[llvm-link] Improve link time for bitcode archives [NFC]

[sdmitriev]

Summary

Linking large bitcode archives currently takes a lot of time with llvm-link,
this patch adds couple improvements which reduce link time for archives

• Use one Linker instance for archive instead of recreating it for each member
• Lazy load archive members

[tra]

LGTM.

Do you have any measurements showing actual impact on performance or memory consumption?

llvm/tools/llvm-link/llvm-link.cpp

149

This should be moved closer to the point where it's used.

[sdmitriev]

Addressed review comment.

[sdmitriev]

In D94643#2498870, @tra wrote:

LGTM.

Do you have any measurements showing actual impact on performance or memory consumption?

On my local experiments with a big bitcode library link time reduced from ~10 minutes to ~9 seconds.

[sdmitriev]

ping

[jdoerfert]

@tra accepted. LGTM too.

llvm/tools/llvm-link/llvm-link.cpp

static std::unique_ptr<Module> loadFile(const char *argv0,
 
   return Result;
 }
 
 static std::unique_ptr<Module> loadArFile(const char *Argv0,
                                           std::unique_ptr<MemoryBuffer> Buffer,
                                           LLVMContext &Context) {
   std::unique_ptr<Module> Result(new Module("ArchiveModule", Context));
   StringRef ArchiveName = Buffer->getBufferIdentifier();
   if (Verbose)
     errs() << "Reading library archive file '" << ArchiveName
            << "' to memory\n";
   Error Err = Error::success();
   object::Archive Archive(*Buffer, Err);
   ExitOnErr(std::move(Err));
+  Linker L(*Result);
   for (const object::Archive::Child &C : Archive.children(Err)) {
     Expected<StringRef> Ename = C.getName();
     if (Error E = Ename.takeError()) {
       errs() << Argv0 << ": ";
       WithColor::error()
           << " failed to read name of archive member"
           << ArchiveName << "'\n";
       return nullptr;
if (!isBitcode(reinterpret_cast<const unsigned char *>
                    reinterpret_cast<const unsigned char *>
                    (MemBuf.get().getBufferEnd()))) {
       errs() << Argv0 << ": ";
       WithColor::error() << "  member of archive is not a bitcode file: '"
                          << ChildName << "'\n";
       return nullptr;
     }
 
-    std::unique_ptr<Module> M = parseIR(MemBuf.get(), ParseErr, Context);
+    std::unique_ptr<Module> M;
+    if (DisableLazyLoad)
+      M = parseIR(MemBuf.get(), ParseErr, Context);
+    else
+      M = getLazyIRModule(MemoryBuffer::getMemBuffer(MemBuf.get(), false),
+                          ParseErr, Context);
 
     if (!M.get()) {
       errs() << Argv0 << ": ";
       WithColor::error() << " parsing member '" << ChildName
                          << "' of archive library failed'" << ArchiveName
                          << "'\n";
       return nullptr;
     }
     if (Verbose)
       errs() << "Linking member '" << ChildName << "' of archive library.\n";
-    if (Linker::linkModules(*Result, std::move(M)))
+    if (L.linkInModule(std::move(M)))
       return nullptr;
   } // end for each child
   ExitOnErr(std::move(Err));
   return Result;
 }
 
 namespace {