Add _GLOBAL_OFFSET_TABLE_ symbol to shared libraries

[davide]

Summary

This fixes libcxxabi bootstrap on FreeBSD for me. Ed, can you please confirm? Thanks.

[emaste]

This does fix libcxxabi build / testrun for me.

Note that linking with gnu ld leaves a local absolute symbol for _GLOBAL_OFFSET_TABLE_ in libc++abi.so:

% nm build-nodebug/lib/libc++abi.so | grep 'GLOBAL_OFFSET'      
00000000002592e0 a _GLOBAL_OFFSET_TABLE_

While with this patch lld produces none:

% nm build-lld-selfhost2/lib/libc++abi.so | grep 'GLOBAL_OFFSET'
%

but I don't know that it matters.

[emaste]

For reference, http://llvm.org/pr26705

[ruiu]

Whether we should submit this or not depends on whether it will fix the issue or not, but in case if we do.

ELF/Driver.cpp

345–351

Reorder the conditions in if so that the most important stuff comes first.

// Add default entry symbol. Note that AMDGPU executables has no entry.
if (Config->Entry.empty() && !Config->Shared && !Config->Relocatable &&
    Config->EMachine != EM_AMDGPU)
  Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";

[emaste]

Whether we should submit this or not depends on whether it will fix the issue or not

It does fix the issue for me.

(I had one failure in a lld-with-lld test at first, because llvm-dis didn't get built as a dependency. In check-libcxxabi for me two libunwind-related failures and one other unrelated failure remain with this patch.)

[• rafael]

Testcase? :-)

[davide]

Address Rui's comments, add a small testcase.

[• rafael]

ELF/Driver.cpp

351

You don't need the '('

test/ELF/global_offset_table_shared.s

2

You are not checking that the symbol is not undefined, so this should pass even with your patch reverted.

[davide]

Rafael LGMT'd this in person. Submitting.

ELF/Driver.cpp

 
   Config->Rela = ELFT::Is64Bits;
 
-  if (!Config->Shared && !Config->Relocatable) {
+  if (Config->Entry.empty() && !Config->Shared && !Config->Relocatable &&
+      Config->EMachine != EM_AMDGPU)
     // Add entry symbol.
     //
     // There is no entry symbol for AMDGPU binaries, so skip adding one to avoid
     // having and undefined symbol.
-    if (Config->Entry.empty() && Config->EMachine != EM_AMDGPU)
-      Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
+    Config->Entry = (Config->EMachine == EM_MIPS) ? "__start" : "_start";
 
+  if (!Config->Relocatable)
     // In the assembly for 32 bit x86 the _GLOBAL_OFFSET_TABLE_ symbol
     // is magical and is used to produce a R_386_GOTPC relocation.
     // The R_386_GOTPC relocation value doesn't actually depend on the

     // Given that the symbol is effectively unused, we just create a dummy
     // hidden one to avoid the undefined symbol error.
     Symtab.addIgnored("_GLOBAL_OFFSET_TABLE_");
-  }
 
   if (!Config->Entry.empty()) {
     // Set either EntryAddr (if S is a number) or EntrySym (otherwise).

test/ELF/global_offset_table_shared.s

+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t
+// RUN: ld.lld -shared %t -o %t2
+.long _GLOBAL_OFFSET_TABLE_