llvm dev - Dec 2010 - [LLVMdev] __used__ attributes in llvm-gcc's crtstuff.c

Hello,

I'm wondering why only some global static variables are marked with
__used__ attributes in llvm-gcc/gcc/crtstuff.c.

GCC compiles crtstuff.c with -fno-toplevel-reorder option, which ensures
that
unused static globals are not removed during optimization. However, since
LLVM does not support that option, I presume __used__ attribute is used
instead.

For example, __CTOR_LIST__[1] definition is changed as follows:

STATIC func_ptr __CTOR_LIST__[1]
/* LLVM LOCAL begin */
#ifdef __llvm__
  __attribute__ ((__used__, aligned(sizeof(func_ptr))))
#else
  __attribute__ ((__unused__, aligned(sizeof(func_ptr))))
#endif
/* LLVM LOCAL end */
  = { (func_ptr) (-1) };

Note that __unused__ just suppresses the warning that __CTOR_LIST__ is not
used.

My question is that why some global static variables, such as
__frame_dummy_init_array_entry
and __do_global_dtors_aux_fini_array_entry, are not given __used__
attributes.
It seems that llvm-gcc removes these variables and causes .init_array and
.fini_array section
to be empty. Linker complains about this as in "ld: warning: .init_array
section has zero size".
Also, I verified that frame_dummy and __do_global_dtors_aux functions are
not executed.

I have configured llvm-gcc for armv6 target and given
--enable-initfini-array option.

I tried to give __used__ attribute to every global static variable in
crtstuff.c and
it caused no problem for me so far.

Would it be any problem in doing so?

Best regards,
Woongsik Choi
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Hello,
> __used__ attributes in llvm-gcc/gcc/crtstuff.c.
> GCC compiles crtstuff.c with -fno-toplevel-reorder option, which ensures
> that
> unused static globals are not removed during optimization. However, since
> LLVM does not support that option,
The problem is not that LLVM does not support this option. Though,
this is indeed so due to certain reason.
All the code is transformed into IR and transformations are performed
on it. It will be really bad for the
transformations to depend on some magic command line options. Even
more, consider e.g. you tried
to link two IR files - one with the code with -fno-toplevev-reorder
and one - without. What should be done
in such case?
> I presume __used__ attribute is used instead.
This is correct, because this is what in fact the code should do here.
> My question is that why some global static variables, such as
> __frame_dummy_init_array_entry
> and __do_global_dtors_aux_fini_array_entry, are not given __used__
> attributes.
Just bug.
> it caused no problem for me so far.
> Would it be any problem in doing so?
Please prepare the patch and submit to llvm-commit. Thanks!

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

I have submitted the patch in the following llvm-commits article
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20101213/113762.html

Best regards,
Woongsik Choi

2010/12/15 Anton Korobeynikov <anton at korobeynikov.info>
> Hello,
>
> > __used__ attributes in llvm-gcc/gcc/crtstuff.c.
> > GCC compiles crtstuff.c with -fno-toplevel-reorder option, which
ensures
> > that
> > unused static globals are not removed during optimization. However,
since
> > LLVM does not support that option,
> The problem is not that LLVM does not support this option. Though,
> this is indeed so due to certain reason.
> All the code is transformed into IR and transformations are performed
> on it. It will be really bad for the
> transformations to depend on some magic command line options. Even
> more, consider e.g. you tried
> to link two IR files - one with the code with -fno-toplevev-reorder
> and one - without. What should be done
> in such case?
>
> > I presume __used__ attribute is used instead.
> This is correct, because this is what in fact the code should do here.
>
> > My question is that why some global static variables, such as
> > __frame_dummy_init_array_entry
> > and __do_global_dtors_aux_fini_array_entry, are not given __used__
> > attributes.
> Just bug.
>
> > it caused no problem for me so far.
> > Would it be any problem in doing so?
> Please prepare the patch and submit to llvm-commit. Thanks!
>
> --
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University
>
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On 2010-12-14 12:26, Woongsik Choi wrote:
> My question is that why some global static variables, such as
> __frame_dummy_init_array_entry
> and __do_global_dtors_aux_fini_array_entry, are not given __used__
> attributes.
...
> I tried to give __used__ attribute to every global static variable in
> crtstuff.c and
> it caused no problem for me so far.
Note that upstream gcc has already marked more globals in crtstuff.c as
__used__:

http://gcc.gnu.org/viewcvs?view=revision&revision=159228
http://gcc.gnu.org/viewcvs?view=revision&revision=159279