[ELF] - Allow producing -r output if only empty archive is given.

[grimar]

Summary

This is used by linux kernel build system.

(https://www.kernel.org/doc/Documentation/kbuild/makefiles.txt "3.2 Built-in object goals")

It has for example next configuration for linking built-in.o files:

drivers-y	:= $(patsubst %/, %/built-in.o, $(drivers-y))

drivers-$(CONFIG_PCI)		+= arch/ia64/pci/
drivers-$(CONFIG_IA64_HP_SIM)	+= arch/ia64/hp/sim/
drivers-$(CONFIG_IA64_HP_ZX1)	+= arch/ia64/hp/common/ arch/ia64/hp/zx1/
drivers-$(CONFIG_IA64_HP_ZX1_SWIOTLB) += arch/ia64/hp/common/ arch/ia64/hp/zx1/
drivers-$(CONFIG_IA64_GENERIC)	+= arch/ia64/hp/common/ arch/ia64/hp/zx1/ arch/ia64/hp/sim/ arch/ia64/sn/ arch/ia64/uv/
drivers-$(CONFIG_OPROFILE)	+= arch/ia64/oprofile/

Im most simple case all CONFIG_* options are off. That means linker is called with empty input archive, emulation option and no inputs
and expected to generate some relocatable output. ld.bfd is able to do that, we - dont, because execution falls into following code
and Files remains empty:

// If an archive file has no symbol table, it is likely that a user
// is attempting LTO and using a default ar command that doesn't
// understand the LLVM bitcode file. It is a pretty common error, so
// we'll handle it as if it had a symbol table.
if (!File->hasSymbolTable()) {
  for (const auto &P : getArchiveMembers(MBRef))
    Files.push_back(make<LazyObjectFile>(P.first, Path, P.second));
  return;
}

I suggest as a simple solution just to allow -r with no inputs to allow producing empty relocatable output here.

[ruiu]

ELF/Driver.cpp

188

You should check for !File->isEmpty() && !File->hasSymbolTable() here instead of making an non-obvious change to LinkerDriver::createFiles.

test/ELF/Inputs/relocatable-empty-archive.s

1 ↗

(On Diff #101551)

This is not an .s file but .a. But you want to avoid checking in an .a file in general. Please create an empty file in the test file using the ar command.

test/ELF/relocatable-empty-archive.s

2

Does this actually create a correct object file? Verify it using objdump.

[grimar]

• Addressed review comments.

ELF/Driver.cpp

188

Thanks. Done.

[grimar]

Planning changes according to Rafael's comments.

[grimar]

• Updated.

[ruiu]

LGTM

ELF/Driver.cpp

case file_magic::archive: {
 
     std::unique_ptr<Archive> File =
         check(Archive::create(MBRef), Path + ": failed to parse archive");
 
     // If an archive file has no symbol table, it is likely that a user
     // is attempting LTO and using a default ar command that doesn't
     // understand the LLVM bitcode file. It is a pretty common error, so
     // we'll handle it as if it had a symbol table.
-    if (!File->hasSymbolTable()) {
+    if (!File->isEmpty() && !File->hasSymbolTable()) {
       for (const auto &P : getArchiveMembers(MBRef))
         Files.push_back(make<LazyObjectFile>(P.first, Path, P.second));
       return;
     }
 
     // Handle the regular case.
     Files.push_back(make<ArchiveFile>(std::move(File)));
     return;

test/ELF/relocatable-empty-archive.s

+# REQUIRES: x86
+# RUN: rm %t.a
+# RUN: llvm-ar rc %t.a
+# RUN: ld.lld -m elf_x86_64 %t.a -o %t -r
+# RUN: llvm-readobj -file-headers %t | FileCheck %s
+
+# CHECK: Format: ELF64-x86-64
+# CHECK: Arch: x86_64
+# CHECK: AddressSize: 64bit
+# CHECK: Type: Relocatable