Use a got to access a hidden weak undefined on MachO

[• espindola]

Summary

Trying to link

attribute((weak, visibility("hidden"))) extern int foo;
int *main(void) {

return &foo;

}

on OS X fails with

final section layout:

__TEXT/__text addr=0x100000FAB, size=0x0000000D, fileOffset=0x00000FAB, type=1
__TEXT/__unwind_info addr=0x100000FB8, size=0x00000048, fileOffset=0x00000FB8, type=22

ld: 32-bit RIP relative reference out of range (-4294971318 max is +/-2GB): from _main (0x100000FAB) to _foo@0x00001000 (0x00000000) in '_main' from test.o for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

The problem being that 0 cannot be computed as a fixed difference from %rip. Exactly the same issue exists on ELF and we can use the same solution.

[rnk]

lgtm, makes sense.

[• espindola]

322739

llvm/lib/Target/TargetMachine.cpp

bool TargetMachine::shouldAssumeDSOLocal(const Module &M,
   // without GOT tables in older clang versions; Keep this behaviour.
   if (TT.isOSBinFormatCOFF() || (TT.isOSWindows() && TT.isOSBinFormatMachO()))
     return true;
 
   // Most PIC code sequences that assume that a symbol is local cannot
   // produce a 0 if it turns out the symbol is undefined. While this
   // is ABI and relocation depended, it seems worth it to handle it
   // here.
-  // FIXME: this is probably not ELF specific.
-  if (GV && isPositionIndependent() && TT.isOSBinFormatELF() &&
-      GV->hasExternalWeakLinkage())
+  if (GV && isPositionIndependent() && GV->hasExternalWeakLinkage())
     return false;
 
   if (GV && !GV->hasDefaultVisibility())
     return true;
 
   if (TT.isOSBinFormatMachO()) {
     if (RM == Reloc::Static)
       return true;

llvm/test/CodeGen/X86/hidden-vis-3.ll

 ; RUN: llc < %s -mtriple=i386-apple-darwin9   | FileCheck %s -check-prefix=X32
 ; RUN: llc < %s -mtriple=x86_64-apple-darwin9 | FileCheck %s -check-prefix=X64
 
 @x = external hidden global i32   ; <i32*> [#uses=1]
 @y = extern_weak hidden global i32  ; <i32*> [#uses=1]
 
 define i32 @t() nounwind readonly {
 entry:
 ; X32: _t:
 ; X32: movl _y, %eax
 
 ; X64: _t:
-; X64: movl _y(%rip), %eax
+; X64: movq _y@GOTPCREL(%rip), %rax
 
   %0 = load i32, i32* @x, align 4    ; <i32> [#uses=1]
   %1 = load i32, i32* @y, align 4    ; <i32> [#uses=1]
   %2 = add i32 %1, %0    ; <i32> [#uses=1]
   ret i32 %2
 }