[llvm] [SSAUpdater] Fix nondeterministic phi instruction order in bitcode (PR #151006)

[Michael Hackner via llvm-commits]

https://github.com/HackAttack created https://github.com/llvm/llvm-project/pull/151006

Currently there can be nondeterminism of the form

```diff
- <INST_PHI op0=307 op1=2 op2=22 op3=67 op4=29 op5=111 op6=57 ...>
+ <INST_PHI op0=307 op1=2 op2=22 op3=67 op4=29 op5=157 op6=71 ...>
```

in generated bitcode. Sorting predecessors here solves that.

>From 142f3db135391d4cb3d94f411b8f182fa4a99776 Mon Sep 17 00:00:00 2001
From: Michael Hackner <mhackner at gmail.com>
Date: Mon, 28 Jul 2025 10:34:27 -0700
Subject: [PATCH] [SSAUpdater] Fix nondeterministic phi instruction order in
 bitcode

Currently there can be nondeterminism of the form

```diff
- <INST_PHI op0=307 op1=2 op2=22 op3=67 op4=29 op5=111 op6=57 ...>
+ <INST_PHI op0=307 op1=2 op2=22 op3=67 op4=29 op5=157 op6=71 ...>
```

in generated bitcode. Sorting predecessors here solves that.
---
 llvm/lib/Transforms/Utils/SSAUpdater.cpp | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Utils/SSAUpdater.cpp b/llvm/lib/Transforms/Utils/SSAUpdater.cpp
index 49d0d9584347e..ed03066a50dca 100644
--- a/llvm/lib/Transforms/Utils/SSAUpdater.cpp
+++ b/llvm/lib/Transforms/Utils/SSAUpdater.cpp
@@ -122,7 +122,14 @@ Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
     }
   } else {
     bool isFirstPred = true;
-    for (BasicBlock *PredBB : predecessors(BB)) {
+
+    // Sort predecessors to get deterministic PHI operand ordering.
+    SmallVector<BasicBlock *, 8> SortedPreds(predecessors(BB));
+    llvm::sort(SortedPreds, [](BasicBlock *A, BasicBlock *B) {
+      return A->getNumber() < B->getNumber();
+    });
+
+    for (BasicBlock *PredBB : SortedPreds) {
       Value *PredVal = GetValueAtEndOfBlock(PredBB);
       PredValues.push_back(std::make_pair(PredBB, PredVal));