From: Sriraman Tallam <tmsriram@google.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: Jakub Jelinek <jakub@redhat.com>, Uros Bizjak <ubizjak@gmail.com>,
"gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,
David Li <davidxl@google.com>,
Cary Coutant <ccoutant@google.com>
Subject: Re: [PATCH x86_64] Optimize access to globals in "-fpie -pie" builds with copy relocations
Date: Tue, 03 Feb 2015 21:36:00 -0000 [thread overview]
Message-ID: <CAAs8Hmw=Z9VeLz+MAY0X-POfkwXLFEsRFy8wEPDdyzVPV8ee=g@mail.gmail.com> (raw)
In-Reply-To: <CAMe9rOrLwHdtsH2z_7dBpVKiJRtfT7iHaHO7wQ23o25TL7cTMw@mail.gmail.com>
On Tue, Feb 3, 2015 at 1:29 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Feb 3, 2015 at 1:20 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>> On Tue, Feb 3, 2015 at 11:36 AM, Jakub Jelinek <jakub@redhat.com> wrote:
>>> On Tue, Feb 03, 2015 at 11:25:38AM -0800, Sriraman Tallam wrote:
>>>> This was the original patch to i386.c to let global accesses take
>>>> advantage of copy relocations and avoid the GOT.
>>>>
>>>>
>>>> @@ -13113,7 +13113,11 @@ legitimate_pic_address_disp_p (rtx disp)
>>>> return true;
>>>> }
>>>> else if (!SYMBOL_REF_FAR_ADDR_P (op0)
>>>> - && SYMBOL_REF_LOCAL_P (op0)
>>>> + && (SYMBOL_REF_LOCAL_P (op0)
>>>> + || (HAVE_LD_PIE_COPYRELOC
>>>> + && flag_pie
>>>> + && !SYMBOL_REF_WEAK (op0)
>>>> + && !SYMBOL_REF_FUNCTION_P (op0)))
>>>> && ix86_cmodel != CM_LARGE_PIC)
>>>>
>>>> I do not understand here why weak global data access must go through
>>>> the GOT and not use copy relocations. Ultimately, there is only going
>>>> to be one copy of the global either defined in the executable or the
>>>> shared object right?
>>>>
>>>> Can we remove the check for SYMBOL_REF_WEAK?
>>>
>>> So, what will then happen if the weak undef symbol isn't defined anywhere?
>>> In non-PIE binaries that is fine, the linker will store 0.
>>> But in PIE binaries, the 0 would be biased by the PIE load bias and thus
>>> wouldn't be NULL.
>>
>> Thanks for clarifying.
>>
>>> You can only optimize weak vars if there is some weak definition in the
>>> current TU.
>>
>> Would this be fine then? Replace !SYMBOL_REF_WEAK (op0) with
>>
>> !(SYMBOL_REF_WEAK (op0) && SYMBOL_REF_EXTERNAL_P (op0))
>>
>
> The full condition is:
>
> && (SYMBOL_REF_LOCAL_P (op0)
> || (HAVE_LD_PIE_COPYRELOC
> && flag_pie
> && !SYMBOL_REF_WEAK (op0)
> && !SYMBOL_REF_FUNCTION_P (op0)))
>
> If the weak op0 is defined in the current TU, shouldn't
> SYMBOL_REF_LOCAL_P (op0) be true for PIE?
Thats not what I see for this:
zap.cc
---------
__attribute__((weak))
int glob;
int main()
{
printf("%d\n", glob);
}
(gdb) p debug_rtx(op0)
(symbol_ref/i:DI ("glob") <var_decl 0x7ffff74f51c8 glob>)
(gdb) p SYMBOL_REF_LOCAL_P(op0)
$4 = false
(gdb) p SYMBOL_REF_WEAK (op0)
$5 = 1
(gdb) p SYMBOL_REF_EXTERNAL_P (op0)
$6 = false
Thanks
Sri
>
> --
> H.J.
next prev parent reply other threads:[~2015-02-03 21:36 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 [this message]
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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