelf(m_Specific(R)),
+                                               m_Trunc(m_Specific(R)))));
+         }) == 1;
+}
 
-    for (const Use &U : I->operands())
-      if (auto *UI = dyn_cast<Instruction>(U))
-        // Don't push Needle back onto the Worklist.
-        if (UI != Needle && L.contains(UI))
-          Worklist.push_back(UI);
-  }
-  return false;
+/// Checks that \p L and \p R are used together in an XOR, which is a recurrence
+/// over both, ignoring zext and trunc. In other words, it checks for the
+/// following pattern:
+///
+/// loop:
+///   %L = phi [_, %entry], [%L.next, %loop]
+///   %R = phi [_, %entry], [%R.next, %loop]
+///   ...
+///   _ = xor ((trunc|zext|self) %L) ((trunc|zext|self) %R)
----------------
pfusik wrote:

```suggestion
///   _ = xor ((trunc|zext|self) %L), ((trunc|zext|self) %R)
```

https://github.com/llvm/llvm-project/pull/144878