This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

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


On Mon, Jun 1, 2015 at 1:33 PM, Ramana Radhakrishnan
<ramana.gcc@googlemail.com> wrote:
> On Mon, Jun 1, 2015 at 7:55 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>> 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.
>
> If you want __attribute__ ((noplt)) to work for non-PIC code, we
> should look to code it in the same place surely by making all
> __attribute__((noplt)) calls, indirect calls irrespective of whether
> it's fpic or not.
>
>
>>>
>>> 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
>
> I don't think that's right, you probably have to allow that along with
> (flag_pic || (decl && attribute_no_plt (decl)) - however it seems odd
> to me that the language extension allows this but the flag doesn't.
>
>> * 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.
>
> Yes, targets need to massage that into place but that's essentially
> the mechanics of retaining indirect calls in each backend. -fno-plt
> doesn't work for ARM / AArch64 with optimizers currently (and I
> suspect on most other targets) because our predicates are too liberal,
> fixed by treating "noplt" or -fno-plt as the equivalent of
> -mlong-calls.
>
>>
>>
>> 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.
>
> If that is the case, then this is a slightly more complicated
> condition in the same place. We then always have indirect calls for
> functions that are marked noplt and just have target generate this
> appropriately.

I have now modified this patch.

This patch does two things:

1) Adds new generic function attribute "no_plt" that is similar in
functionality  to -fno-plt except that it applies only to calls to
functions that are marked  with this attribute.
2) For x86_64, it makes -fno-plt(and the attribute) also work for
non-PIC code by  directly generating an indirect call via a GOT entry.

For PIC code, no_plt merely shadows the implementation of -fno-plt, no
surprises here.

* c-family/c-common.c (no_plt): New attribute.
(handle_no_plt_attribute): New handler.
* calls.c (prepare_call_address): Check for no_plt
attribute.
* config/i386/i386.c (ix86_function_ok_for_sibcall): Check
for no_plt attribute.
(ix86_expand_call):  Ditto.
(nopic_no_plt_attribute): New function.
(ix86_output_call_insn): Output indirect call for non-pic
no plt calls.
* doc/extend.texi (no_plt): Document new attribute.
* testsuite/gcc.target/i386/noplt-1.c: New test.
* testsuite/gcc.target/i386/noplt-2.c: New test.
* testsuite/gcc.target/i386/noplt-3.c: New test.
* testsuite/gcc.target/i386/noplt-4.c: New test.


Please review.

Thanks
Sri


>
> To be honest, this is trivial to implement in the ARM backend as one
> would just piggy back on the longcalls work - despite that, IMNSHO
> it's best done in a target independent manner.
>
> regards
> Ramana
>
>>
>> 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
>>>>>>
>>>>>>
>>>>>

Attachment: noplt_attrib_patch_new.txt
Description: Text document


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]