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

[Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>]

>> 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.

> 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
>