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

[Ian Lance Taylor <iant at google dot com>]

On Mon, Aug 13, 2012 at 9:53 PM, WANG.Jiong <wong.kwongyuan@gmail.com> wrote:
> 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.

I see.  So you have a relocation type that refers to a symbol with a
magic name.  That seems like a bad design to me.  Can you change it?

If you can't change it, there is a phase ordering problem.  Gold reads
all the symbol tables, including fetching objects out of archives,
before it does the relocation processing.  You will only know that you
need the symbol when you are doing relocation processing.  At that
time, it is too late.

As far as I can see, you will have to make the symbol always be
undefined, as though the linker were always invoke with -u
__tls_get_addr.  That will cause the object defining the symbol to
always be brought into the link, but I don't see how to avoid that.
You just need to call symtab->add_undefined_symbol_from_command_line,
one way or another, and you need to do it early in the link, e.g.,
when your Target is constructed.

Ian