Warn on empty archive files.

[ruiu]

Summary

Warn on empty archive files.

[ruiu]

• Fix the error message.

[davide]

ELF/InputFiles.cpp

437–438

This is not entirely correct. In fact, you can use ar with a plugin and it will work just fine. Can you rephrase that in a different way? (e.g. you need to have a version of the archives which can create a bitcode armap?)

[ruiu]

• Updated the message.

[davide]

lgtm

[davide]

If this gets too spammy for any reason, we can still disable it.

[grimar]

In D25044#555989, @davide wrote:

If this gets too spammy for any reason, we can still disable it.

We may not disable, but log() it instead if warning().

ELF/InputFiles.cpp

case STB_GNU_UNIQUE:
     return elf::Symtab<ELFT>::X->addRegular(Name, *Sym, Sec)->body();
   }
 }
 
 template <class ELFT> void ArchiveFile::parse() {
   File = check(Archive::create(MB), "failed to parse archive");
 
   // Read the symbol table to construct Lazy objects.
-  for (const Archive::Symbol &Sym : File->symbols())
+  bool IsEmpty = true;
+  for (const Archive::Symbol &Sym : File->symbols()) {
     Symtab<ELFT>::X->addLazyArchive(this, Sym);
+    IsEmpty = false;
+  }
+
+  if (IsEmpty)
+    warning(getName() + " has no symbol. If you are doing an LTO build, "
+            "make sure you are using llvm-ar instead of ar.");
 }
 
 // Returns a buffer pointing to a member file containing a given symbol.
 MemoryBufferRef ArchiveFile::getMember(const Archive::Symbol *Sym) {
   Archive::Child C =
       check(Sym->getMember(),
             "could not get the member for symbol " + Sym->getName());
 

test/ELF/empty-archive.s

 // RUN: llvm-ar rc %t.a
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-// RUN: ld.lld -shared %t.o %t.a -o t
+// RUN: ld.lld -shared %t.o %t.a -o t 2>&1 | FileCheck %s
+
+// CHECK: has no symbol. If you are doing an LTO build, make sure you are using llvm-ar instead of ar.