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 ↗

(On Diff #50680)

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 ↗

(On Diff #50751)

You don't need the '('

test/ELF/global_offset_table_shared.s

2 ↗

(On Diff #50751)

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.

lld/trunk/ELF/Driver.cpp

template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
   initSymbols<ELFT>();
 
   SymbolTable<ELFT> Symtab;
   std::unique_ptr<TargetInfo> TI(createTarget());
   Target = TI.get();
 
   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
     // symbol value, so it could use an index of STN_UNDEF which, according
     // to the spec, means the symbol value is 0.
     // Unfortunately both gas and MC keep the _GLOBAL_OFFSET_TABLE_ symbol in
     // the object file.
     // The situation is even stranger on x86_64 where the assembly doesn't
     // need the magical symbol, but gas still puts _GLOBAL_OFFSET_TABLE_ as
     // an undefined symbol in the .o files.
     // 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).
     StringRef S = Config->Entry;
     if (S.getAsInteger(0, Config->EntryAddr))
       Config->EntrySym = Symtab.addUndefined(S);
   }
 

lld/trunk/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
+// RUN: llvm-readobj -t %t2 | Filecheck %s
+.long _GLOBAL_OFFSET_TABLE_
+
+// CHECK-NOT: Name: _GLOBAL_OFFSET_TABLE_