[cfe-dev] alias attributes in clang on darwin

[Jack Howarth]

On Mon, May 10, 2010 at 09:28:14PM -0700, Chris Lattner wrote:
> 
> On May 10, 2010, at 4:40 PM, Jack Howarth wrote:
> > Clang however compiles this without complaint. Is this an enhancement in clang
> > on the alias attribute handling on darwin or a bug?
> 
> I don't know.

Chris,
   Peter O'Gorman pointed out that Mac OS X prepends an underscore to C symbols 
while whereas many linux systems don't. So, for Apple's gcc-4.2, if I change
the testcase to...

cat 20081222_0.h
int x();

cat 20081222_1.c
#include "20081222_0.h"

/* Actually, call "x" "INT_X", and make it hidden.  */
extern __typeof (x) x
	__asm__ ("_INT_x")
	__attribute__ (( __visibility__ ("hidden")));

int x ()
{
  return 7;
}

/* Make an externally-visible symbol "X" that's an alias for INT_x.  */
extern __typeof (x) EXT_x
	__asm__ ("_x")
	__attribute__ (( weak, __alias__ ("INT_x")));

by prepending "_" for the __asm__(), it also produces the expected warning of...

gcc-4.2 -O0 -c -o c_lto_20081222_1.o 20081222_1.c
20081222_1.c:16: warning: alias definitions not supported in Mach-O; ignored

Interestingly, clang accepts both the original and the modified test
case without complaint. Considering that Mach-O doesn't support alias
definitions this should probably be looked into as a potential bug in
clang on darwin.
                   Jack
ps Unless I am confused, these issues were addressed by Geoff Keating 
with weakref's but it is unclear if a bug-free implementation was ever
achieved...

http://gcc.gnu.org/ml/gcc/2005-12/msg00009.html

Note the comment "I couldn't fully test this patch because weakrefs
still don't seem to be working properly on Darwin".


> 
> > If it is that latter, can this enhancement be backported into llvm-gcc-4.2?
> 
> No, in general, we don't plan to backport any clang enhancements into llvm-gcc.
> 
> -Chris