[clang][DebugInfo] Re-enable VTable debug info on COFF platforms (#158450)

The debug info for VTables introduced in #130255 was temporarily
disabled on COFF platforms by #151684, due to the risk of emitting
dangling relocations (see also:
https://github.com/llvm/llvm-project/issues/149639#issuecomment-3114257062
).

This patch re-enables that debug info and adds a guard to prevent
emitting dangling relocations by checking whether the VTable definition
is actually emitted.

Resolves #149639

1 files changed, 12 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index 578d09f..12c7d48 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -2657,12 +2657,22 @@ StringRef CGDebugInfo::getVTableName(const CXXRecordDecl *RD) {
 // existing information in the DWARF. The type is assumed to be 'void *'.
 void CGDebugInfo::emitVTableSymbol(llvm::GlobalVariable *VTable,
                                    const CXXRecordDecl *RD) {
-  if (!CGM.getTarget().getCXXABI().isItaniumFamily() ||
-      CGM.getTarget().getTriple().isOSBinFormatCOFF())
+  if (!CGM.getTarget().getCXXABI().isItaniumFamily())
     return;
   if (DebugKind <= llvm::codegenoptions::DebugLineTablesOnly)
     return;
 
+  // On COFF platform, we shouldn't emit a reference to an external entity (i.e.
+  // VTable) into debug info, which is constructed within a discardable section.
+  // If that entity ends up implicitly dllimported from another DLL, the linker
+  // may produce a runtime pseudo-relocation for it (BFD-ld only. LLD prohibits
+  // to emit such relocation). If the debug section is stripped, the runtime
+  // pseudo-relocation points to memory space outside of the module, causing an
+  // access violation.
+  if (CGM.getTarget().getTriple().isOSBinFormatCOFF() &&
+      VTable->isDeclarationForLinker())
+    return;
+
   ASTContext &Context = CGM.getContext();
   StringRef SymbolName = "_vtable$";
   SourceLocation Loc;