[llvm] [RemoveDIs][DebugInfo] Handle RAUW from dead constants (PR #80837)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-debuginfo

Author: Stephen Tozer (SLTozer)

<details>
<summary>Changes</summary>

Currently when a constant value dies, all `ValueAsMetadata` uses of that constant will be replaced with `nullptr`. For debug info intrinsics, this `RAUW` operation ends up being passed to `MetadataAsValue`, which canonicalizes the `nullptr` argument, replacing it with an empty `MDNode`, `!{}` (this is one of the only ways that an `MDNode` can be the first argument of a debug intrinsic). Since DPValues don't use `MetadataAsValue`, this is skipped and we end up with just a `nullptr`; currently this doesn't cause any problems because for most operations we use a wrapper that translates nullness, but will cause problems when we start printing DPValues, and should be considered an invalid state.

There are no tests for this change because currently any `nullptr` argument in a DPValue will be replaced with `!{}` when we convert it back to a debug intrinsic before printing, meaning this change is "NFC" for now. Instead, this patch adds an assert before converting back to debug intrinsics, which will cause the following tests to fail without the functional change:
```
llvm/test/Transforms/Inline/delete-function-with-metadata-use.ll
llvm/test/DebugInfo/X86/array2.ll
```

Hopefully this also paves the way to eliminating instances of empty `MDNodes` appearing in DPValues at any time as well, but there may be other cases that introduce them as well (@<!-- -->OCHyams may be familiar?).

---
Full diff: https://github.com/llvm/llvm-project/pull/80837.diff


2 Files Affected:

- (modified) llvm/lib/IR/DebugProgramInstruction.cpp (+1) 
- (modified) llvm/lib/IR/Metadata.cpp (+6) 


``````````diff
diff --git a/llvm/lib/IR/DebugProgramInstruction.cpp b/llvm/lib/IR/DebugProgramInstruction.cpp
index 03085b3bfd1279..5b9382fec88fdd 100644
--- a/llvm/lib/IR/DebugProgramInstruction.cpp
+++ b/llvm/lib/IR/DebugProgramInstruction.cpp
@@ -286,6 +286,7 @@ DPValue::createDebugIntrinsic(Module *M, Instruction *InsertBefore) const {
   // Create the intrinsic from this DPValue's information, optionally insert
   // into the target location.
   DbgVariableIntrinsic *DVI;
+  assert(getRawLocation() && "DPValue's RawLocation should be non-null.");
   if (isDbgAssign()) {
     Value *AssignArgs[] = {
         MetadataAsValue::get(Context, getRawLocation()),
diff --git a/llvm/lib/IR/Metadata.cpp b/llvm/lib/IR/Metadata.cpp
index 37017a222d4853..06db91fe4f8e74 100644
--- a/llvm/lib/IR/Metadata.cpp
+++ b/llvm/lib/IR/Metadata.cpp
@@ -158,6 +158,12 @@ void DebugValueUser::handleChangedValue(void *Old, Metadata *New) {
   // getOwner, if needed.
   auto OldMD = static_cast<Metadata **>(Old);
   ptrdiff_t Idx = std::distance(&*DebugValues.begin(), OldMD);
+  // If replacing a ValueAsMetadata with a nullptr, replace it with a
+  // PoisonValue instead.
+  if (OldMD && isa<ValueAsMetadata>(*OldMD) && !New) {
+    auto *OldVAM = cast<ValueAsMetadata>(*OldMD);
+    New = ValueAsMetadata::get(PoisonValue::get(OldVAM->getValue()->getType()));
+  }
   resetDebugValue(Idx, New);
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/80837