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

[Sriraman Tallam <tmsriram at google dot com>]

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.

>
>> 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
>>
>>
>