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 11:41 AM, Ramana Radhakrishnan
<ramana.gcc@googlemail.com> wrote:
> 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 ?

We can and and please see this thread, that is the exact patch I proposed :
https://gcc.gnu.org/ml/gcc-patches/2015-05/msg02682.html

However, there was one caveat.  I want this working without -fPIC too.
non-PIC code also generates PLT calls and I want them eliminated.

>
> 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.

You are right.  This is the way I wanted it too but I also wanted the
attribute to work without PIC. PLT calls are generated without -fPIC
and -fPIE too and I wanted a solution for that.  On looking at the
code in more detail,

* -fno-plt is made to work with -fPIC, is there a reason to not make
it work for non-PIC code?  I can remove the flag_pic check from
calls.c
* Then, I add the generic attribute "noplt" and everything is fine.

There is just one caveat with the above approach, for x86_64
(*call_insn) will not generate indirect-calls for *non-PIC* code
because constant_call_address_operand in predicates.md will evaluate
to false.  This can be fixed appropriately in ix86_output_call_insn in
i386.c.


Is this alright?  Sorry for the confusion, but the primary reason why
I did not do it the way you suggested is because we wanted "noplt"
attribute to work for non-PIC code also.

Thanks
Sri

>
> 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
>>>>
>>>>
>>>