Re: [GOLD] How can I add a undefined symbol in target implementation, not by "-u SYM" from command line ?

["WANG.Jiong" <wong dot kwongyuan at gmail dot com>]

On 08/14/2012 12:39 PM, Ian Lance Taylor wrote:
> On Mon, Aug 13, 2012 at 8:30 PM, Jiong WANG <wong.kwongyuan@gmail.com> wrote:
> > 2012/8/13 Ian Lance Taylor <iant@google.com>:
> > > On Mon, Aug 13, 2012 at 3:25 AM, Jiong WANG <wong.kwongyuan@gmail.com> wrote:
> > > >    How can I predefine a undefined symbol in target implementation?
> > > >
> > > >    actually, I am porting gold for tilegx, and our arch's tls
> > > > implementation requires predefiniation of "_tls_get_addr" because the
> > > > assembler generate relocation which use this symbol implicitly.
> > > >
> > > >     I know there are interfaces,  "define_in_output_data/segment" to
> > > > predefined symbol with value, but there is no interface to predefine
> > > > "undefined" symbol ?
> > > >
> > > >     gold linker do support this by command line "-u SYMBOL",  but it's
> > > > a compile time decision not link time.
> > > >
> > > >     currently, I managed to support this by the following ugly code:
> > > >
> > > >      options::parse_set(NULL, "_tls_get_addr",
> > > > (gold::options::String_set*)&parameters->options().undefined());
> > > >
> > > >     which is bad, so, could anyone give me some suggestion on this?
> > >
> > > I assume that the symbol is defined somewhere.  You probably want to
> > > add a do_is_defined_by_abi method to your Target.  See the examples in
> > > existing targets.
> > Hi Ian & all, thanks for your suggestion.
> >
> > I have explored do_is_defined_by_abi, and found it's mostly to avoid
> > warning, but not for creating such a symbol
> >
> > do_is_defined_by_abi has one argument of the type "const Symbol*",  so
> > when it's invoked, that symbol should already existed.
> > ...
> >    basically, I want to create a symbol which is neither against
> > section or segment, just a normally external function symbol.
> I'm sorry, I don't understand what you mean.
>
> If there is no reference to __tls_get_addr in the object files, why do
> you care whether it is defined?
>
> If there is a reference in the object files, then where is it defined?
>
> You seem to be asking how to create an undefined symbol, as though
> used with the -u option.  But that makes no sense.  Why would you want
> that?  The purpose of the -u option is to add a reference to a symbol
> in order to fetch the definition from an archive.
Hi Ian,

  thanks for reply.

  suppose the following code:

  __thread int gd_v = 0x10;

  int cal(int a)
  {
     return a + gd_v;
  }

 tilegx gcc will generate the following relocation:

jal     tls_gd_call(gd_v),   tls_gd_call will actually need linker to 
treat it like plt@__tls_get_addr

while all other arch, arm/mips/x86 etc, will generate relocation against 
__tls_get_addr explictly

so, for tilegx arch,  the symbol "__tls_get_addr" will not exist in the 
.o file, while for other arches, it will.

from my understanding, I need to do the following two thing for tilegx arch:

  1.  when scaned the tls_gd_call relocation, I need to create the 
symbol "__tls_get_addr", and
       make a plt entry for it.
  2.  when apply relation for tls_gd_call, make the jal instruction 
jump to plt entry for "__tls_get_addr"

I found there are  interfaces like define_in_output_data etc which could 
predefine symbol, but It seems
they can not defined a symbol with GLOBAL/UND type which is the type of 
"__tls_get_addr"

this is my problem.

please point out if there are any mistunderstood

thanks

---
Regards,
WANG.Jiong


> Ian