Make `LLVMRustGetOrInsertGlobal` always return a `GlobalVariable`

`Module::getOrInsertGlobal` returns a `Constant*`, which is a super
class of `GlobalVariable`, but if the given type doesn't match an
existing declaration, it returns a bitcast of that global instead.
This causes UB when we pass that to `LLVMGetVisibility` which
unconditionally casts the opaque argument to a `GlobalValue*`.

Instead, we can do our own get-or-insert without worrying whether
existing types match exactly. It's not relevant when we're just trying
to get/set the linkage and visibility, and if types are needed we can
bitcast or error nicely from `rustc_codegen_llvm` instead.

1 files changed, 11 insertions, 1 deletions

diff --git a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
index e77d29bed71..f3d8eb2602a 100644
--- a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
@@ -124,8 +124,18 @@ extern "C" LLVMValueRef LLVMRustGetOrInsertFunction(LLVMModuleRef M,
 
 extern "C" LLVMValueRef
 LLVMRustGetOrInsertGlobal(LLVMModuleRef M, const char *Name, size_t NameLen, LLVMTypeRef Ty) {
+  Module *Mod = unwrap(M);
   StringRef NameRef(Name, NameLen);
-  return wrap(unwrap(M)->getOrInsertGlobal(NameRef, unwrap(Ty)));
+
+  // We don't use Module::getOrInsertGlobal because that returns a Constant*,
+  // which may either be the real GlobalVariable*, or a constant bitcast of it
+  // if our type doesn't match the original declaration. We always want the
+  // GlobalVariable* so we can access linkage, visibility, etc.
+  GlobalVariable *GV = Mod->getGlobalVariable(NameRef, true);
+  if (!GV)
+    GV = new GlobalVariable(*Mod, unwrap(Ty), false,
+                            GlobalValue::ExternalLinkage, nullptr, NameRef);
+  return wrap(GV);
 }
 
 extern "C" LLVMValueRef