From: Sriraman Tallam <tmsriram@google.com>
To: GCC Patches <gcc-patches@gcc.gnu.org>,
David Li <davidxl@google.com>,
Cary Coutant <ccoutant@google.com>,
Ian Lance Taylor <iant@google.com>,
Paul Pluzhnikov <ppluzhnikov@google.com>
Subject: Re: [PATCH x86_64] Optimize access to globals in "-fpie -pie" builds with copy relocations
Date: Mon, 09 Jun 2014 22:55:00 -0000 [thread overview]
Message-ID: <CAAs8HmxZJ0LdWjXb8u+yRnqjGp4y1C4GUTVQy_S7-tKaCMZVVA@mail.gmail.com> (raw)
In-Reply-To: <CAAs8Hmz8-tQ+BoHvSxpKErf9FG+-F7Mho9hv3FpwxLE0aBGcGQ@mail.gmail.com>
Ping.
On Mon, May 19, 2014 at 11:11 AM, Sriraman Tallam <tmsriram@google.com> wrote:
> Ping.
>
> On Thu, May 15, 2014 at 11:34 AM, Sriraman Tallam <tmsriram@google.com> wrote:
>> Optimize access to globals with -fpie, x86_64 only:
>>
>> Currently, 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 patch to the gold linker :
>> https://sourceware.org/ml/binutils/2014-05/msg00092.html
>> submitted recently allows gold to generate copy relocations for -pie mode when
>> necessary.
>>
>> I have added option -mld-pie-copyrelocs which when combined with -fpie would do
>> this. Note that the BFD linker does not support pie copyrelocs yet and this
>> option cannot be used there.
>>
>> Please review.
>>
>>
>> ChangeLog:
>>
>> * config/i386/i36.opt (mld-pie-copyrelocs): New option.
>> * config/i386/i386.c (legitimate_pic_address_disp_p): Check if this
>> address is still legitimate in the presence of copy relocations
>> and -fpie.
>> * testsuite/gcc.target/i386/ld-pie-copyrelocs-1.c: New test.
>> * testsuite/gcc.target/i386/ld-pie-copyrelocs-2.c: New test.
>>
>>
>>
>> Patch attached.
>> Thanks
>> Sri
next prev parent reply other threads:[~2014-06-09 22:55 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-15 18:34 Sriraman Tallam
2014-05-19 18:11 ` Sriraman Tallam
2014-06-09 22:55 ` Sriraman Tallam [this message]
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
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
2014-12-04 22:19 Dominique Dhumieres
2014-12-04 23:54 ` H.J. Lu
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