[COFF] Use COFF stubs for extern_weak functions

[rnk]

Summary

A COFF stub indirects the reference to a symbol through memory. A
.refptr.$sym global variable pointer is created to refer to $sym.
Typically mingw uses these for external global variable declarations,
but we can use them for weak function declarations as well.

Updates the dso_local classification to add a special case for
extern_weak symbols on COFF in both clang and LLVM.

Fixes PR37598

[rnk]

• fix some tests

[mstorsjo]

LGTM, it's nice if this mechanism comes in handy in other cases than the one it was originally made for.

[smeenai]

LGTM, neat!

llvm/test/CodeGen/X86/extern_weak.ll

20–21 ↗

(On Diff #198359)

I'm not parsing this too well (it's not quite clear what the relation between L_foo$non_lazy_ptr and _foo is, but I'm assuming $non_lazy_ptr is some Darwin thing that gets synthesized by the .weak_reference _foo), but I think you're just enhancing the Darwin test rather than changing that codegen in any way?

[mstorsjo]

Ideally it'd be nice to have this consistent on arm and aarch64 as well, but it's not a blocker (it's not something I'm running into so far).

[rnk]

Thanks! I'll see about AArch64. It would be nice for this use of __attribute__((weak)) declarations to work there as well.

llvm/test/CodeGen/X86/extern_weak.ll

20–21 ↗

(On Diff #198359)

Yes, it's basically the same thing as a COFF stub or a GOT entry: a global pointer that points to _foo.

lib/CodeGen/CodeGenModule.cpp

static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
   if (TT.isWindowsGNUEnvironment()) {
     // In MinGW, variables without DLLImport can still be automatically
     // imported from a DLL by the linker; don't mark variables that
     // potentially could come from another DLL as DSO local.
     if (GV->isDeclarationForLinker() && isa<llvm::GlobalVariable>(GV) &&
         !GV->isThreadLocal())
       return false;
   }
+
+  // On COFF, don't mark 'extern_weak' symbols as DSO local. If these symbols
+  // remain unresolved in the link, they can be resolved to zero, which is
+  // outside the current DSO.
+  if (TT.isOSBinFormatCOFF() && GV->hasExternalWeakLinkage())
+    return false;
+
   // Every other GV is local on COFF.
   // Make an exception for windows OS in the triple: Some firmware builds use
   // *-win32-macho triples. This (accidentally?) produced windows relocations
   // without GOT tables in older clang versions; Keep this behaviour.
   // FIXME: even thread local variables?
   if (TT.isOSBinFormatCOFF() || (TT.isOSWindows() && TT.isOSBinFormatMachO()))
     return true;
 

test/CodeGen/dso-local-executable.c

 // RUN: %clang_cc1 -triple x86_64-pc-win32 -emit-llvm %s -o - | FileCheck -allow-deprecated-dag-overlap --check-prefix=COFF %s
 // COFF-DAG: @bar = external dso_local global i32
-// COFF-DAG: @weak_bar = extern_weak dso_local global i32
+// COFF-DAG: @weak_bar = extern_weak global i32
 // COFF-DAG: declare dso_local void @foo()
 // COFF-DAG: @baz = dso_local global i32 42
 // COFF-DAG: define dso_local i32* @zed()
 // COFF-DAG: @thread_var = external dso_local thread_local global i32
 // COFF-DAG: @local_thread_var = dso_local thread_local global i32 42
 // COFF-DAG: @import_var = external dllimport global i32
 // COFF-DAG: declare dllimport void @import_func()