[COFF] Fix to not add archive name to buffer identifiers when they come from thin archives.

[akhuang]

Summary

Currently lld adds the archive name to MemoryBufferRef identifiers in order to
ensure they are unique. For thin archives, since the file name is already unique and we
want to keep the original path to the file, don't add the archive name.

[rnk]

Looks right to me, but let's wait for @inlgorion to take a look on Monday.

[inglorion]

lgtm, thanks!

[akhuang]

rebase

lld/COFF/Driver.cpp

std::string childName = CHECK(
       "could not get the filename for the member defining symbol " +
       toCOFFString(sym));
   auto future = std::make_shared<std::future<MBErrPair>>(
       createFutureForFile(childName));
   enqueueTask([=]() {
     auto mbOrErr = future->get();
     if (mbOrErr.second)
       reportBufferError(errorCodeToError(mbOrErr.second), childName);
+    // Pass empty string as archive name so that the original filename is
+    // used as the buffer identifier.
     driver->addArchiveBuffer(takeBuffer(std::move(mbOrErr.first)),
-                             toCOFFString(sym), parentName,
-                             /*OffsetInArchive=*/0);
+                             toCOFFString(sym), "", /*OffsetInArchive=*/0);
   });
 }
 
 static bool isDecorated(StringRef sym) {
   return sym.startswith("@") || sym.contains("@@") || sym.startswith("?") ||
          (!config->mingw && sym.contains('@'));
 }
 

lld/test/COFF/thinlto-index-only.ll

 ; BACKEND2: </MODULE_STRTAB_BLOCK
 ; BACKEND2-NEXT: <GLOBALVAL_SUMMARY_BLOCK
 ; BACKEND2-NEXT: <VERSION
 ; BACKEND2-NEXT: <FLAGS
 ; BACKEND2-NEXT: <VALUE_GUID op0=1 op1=-5300342847281564238
 ; BACKEND2-NEXT: <COMBINED
 ; BACKEND2-NEXT: </GLOBALVAL_SUMMARY_BLOCK
 
+; Thin archive tests. Check that the module paths point to the original files.
+; RUN: rm -rf %t
+; RUN: mkdir %t
+; RUN: opt -thinlto-bc -o %t/foo.obj < %s
+; RUN: opt -thinlto-bc -o %t/bar.obj < %p/Inputs/thinlto.ll
+; RUN: llvm-ar rcsT %t5.lib %t/bar.obj %t3.obj
+; RUN: lld-link -thinlto-index-only -entry:main %t/foo.obj %t5.lib
+; RUN: llvm-dis -o - %t/foo.obj.thinlto.bc | FileCheck %s --check-prefix=THINARCHIVE
+; THINARCHIVE: ^0 = module: (path: "{{.*}}foo.obj",
+; THINARCHIVE: ^1 = module: (path: "{{.*}}bar.obj",
+
 target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-pc-windows-msvc19.0.24215"
 
 declare void @g(...)
 
 define void @main() {
   call void (...) @g()
   ret void
 }