[llvm] Avoid dbg.value between phi nodes when converting from DbgRecords (PR #83620)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-ir

Author: Daniel Sanders (dsandersllvm)

<details>
<summary>Changes</summary>

We have a downstream pass that at some point clones a phi instruction that is carrying some DPValues. When the verifier is called on IR containing phi nodes like:
```
%0 = phi ... ; has DPValues
%1 = phi ... ; cloned from previous phi
```
it currently converts to the debug intrinsics like so:
```
%0 = phi ...
call @<!-- -->llvm.dbg.value(...)
call @<!-- -->llvm.dbg.value(...)
...
%1 = phi ...
```
and then fails to verify because the phi's aren't first.

This patch moves the insertion point to a position after the phi nodes and any dbg.value calls it has already added.

It's surprising that the verifier modifies the IR but this is presumably temporary until RemoveDI's finishes. More surprising is that F.dump() performs this conversion but BB.dump() does not.

Unfortunately, I don't have a test case I can share as this problem occurred in a downstream pass and I'm not aware of any upstream that can be made to similarly clone a phi node.

---
Full diff: https://github.com/llvm/llvm-project/pull/83620.diff


1 Files Affected:

- (modified) llvm/lib/IR/BasicBlock.cpp (+7-2) 


``````````diff
diff --git a/llvm/lib/IR/BasicBlock.cpp b/llvm/lib/IR/BasicBlock.cpp
index 25aa3261164513..2001804e709fc8 100644
--- a/llvm/lib/IR/BasicBlock.cpp
+++ b/llvm/lib/IR/BasicBlock.cpp
@@ -109,9 +109,14 @@ void BasicBlock::convertFromNewDbgValues() {
       continue;
 
     DPMarker &Marker = *Inst.DbgMarker;
-    for (DbgRecord &DR : Marker.getDbgValueRange())
-      InstList.insert(Inst.getIterator(),
+    for (DbgRecord &DR : Marker.getDbgValueRange()) {
+      auto I = Inst.getIterator();
+      // Avoid inserting debug info between phi nodes.
+      if (isa<PHINode>(Inst))
+        I = getFirstNonPHIOrDbg()->getIterator();
+      InstList.insert(I,
                       DR.createDebugIntrinsic(getModule(), nullptr));
+    }
 
     Marker.eraseFromParent();
   }

``````````

</details>


https://github.com/llvm/llvm-project/pull/83620