[LLD] [COFF] Fix the mingw --export-all-symbols behaviour with comdat symbols

[mstorsjo]

Summary

When looking for the "all" symbols that are supposed to be exported,
we can't look at the live flag - the symbols we mark as to be
exported will become GC roots even if they aren't yet marked as live.

With this in place, building an LLVM library with BUILD_SHARED_LIBS
produces the same set of symbols exported regardless of whether the
--gc-symbols flag is specified, both with and without being built
with -ffunction-sections.

[rnk]

lgtm

Yeah, this would be an issue for explicit / extern template instantiations.

lld/COFF/MinGW.cpp

 void AutoExporter::addWholeArchive(StringRef path) {
   StringRef libName = sys::path::filename(path);
   // Drop the file extension, to match the processing below.
   libName = libName.substr(0, libName.rfind('.'));
   excludeLibs.erase(libName);
 }
 
 bool AutoExporter::shouldExport(Defined *sym) const {
-  if (!sym || !sym->isLive() || !sym->getChunk())
+  if (!sym || !sym->getChunk())
     return false;
 
   // Only allow the symbol kinds that make sense to export; in particular,
   // disallow import symbols.
   if (!isa<DefinedRegular>(sym) && !isa<DefinedCommon>(sym))
     return false;
   if (excludeSymbols.count(sym->getName()))
     return false;

lld/test/COFF/export-all.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -triple=i686-windows-gnu %s -filetype=obj -o %t.obj
 
 # RUN: lld-link -lldmingw -dll -out:%t.dll -entry:DllMainCRTStartup@12 %t.obj -implib:%t.lib
 # RUN: llvm-readobj --coff-exports %t.dll | grep Name: | FileCheck %s
 # RUN: llvm-readobj %t.lib | FileCheck -check-prefix=IMPLIB %s
 
 # CHECK: Name:
+# CHECK-NEXT: Name: comdatFunc
 # CHECK-NEXT: Name: dataSym
 # CHECK-NEXT: Name: foobar
 # CHECK-EMPTY:
 
+# IMPLIB: Symbol: __imp__comdatFunc
+# IMPLIB: Symbol: _comdatFunc
 # IMPLIB: Symbol: __imp__dataSym
 # IMPLIB-NOT: Symbol: _dataSym
 # IMPLIB: Symbol: __imp__foobar
 # IMPLIB: Symbol: _foobar
 
 .global _foobar
 .global _DllMainCRTStartup@12
 .global _dataSym
 .global _unexported
 .global __imp__unexported
 .global .refptr._foobar
+.global _comdatFunc
 .text
 _DllMainCRTStartup@12:
   ret
 _foobar:
   ret
 _unexported:
   ret
+.section .text@_comdatFunc,"xr",one_only,_comdatFunc
+_comdatFunc:
+  ret
 .data
 _dataSym:
   .int 4
 __imp__unexported:
   .int _unexported
 .refptr._foobar:
   .int _foobar