Re: MIPS stubs in Gold

[Doug Kwan (éæå) <dougkwan at google dot com>]

I looked at the first e-mail again.  It seems that you can allocate
the PIC stubs before computing the final addresses.  That should be
simple than the ARM case, there is no need for relaxation.  In PIC
prologue is a bit tricky without -ffunction section, like Ian said.
You can look at the Arm_input_section to see how I insert the stub
table.  I image you can do something similar to MIPS text sections
that need to be converted.

-Doug

On Fri, Jun 24, 2011 at 4:17 PM, Doug Kwan (Ãö®¶¼w) <dougkwan@google.com> wrote:
> In ARM, this is done in a relaxing pass.  We don't know if we need a
> stub until the addresses are finalized but adding stub also changes
> addresses.  So this is done in iteratively no address is change.  The
> main hook is Target::do_relax.  The main function to look at is
> Target_arm::do_relax.
>
> I am not very familiar with the MIPS architecture. When I looked at
> the 32-bit ISA, it seems to me that the j and jal instructions cannot
> reach the whole 4G address range.  I suspect you might want to do what
> I did for ARM.
>
> -Doug
>
>
> On Fri, Jun 24, 2011 at 7:35 AM, Aleksandar Simeonov
> <Aleksandar.Simeonov@rt-rk.com> wrote:
>> Hi gain,
>> After some time I have to bother you again. I need some additional
>> details about ARM implementation of stubs and few tips how to implement
>> them for MIPS.
>>
>> Situation is as follows:
>> Some function calls need special handling depend on flags and relocation
>> type. In that case, some additional stub code is needed. There are two
>> types of stubs. One that goes immediately before function and it only
>> contains loading of function address in some register and second that
>> goes on arbitrary place in .text section (usually at the end) and
>> contains loading of function address in some register and jump to that
>> function. Also, relocations that are related to that function call has
>> to be changed in a way to point to that new code. New relocation has to
>> be added to the stub code to fix function address in it. Additional
>> problem for the first type of stub is that is has to be just before
>> function in .text section (it is only used if function is at the
>> beginning of the .text section).
>>
>> I looked at ARM code, also I looked at implementation of plt, but I
>> didn't manage to find what I needed.
>>
>> Can someone give me some hints or ideas what to do and how to proceed
>> with this? Also, which details can help and which can't (there are quite
>> some code in ARM implementation of Gold :-)).
>>
>> Thank you in advance for your help,
>> Aleksandar
>>
>>
>> On 01/06/2011 20:21, Doug Kwan (Ãö®¶¼w) wrote:
>>> The ARM stubs do a number of things
>>>
>>> -extending branch range
>>> -handling transistion between ARM and THUMB with BX and BLX
>>> instructions cannot be used.
>>> -avoid a hardware erratum in involving page straddling long branches
>>> in THUMB2 mode.
>>>
>>> The base class is Stub, from which other stub kinds are derived.  Stub
>>> generation in ARM is a relaxation.  I need to iterate a number of time
>>> because addresses are changed by the inserted stubs.  I hope that this
>>> is not the case in MIPS.  There are hooks for doing relaxation.  Look
>>> for do_relax() in the ARM backend.
>>>
>>> -Doug
>>>
>>> On Wed, Jun 1, 2011 at 8:43 AM, Ian Lance Taylor <iant@google.com> wrote:
>>>> Aleksandar Simeonov <Aleksandar.Simeonov@RT-RK.com> writes:
>>>>
>>>>> Again I have problem that occupy me for some time and for what I can't
>>>>> find solution. Situation is as follows (next text is taken from
>>>>> elfxx-mips.c from ld):
>>>>>
>>>>> "Information about a non-PIC interface to a PIC function.  There are
>>>>>    two ways of creating these interfaces.  The first is to add:
>>>>>
>>>>>       lui     $25,%hi(func)
>>>>>       addiu   $25,$25,%lo(func)
>>>>>
>>>>>    immediately before a PIC function "func".  The second is to add:
>>>>>
>>>>>       lui     $25,%hi(func)
>>>>>       j       func
>>>>>       addiu   $25,$25,%lo(func)
>>>>>
>>>>>    to a separate trampoline section.
>>>>>
>>>>>    Stubs of the first kind go in a new section immediately before the
>>>>>    target function.  Stubs of the second kind go in a single section
>>>>>    pointed to by the hash table's "strampoline" field."
>>>>>
>>>>> I tried to find something similar in Gold for other architectures, but
>>>>> without success. Can you give me some suggestions where to look and what
>>>>> to do to implement this.
>>>>
>>>> The ARM backend implements stubs, e.g., for calls between ARM and Thumb
>>>> code.  Search for "stub" and "Stub".  You probably don't have to do
>>>> anything as complex for the MIPS backend.  At least, I hope not.
>>>>
>>>> Ian
>>>>
>>>
>>
>