From: Uros Bizjak <ubizjak@gmail.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,
Sriraman Tallam <tmsriram@google.com>,
Jakub Jelinek <jakub@redhat.com>
Subject: Re: [PATCH x86_64] Optimize access to globals in "-fpie -pie" builds with copy relocations
Date: Thu, 04 Dec 2014 19:32:00 -0000 [thread overview]
Message-ID: <CAFULd4Ztjg_h0z3=g3nHVoyj6L5R9TcLxBAaKFqFnfbqXF3muQ@mail.gmail.com> (raw)
In-Reply-To: <CAMe9rOpkZZmuhL_rqYrAE6wx6u7QYWe-DjuKp11u5wPh1dGztA@mail.gmail.com>
On Thu, Dec 4, 2014 at 5:46 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>>> It would probably help reviewers if you pointed to actual path
>>>>>>>> submission [1], which unfortunately contains the explanation in the
>>>>>>>> patch itself [2], which further explains that this functionality is
>>>>>>>> currently only supported with gold, patched with [3].
>>>>>>>>
>>>>>>>> [1] https://gcc.gnu.org/ml/gcc-patches/2014-09/msg00645.html
>>>>>>>> [2] https://gcc.gnu.org/ml/gcc-patches/2014-09/txt2CHtu81P1O.txt
>>>>>>>> [3] https://sourceware.org/ml/binutils/2014-05/msg00092.html
>>>>>>>>
>>>>>>>> After a bit of the above detective work, I think that new gcc option
>>>>>>>> is not necessary. The configure should detect if new functionality is
>>>>>>>> supported in the linker, and auto-configure gcc to use it when
>>>>>>>> appropriate.
>>>>>>>
>>>>>>> I think GCC option is needed since one can use -fuse-ld= to
>>>>>>> change linker.
>>>>>>
>>>>>> IMO, nobody will use this highly special x86_64-only option. It would
>>>>>> be best for gnu-ld to reach feature parity with gold as far as this
>>>>>> functionality is concerned. In this case, the optimization would be
>>>>>> auto-configured, and would fire automatically, without any user
>>>>>> intervention.
>>>>>>
>>>>>
>>>>> Let's do it. I implemented the same feature in bfd linker on both
>>>>> master and 2.25 branch.
>>>>>
>>>>
>>>> +bool
>>>> +i386_binds_local_p (const_tree exp)
>>>> +{
>>>> + /* Globals marked extern are treated as local when linker copy relocations
>>>> + support is available with -f{pie|PIE}. */
>>>> + if (TARGET_64BIT && ix86_copyrelocs && flag_pie
>>>> + && TREE_CODE (exp) == VAR_DECL
>>>> + && DECL_EXTERNAL (exp) && !DECL_WEAK (exp))
>>>> + return true;
>>>> + return default_binds_local_p (exp);
>>>> +}
>>>> +
>>>>
>>>> It returns true with -fPIE and false without -fPIE. It is lying to compiler.
>>>> Maybe legitimate_pic_address_disp_p is a better place.
>>
>> Agreed.
>>
>>> Something like this?
>>
>> Yes.
>>
>> OK, if Jakub doesn't have any objections here. Please also add
>> Sriraman as author to ChangeLog entry.
>>
>> Thanks,
>> Uros.
>
> Here is the patch. OK to install?
>
> Thanks.
>
> --
> H.J.
> ---
> Normally, with -fPIE/-fpie, GCC accesses globals that are extern to the
> module using the GOT. This is two instructions, one to get the address
> of the global from the GOT and the other to get the value. If it turns
> out that the global gets defined in the executable at link-time, it still
> needs to go through the GOT as it is too late then to generate a direct
> access.
>
> Examples:
>
> foo.cc
> ------
> int a_glob;
> int main () {
> return a_glob; // defined in this file
> }
>
> With -O2 -fpie -pie, the generated code directly accesses the global via
> PC-relative insn:
>
> 5e0 <main>:
> mov 0x165a(%rip),%eax # 1c40 <a_glob>
>
> foo.cc
> ------
>
> extern int a_glob;
> int main () {
> return a_glob; // defined in this file
> }
>
> With -O2 -fpie -pie, the generated code accesses global via GOT using
> two memory loads:
>
> 6f0 <main>:
> mov 0x1609(%rip),%rax # 1d00 <_DYNAMIC+0x230>
> mov (%rax),%eax
>
> This is true even if in the latter case the global was defined in the
> executable through a different file.
>
> Some experiments on google benchmarks shows that the extra memory loads
> affects performance by 1% to 5%.
>
> Solution - Copy Relocations:
>
> When the linker supports copy relocations, GCC can always assume that
> the global will be defined in the executable. For globals that are truly
> extern (come from shared objects), the linker will create copy relocations
> and have them defined in the executable. Result is that no global access
> needs to go through the GOT and hence improves performance.
>
> This optimization only applies to undefined, non-weak global data.
> Undefined, weak global data access still must go through the GOT.
>
> This patch checks if linker supports PIE with copy reloc, which is
> enabled in gold and bfd linker in bininutils 2.25, at configure time
> and enables this optimization if the linker support is available.
>
> gcc/
>
> * configure.ac (HAVE_LD_PIE_COPYRELOC): Defined to 1 if
> Linux/x86-64 linker supports PIE with copy reloc.
> * config.in: Regenerated.
> * configure: Likewise.
>
> * config/i386/i386.c (legitimate_pic_address_disp_p): Allow
> pc-relative address for undefined, non-weak, non-function
> symbol reference in 64-bit PIE if linker supports PIE with
> copy reloc.
>
> * doc/sourcebuild.texi: Document pie_copyreloc target.
>
> gcc/testsuite/
>
> * gcc.target/i386/pie-copyrelocs-1.c: New test.
> * gcc.target/i386/pie-copyrelocs-2.c: Likewise.
> * gcc.target/i386/pie-copyrelocs-3.c: Likewise.
> * gcc.target/i386/pie-copyrelocs-4.c: Likewise.
>
> * lib/target-supports.exp (check_effective_target_pie_copyreloc):
> New procedure.
OK.
Thanks,
Uros.
next prev parent reply other threads:[~2014-12-04 19:32 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-12-02 19:19 Uros Bizjak
2014-12-02 19:39 ` H.J. Lu
2014-12-02 19:40 ` H.J. Lu
2014-12-02 20:01 ` Uros Bizjak
2014-12-02 20:43 ` H.J. Lu
2014-12-02 20:19 ` Jakub Jelinek
2014-12-02 22:14 ` H.J. Lu
2014-12-02 23:21 ` H.J. Lu
2014-12-03 13:47 ` H.J. Lu
2014-12-03 15:01 ` H.J. Lu
2014-12-03 21:35 ` H.J. Lu
2014-12-04 12:44 ` Uros Bizjak
2014-12-04 16:46 ` H.J. Lu
2014-12-04 19:32 ` Uros Bizjak [this message]
2015-02-03 19:25 ` Sriraman Tallam
2015-02-03 19:26 ` Sriraman Tallam
2015-02-03 19:36 ` Jakub Jelinek
2015-02-03 21:20 ` Sriraman Tallam
2015-02-03 21:29 ` H.J. Lu
2015-02-03 21:36 ` Sriraman Tallam
2015-02-03 22:03 ` H.J. Lu
2015-02-03 22:19 ` Jakub Jelinek
2015-02-04 1:16 ` H.J. Lu
2015-02-04 18:27 ` Sriraman Tallam
2015-02-04 18:31 ` Jakub Jelinek
2015-02-04 18:38 ` H.J. Lu
2015-02-04 18:42 ` Jakub Jelinek
2015-02-04 18:45 ` H.J. Lu
2015-02-04 18:51 ` Sriraman Tallam
2015-02-04 18:57 ` H.J. Lu
2015-02-04 21:53 ` Sriraman Tallam
2015-02-04 22:37 ` H.J. Lu
2015-02-04 22:47 ` Bernhard Reutner-Fischer
2015-02-04 23:10 ` H.J. Lu
2015-02-04 23:29 ` H.J. Lu
2015-02-05 16:57 ` Bernhard Reutner-Fischer
2015-02-05 18:54 ` Richard Henderson
2015-02-05 19:01 ` H.J. Lu
2015-02-05 19:59 ` Richard Henderson
2015-02-05 22:05 ` Sriraman Tallam
2015-02-05 22:47 ` H.J. Lu
2015-02-05 22:48 ` Sriraman Tallam
2015-02-06 16:25 ` H.J. Lu
2015-02-27 23:39 ` H.J. Lu
2015-02-27 23:46 ` H.J. Lu
-- strict thread matches above, loose matches on Subject: below --
2014-12-04 22:19 Dominique Dhumieres
2014-12-04 23:54 ` H.J. Lu
2014-05-15 18:34 Sriraman Tallam
2014-05-19 18:11 ` Sriraman Tallam
2014-06-09 22:55 ` Sriraman Tallam
2014-06-21 0:17 ` Sriraman Tallam
2014-06-26 17:55 ` Sriraman Tallam
2014-07-11 17:42 ` Sriraman Tallam
2014-09-02 18:15 ` Sriraman Tallam
2014-09-02 20:40 ` Richard Henderson
2014-09-03 7:25 ` Bernhard Reutner-Fischer
2014-09-08 22:19 ` Sriraman Tallam
2014-09-19 21:11 ` Sriraman Tallam
2014-09-29 17:57 ` Sriraman Tallam
2014-10-06 20:43 ` Sriraman Tallam
2014-11-10 23:35 ` Sriraman Tallam
2014-12-02 18:01 ` Sriraman Tallam
2014-12-02 19:06 ` H.J. Lu
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