COFF: Do not create empty temporary archives for /msvclto.

[pcc]

Summary

The MSVC linker doesn't support them.

[ruiu]

LGTM

lld/COFF/Driver.cpp

filterBitcodeFiles(StringRef Path, std::vector<std::string> &TemporaryFiles) {
 
   if (Magic == file_magic::bitcode)
     return None;
   if (Magic != file_magic::archive)
     return Path.str();
   if (!needsRebuilding(MBRef))
     return Path.str();
 
-  log("Creating a temporary archive for " + Path +
-      " to remove bitcode files");
-
   std::unique_ptr<Archive> File =
       check(Archive::create(MBRef),
             MBRef.getBufferIdentifier() + ": failed to parse archive");
 
   std::vector<NewArchiveMember> New;
   for (MemoryBufferRef Member : getArchiveMembers(File.get()))
     if (identify_magic(Member.getBuffer()) != file_magic::bitcode)
       New.emplace_back(Member);
 
+  if (New.empty())
+    return None;
+
+  log("Creating a temporary archive for " + Path + " to remove bitcode files");
+
   SmallString<128> S;
   if (auto EC = sys::fs::createTemporaryFile("lld-" + sys::path::stem(Path),
                                              ".lib", S))
     fatal(EC, "cannot create a temporary file");
   std::string Temp = S.str();
   TemporaryFiles.push_back(Temp);
 
   std::pair<StringRef, std::error_code> Ret =

lld/test/COFF/msvclto-archive.ll

 ;; Make sure we re-create archive files to strip bitcode files.
 
+;; Do not create empty archives because the MSVC linker
+;; doesn't support them.
 ; RUN: llvm-as -o %t.obj %s
 ; RUN: rm -f %t-main1.a
 ; RUN: llvm-ar cru %t-main1.a %t.obj
 ; RUN: mkdir -p %t.dir
 ; RUN: llvm-mc -triple=x86_64-pc-windows-msvc -filetype=obj -o %t.dir/bitcode.obj %p/Inputs/msvclto.s
 ; RUN: lld-link %t-main1.a %t.dir/bitcode.obj /msvclto /out:%t.exe /opt:lldlto=1 /opt:icf \
 ; RUN:   /entry:main /verbose > %t.log || true
 ; RUN: FileCheck -check-prefix=BC %s < %t.log
-; BC: Creating a temporary archive for
+; BC-NOT: Creating a temporary archive for
 
 ; RUN: rm -f %t-main2.a
 ; RUN: llvm-ar cru %t-main2.a %t.dir/bitcode.obj
 ; RUN: lld-link %t.obj %t-main2.a /msvclto /out:%t.exe /opt:lldlto=1 /opt:icf \
 ; RUN:   /entry:main /verbose > %t.log || true
 ; RUN: FileCheck -check-prefix=OBJ %s < %t.log
 ; OBJ-NOT: Creating a temporary archive