From: "H.J. Lu" <hjl.tools@gmail.com>
To: Uros Bizjak <ubizjak@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: Fri, 27 Feb 2015 23:46:00 -0000 [thread overview]
Message-ID: <CAMe9rOqHeNfS_JVy89bhb-Dv8oViWHxrABWkYOA_+1YbJmc4wA@mail.gmail.com> (raw)
In-Reply-To: <CAMe9rOq5CKSTLYhVuC_Q780nVX-5FDXLusKJ9-cdVnBq4gGC+w@mail.gmail.com>
On Fri, Feb 27, 2015 at 3:23 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Dec 4, 2014 at 8:46 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Thu, Dec 4, 2014 at 4:44 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
>>> On Wed, Dec 3, 2014 at 10:35 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.
>
> This caused:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65248
>
> Should we turn it off by default?
>
Or we can provide a command line option to turn it off.
--
H.J.
next prev parent reply other threads:[~2015-02-27 23:26 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
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 [this message]
-- 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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