[llvm] [CodeGen] TailDuplicator should remove PHI when single predecessor block remains (PR #158533)

[Afanasyev Ivan via llvm-commits]

https://github.com/ivafanas created https://github.com/llvm/llvm-project/pull/158533

If tail block has only single predecessor after tail duplication, `TailDuplicator` saves PHI instructions with single block input.

MIR after `TailDuplicator` looks like:

```
  bb.1:
    %1 = PHI %0, %bb.0
    ...
```

Fix proposal is to replace PHI instruction with single input by COPY instruction.

Similar to https://github.com/llvm/llvm-project/pull/158265, bug appears only in the scenario of pre-ra partial tail duplication and does not affect in-tree backends.

>From c36b206a7b59e7549f67d4deac8674e0458f6bae Mon Sep 17 00:00:00 2001
From: Ivan Afanasyev <ivafanas at gmail.com>
Date: Mon, 15 Sep 2025 11:36:50 +0700
Subject: [PATCH] [CodeGen] TailDuplicator should remove PHI when single
 predecessor block remains

---
 llvm/lib/CodeGen/TailDuplicator.cpp | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/llvm/lib/CodeGen/TailDuplicator.cpp b/llvm/lib/CodeGen/TailDuplicator.cpp
index 9b1420a94142d..f94ada4e68e9d 100644
--- a/llvm/lib/CodeGen/TailDuplicator.cpp
+++ b/llvm/lib/CodeGen/TailDuplicator.cpp
@@ -387,6 +387,16 @@ void TailDuplicator::processPHI(
     MI->eraseFromParent();
   else if (MI->getNumOperands() == 1)
     MI->setDesc(TII->get(TargetOpcode::IMPLICIT_DEF));
+  else if (TailBB->pred_size() == 2) {
+    // According to callers implementation, PredBB has not yet been removed from
+    // TailBB predecessors list. After removal TailBB will have the only one
+    // predecessor and all phi instructions should be replaced with copies.
+    assert(PredBB->isSuccessor(TailBB));
+    BuildMI(*TailBB, MI->getIterator(), MI->getDebugLoc(),
+            TII->get(TargetOpcode::COPY), MI->getOperand(0).getReg())
+        .add(MI->getOperand(1));
+    MI->eraseFromParent();
+  }
 }
 
 /// Duplicate a TailBB instruction to PredBB and update