Re: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt=

[Ramana Radhakrishnan <ramana.gcc@googlemail.com>]

On Mon, Jun 1, 2015 at 7:01 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> On Mon, Jun 1, 2015 at 1:24 AM, Ramana Radhakrishnan
> <ramana.radhakrishnan@arm.com> wrote:
>>
>>>> Why isn't it just an indirect call in the cases that would require a GOT
>>>> slot and a direct call otherwise ? I'm trying to work out what's so
>>>> different on each target that mandates this to be in the target backend.
>>>> Also it would be better to push the tests into gcc.dg if you can and
>>>> check
>>>> for the absence of a relocation so that folks at least see these as being
>>>> UNSUPPORTED on their target.
>>>
>>>
>>
>>
>> To be even more explicit, shouldn't this be handled similar to the way in
>> which -fno-plt is handled in a target agnostic manner ? After all, if you
>> can handle this for the command line, doing the same for a function which
>> has been decorated with attribute((noplt)) should be simple.
>
> -fno-plt does not work for non-PIC code, having non-PIC code not use
> PLT was my primary motivation.  Infact, if you go back in this thread,
> I suggested to HJ if I should piggyback on -fno-plt.  I tried using
> the -fno-plt implementation to do this by removing the flag_pic check
> in calls.c, but that does not still work for non-PIC code.

You're missing my point, unless I'm missing something basic here - I
should have been even more explicit and said -fPIC was a given in all
this discussion.

calls.c:229 has

else if (flag_pic && !flag_plt && fndecl_or_type
           && TREE_CODE (fndecl_or_type) == FUNCTION_DECL
           && !targetm.binds_local_p (fndecl_or_type))

why can't we merge the check in here for the attribute noplt ?

If a new attribute is added to the "GNU language" in this case, why
isn't this being treated in the same way as the command line option
has been treated ? All this means is that we add an attribute and a
command line option to common code and then not implement it in a
proper target agnostic fashion.

regards
Ramana


>
>>
>>> I am not familiar with PLT calls for other targets.  I can move the
>>> tests to gcc.dg but what relocation are you suggesting I check for?
>>
>>
>> Move the test to gcc.dg, add a target_support_no_plt function in
>> testsuite/lib/target-supports.exp and mark this as being supported only on
>> x86 and use scan-assembler to scan for PLT relocations for x86. Other
>> targets can add things as they deem fit.
>
>>
>> In any case, on a large number of elf/ linux targets I would have thought
>> the absence of a JMP_SLOT relocation would be good enough to check that this
>> is working correctly.
>>
>> regards
>> Ramana
>>
>>
>>
>>
>>>
>>> Thanks
>>> Sri
>>>
>>>
>>>>
>>>>
>>>>
>>>> Ramana
>>>>>
>>>>>
>>>>>>
>>>>>> Also I think the PLT calls have EBX in call fusage wich is added by
>>>>>> ix86_expand_call.
>>>>>>    else
>>>>>>      {
>>>>>>        /* Static functions and indirect calls don't need the pic
>>>>>> register.  */
>>>>>>        if (flag_pic
>>>>>>            && (!TARGET_64BIT
>>>>>>                || (ix86_cmodel == CM_LARGE_PIC
>>>>>>                    && DEFAULT_ABI != MS_ABI))
>>>>>>            && GET_CODE (XEXP (fnaddr, 0)) == SYMBOL_REF
>>>>>>            && ! SYMBOL_REF_LOCAL_P (XEXP (fnaddr, 0)))
>>>>>>          {
>>>>>>            use_reg (&use, gen_rtx_REG (Pmode,
>>>>>> REAL_PIC_OFFSET_TABLE_REGNUM));
>>>>>>            if (ix86_use_pseudo_pic_reg ())
>>>>>>              emit_move_insn (gen_rtx_REG (Pmode,
>>>>>> REAL_PIC_OFFSET_TABLE_REGNUM),
>>>>>>                              pic_offset_table_rtx);
>>>>>>          }
>>>>>>
>>>>>> I think you want to take that away from FUSAGE there just like we do
>>>>>> for
>>>>>> local calls
>>>>>> (and in fact the code should already check flag_pic && flag_plt I
>>>>>> suppose.
>>>>>
>>>>>
>>>>> Done that now and patch attached.
>>>>>
>>>>> Thanks
>>>>> Sri
>>>>>
>>>>>>
>>>>>> Honza
>>>
>>>
>>