[llvm] [InstCombine] Combine interleaved PHI reduction chains. (PR #143878)

[Antonio Frighetto via llvm-commits]

================
@@ -996,6 +997,154 @@ Instruction *InstCombinerImpl::foldPHIArgOpIntoPHI(PHINode &PN) {
   return NewCI;
 }
 
+/// Try to fold reduction ops interleaved through two PHIs to a single PHI.
+///
+/// For example, combine:
+///   %phi1 = phi [init1, %BB1], [%op1, %BB2]
+///   %phi2 = phi [init2, %BB1], [%op2, %BB2]
+///   %op1 = binop %phi1, constant1
+///   %op2 = binop %phi2, constant2
+///   %rdx = binop %op1, %op2
+/// =>
+///   %phi_combined = phi [init_combined, %BB1], [%op_combined, %BB2]
+///   %rdx_combined = binop %phi_combined, constant_combined
+///
+/// For now, we require init1, init2, constant1 and constant2 to be constants.
+Instruction *InstCombinerImpl::foldPHIReduction(PHINode &PN) {
+  BinaryOperator *BO1;
+  Value *Start1;
+  Value *Step1;
+
+  // Find the first recurrence.
+  if (!PN.hasOneUse() || !matchSimpleRecurrence(&PN, BO1, Start1, Step1))
+    return nullptr;
+
+  // Ensure BO1 has two uses (PN and the reduction op) and can be reassociated.
+  if (!BO1->hasNUses(2) || !BO1->isAssociative())
+    return nullptr;
+
+  // Convert Start1 and Step1 to constants.
+  auto *Init1 = dyn_cast<Constant>(Start1);
+  auto *C1 = dyn_cast<Constant>(Step1);
+  if (!Init1 || !C1)
+    return nullptr;
+
+  // Find the reduction operation.
+  auto Opc = BO1->getOpcode();
+  BinaryOperator *Rdx = nullptr;
+  for (User *U : BO1->users())
+    if (U != &PN) {
+      Rdx = dyn_cast<BinaryOperator>(U);
+      break;
+    }
----------------
antoniofrighetto wrote:

```suggestion
  auto It = find_if(BO1->users(), [&](auto *U) { return U != &PN; });
  if (It != BO1->users().end())
    Rdx = dyn_cast<BinaryOperator>(*It);
```

https://github.com/llvm/llvm-project/pull/143878