[clang] [clang][CodeGen] Skip deleted globals in emitUsed (PR #210959)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-codegen

Author: Emery Conrad (conrade-ctc)

<details>
<summary>Changes</summary>

A global on the `llvm.used`/`llvm.compiler.used` list can be deleted before the module is released: CodeGen erases an unreferenced `GlobalValue` without RAUW when the same mangled name is redefined with a different type (`GetOrCreateLLVMGlobal`), which nulls the `WeakTrackingVH` on the used list — value handles are not uses, so the `use_empty()` guard does not protect them.

In whole-TU compilation this is unobservable because the accompanying `err_duplicate_mangled_name` suppresses `Release()`. The incremental interpreter, however, clears the diagnostic state after the failed parse and keeps building the same module, so the next successful parse runs `Release()` and crashes in `emitUsed` dereferencing the dead handle:

```
clang-repl> __attribute__((used)) int a asm("sym") = 1; float b asm("sym") = 2.0f;
clang-repl> int ok = 0;
clang-repl: .../ValueHandle.h:95: llvm::Value& llvm::ValueHandleBase::operator*() const: Assertion `V && "Dereferencing deleted ValueHandle"' failed.
```

Out-of-tree incremental clients hit the same crash through other routes that delete an emitted used global before module release (e.g. CppInterOp's force-emission of reflection queries, see compiler-research/CppInterOp#<!-- -->1068).

Fix: skip null handles when building the used array. The repro above is included as a lit test; `clang/test/Interpreter`, `clang/test/CodeGen`, and `clang/test/CodeGenCXX` pass locally.

🤖 Done with the help of [Claude Code](https://claude.com/claude-code) (Fable 5, human in the loop)

---
Full diff: https://github.com/llvm/llvm-project/pull/210959.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CodeGenModule.cpp (+10-7) 
- (added) clang/test/Interpreter/used-global-after-error.cpp (+15) 


``````````diff
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 78627047b19ad..ead29559ced51 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -3701,13 +3701,16 @@ static void emitUsed(CodeGenModule &CGM, StringRef Name,
   if (List.empty())
     return;
 
-  // Convert List to what ConstantArray needs.
-  SmallVector<llvm::Constant*, 8> UsedArray;
-  UsedArray.resize(List.size());
-  for (unsigned i = 0, e = List.size(); i != e; ++i) {
-    UsedArray[i] =
-        llvm::ConstantExpr::getPointerBitCastOrAddrSpaceCast(
-            cast<llvm::Constant>(&*List[i]), CGM.Int8PtrTy);
+  // Convert List to what ConstantArray needs. A used global may have been
+  // deleted after it was added to the list (e.g. when its home module keeps
+  // accumulating declarations after an erroneous incremental parse), leaving
+  // a null value handle behind; skip those entries.
+  SmallVector<llvm::Constant *, 8> UsedArray;
+  UsedArray.reserve(List.size());
+  for (const llvm::WeakTrackingVH &VH : List) {
+    if (llvm::Value *V = VH)
+      UsedArray.push_back(llvm::ConstantExpr::getPointerBitCastOrAddrSpaceCast(
+          cast<llvm::Constant>(V), CGM.Int8PtrTy));
   }
 
   if (UsedArray.empty())
diff --git a/clang/test/Interpreter/used-global-after-error.cpp b/clang/test/Interpreter/used-global-after-error.cpp
new file mode 100644
index 0000000000000..fc1043042d5cc
--- /dev/null
+++ b/clang/test/Interpreter/used-global-after-error.cpp
@@ -0,0 +1,15 @@
+// REQUIRES: host-supports-jit
+// UNSUPPORTED: system-aix
+// RUN: cat %s | clang-repl | FileCheck %s
+
+// A cast<> of a null WeakTrackingVH used to crash emitUsed when a global on
+// the llvm.used list was deleted before the module was released: the
+// duplicate definition below is diagnosed by CodeGen, which then replaces the
+// used global's unreferenced GV with a differently-typed one; the failed
+// parse keeps the module alive, and the next successful parse finalizes it.
+extern "C" int printf(const char *, ...);
+__attribute__((used)) int a asm("sym") = 1; float b asm("sym") = 2.0f;
+auto r1 = printf("ok = %d\n", 42);
+// CHECK: ok = 42
+
+%quit

``````````

</details>


https://github.com/llvm/llvm-project/pull/210959