[clang] [clang][bytecode] Fix definining extern variables (PR #108940)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

<details>
<summary>Changes</summary>

At the point of defintion of the variable, a function might already refert to the variable by its index. Replace the index with the new one.

---
Full diff: https://github.com/llvm/llvm-project/pull/108940.diff


2 Files Affected:

- (modified) clang/lib/AST/ByteCode/Program.cpp (+10-1) 
- (added) clang/test/AST/ByteCode/extern.cpp (+13) 


``````````diff
diff --git a/clang/lib/AST/ByteCode/Program.cpp b/clang/lib/AST/ByteCode/Program.cpp
index bd5860beabaecd..79c645257306e0 100644
--- a/clang/lib/AST/ByteCode/Program.cpp
+++ b/clang/lib/AST/ByteCode/Program.cpp
@@ -204,9 +204,18 @@ std::optional<unsigned> Program::createGlobal(const ValueDecl *VD,
     IsStatic = false;
     IsExtern = true;
   }
+
+  // Register all previous declarations as well. For extern blocks, just replace
+  // the index with the new variable.
   if (auto Idx = createGlobal(VD, VD->getType(), IsStatic, IsExtern, Init)) {
-    for (const Decl *P = VD; P; P = P->getPreviousDecl())
+    for (const Decl *P = VD; P; P = P->getPreviousDecl()) {
+      if (P != VD) {
+        unsigned PIdx = GlobalIndices[P];
+        if (Globals[PIdx]->block()->isExtern())
+          Globals[PIdx] = Globals[*Idx];
+      }
       GlobalIndices[P] = *Idx;
+    }
     return *Idx;
   }
   return std::nullopt;
diff --git a/clang/test/AST/ByteCode/extern.cpp b/clang/test/AST/ByteCode/extern.cpp
new file mode 100644
index 00000000000000..a616269911a7ec
--- /dev/null
+++ b/clang/test/AST/ByteCode/extern.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify=both,expected %s
+// RUN: %clang_cc1 -verify=both,ref %s
+
+
+// both-no-diagnostics
+
+extern const int E;
+constexpr int getE() {
+  return E;
+}
+const int E = 10;
+static_assert(getE() == 10);
+

``````````

</details>


https://github.com/llvm/llvm-project/pull/108940