Improve error check for an empty archive.

[ruiu]

Summary

Previously, it warned on any archive file that has no symbol.
With this patch, it warns on such archive file that contains only
LLVM bitcode files.

[ruiu]

• Updated as per Rafael's comment. This patch needs another patch to LLVM to define Archive::isEmpty which is just this.
  
  bool Archive::isEmpty() const { return Data.getBufferSize() == 8; }

[• rafael]

LGTM

lld/trunk/ELF/InputFiles.cpp

if (Sec == &InputSection<ELFT>::Discarded)
           ->body();
     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");
 
+  // Checks for a common usage error of an ar command.
+  if (File->getNumberOfSymbols() == 0 && !File->isEmpty())
+    warn(getName() + " has no symbol."
+         " Chances are you are doing an LTO build and forgot to use an ar"
+         " command that can create a symbol table for LLVM bitcode files."
+         " If so, use llvm-ar or GNU ar + plugin.");
+
   // Read the symbol table to construct Lazy objects.
-  bool IsEmpty = true;
-  for (const Archive::Symbol &Sym : File->symbols()) {
+  for (const Archive::Symbol &Sym : File->symbols())
     Symtab<ELFT>::X->addLazyArchive(this, Sym);
-    IsEmpty = false;
-  }
-
-  if (IsEmpty)
-    warn(getName() + " has no symbol. Chances are you are doing "
-         "an LTO build and forgot to use an ar command that can create "
-         "a symbol table for LLVM bitcode files. If so, use llvm-ar or "
-         "GNU ar + plugin.");
 }
 
 // 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());
 

lld/trunk/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 2>&1 | FileCheck %s
-
-// CHECK: has no symbol.
+// RUN: ld.lld -shared %t.o %t.a -o t

lld/trunk/test/ELF/lto/archive.ll

 ; REQUIRES: x86
 ; RUN: llvm-as %S/Inputs/archive.ll -o %t1.o
 ; RUN: rm -f %t.a
 ; RUN: llvm-ar rcs %t.a %t1.o
 ; RUN: llvm-as %s -o %t2.o
 ; RUN: ld.lld -m elf_x86_64 %t2.o %t.a -o %t3 -shared
 ; RUN: llvm-readobj -t %t3 | FileCheck %s
 ; RUN: ld.lld -m elf_x86_64 %t2.o --whole-archive %t.a -o %t3 -shared
 ; RUN: llvm-readobj -t %t3 | FileCheck %s
 
+; RUN: llvm-ar rcS %t4.a %t1.o
+; RUN: ld.lld -m elf_x86_64 %t2.o %t4.a -o %t3 -shared 2>&1 \
+; RUN:   | FileCheck -check-prefix=WARN %s
 
 ; CHECK:      Name: g (
 ; CHECK-NEXT: Value:
 ; CHECK-NEXT: Size:
 ; CHECK-NEXT: Binding: Global
 ; CHECK-NEXT: Type: Function
 ; CHECK-NEXT: Other: 0
 ; CHECK-NEXT: Section: .text
 
 ; CHECK:      Name: f (
 ; CHECK-NEXT: Value:
 ; CHECK-NEXT: Size:
 ; CHECK-NEXT: Binding: Global
 ; CHECK-NEXT: Type: Function
 ; CHECK-NEXT: Other: 0
 ; CHECK-NEXT: Section: .text
 
+; WARN: has no symbol.
+
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 define void @g() {
   call void @f()
   ret void
 }
 
 declare void @f()