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

[WANG.Jiong]

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