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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Michael Hackner (HackAttack)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/151006.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SSAUpdater.cpp (+8-1) 


``````````diff
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));
 

``````````

</details>


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