From: Sriraman Tallam <tmsriram@google.com>
To: ramrad01@arm.com
Cc: Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
Jan Hubicka <hubicka@ucw.cz>, "H.J. Lu" <hjl.tools@gmail.com>,
Pedro Alves <palves@redhat.com>, Michael Matz <matz@suse.de>,
David Li <davidxl@google.com>,
GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt=
Date: Mon, 01 Jun 2015 18:55:00 -0000 [thread overview]
Message-ID: <CAAs8Hmy6gFfoKrVWJ2tCWQZ6Lkez2DCikWozVgwFcY-peDtOhw@mail.gmail.com> (raw)
In-Reply-To: <CAJA7tRbHAHi7i1xboTZadrJLE_Ry628pwLot6f3wdK4KzawqCQ@mail.gmail.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
>>>>
>>>>
>>>
next prev parent reply other threads:[~2015-06-01 18:55 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-10 15:19 H.J. Lu
[not found] ` <CAAs8HmwWSDY+KjKcB4W=TiYV0Pz7NSvfL_8igp+hPT-LU1utTg@mail.gmail.com>
2015-05-21 21:31 ` Sriraman Tallam
2015-05-21 21:39 ` Sriraman Tallam
2015-05-21 22:02 ` Pedro Alves
2015-05-21 22:02 ` Jakub Jelinek
2015-05-22 1:47 ` H.J. Lu
2015-05-22 3:38 ` Xinliang David Li
2015-05-21 22:34 ` Sriraman Tallam
2015-05-22 9:22 ` Pedro Alves
2015-05-22 15:13 ` Sriraman Tallam
2015-05-28 18:53 ` Sriraman Tallam
2015-05-28 19:05 ` H.J. Lu
2015-05-28 19:48 ` Sriraman Tallam
2015-05-28 20:19 ` H.J. Lu
2015-05-28 21:27 ` Sriraman Tallam
2015-05-28 21:31 ` H.J. Lu
2015-05-28 21:52 ` Sriraman Tallam
2015-05-28 22:48 ` H.J. Lu
2015-05-29 3:51 ` Sriraman Tallam
2015-05-29 5:13 ` H.J. Lu
2015-05-29 7:13 ` Sriraman Tallam
2015-05-29 17:36 ` Sriraman Tallam
2015-05-29 17:52 ` H.J. Lu
2015-05-29 18:33 ` Sriraman Tallam
2015-05-29 20:50 ` Jan Hubicka
2015-05-29 22:56 ` Sriraman Tallam
2015-05-29 23:08 ` Sriraman Tallam
[not found] ` <CAJA7tRYsMiq7rx34c=z6KwRdwYxxaeP6Z6qzA4XEwnJSMT7z=Q@mail.gmail.com>
2015-05-30 4:44 ` Sriraman Tallam
2015-06-01 8:24 ` Ramana Radhakrishnan
2015-06-01 18:01 ` Sriraman Tallam
2015-06-01 18:41 ` Ramana Radhakrishnan
2015-06-01 18:55 ` Sriraman Tallam [this message]
2015-06-01 20:33 ` Ramana Radhakrishnan
2015-06-02 18:27 ` Sriraman Tallam
2015-06-02 19:59 ` Bernhard Reutner-Fischer
2015-06-02 20:09 ` Sriraman Tallam
2015-06-02 21:18 ` Bernhard Reutner-Fischer
2015-06-02 21:09 ` Ramana Radhakrishnan
2015-06-02 21:25 ` Xinliang David Li
2015-06-02 21:52 ` Bernhard Reutner-Fischer
2015-06-02 21:40 ` Sriraman Tallam
2015-06-03 14:37 ` Ramana Radhakrishnan
2015-06-03 18:53 ` Sriraman Tallam
2015-06-03 20:16 ` Richard Henderson
2015-06-03 20:59 ` Sriraman Tallam
2015-06-04 16:56 ` Sriraman Tallam
2015-06-04 17:30 ` Richard Henderson
2015-06-04 21:34 ` Sriraman Tallam
2015-07-24 19:02 ` H.J. Lu
2015-06-03 19:57 ` Richard Henderson
-- strict thread matches above, loose matches on Subject: below --
2015-05-01 0:31 Sriraman Tallam
2015-05-01 3:21 ` Alan Modra
2015-05-01 3:26 ` Sriraman Tallam
2015-05-01 15:01 ` Andi Kleen
2015-05-01 16:19 ` Xinliang David Li
2015-05-01 16:23 ` H.J. Lu
2015-05-01 16:26 ` Xinliang David Li
2015-05-01 18:06 ` Sriraman Tallam
2015-05-02 12:12 ` Andi Kleen
2015-05-01 17:50 ` Sriraman Tallam
2015-05-04 14:45 ` Michael Matz
2015-05-04 16:43 ` Xinliang David Li
2015-05-04 16:58 ` Michael Matz
2015-05-04 17:22 ` Xinliang David Li
2015-05-09 16:35 ` H.J. Lu
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