From: Sriraman Tallam <tmsriram@google.com>
To: Andi Kleen <andi@firstfloor.org>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>,
"H.J. Lu" <hjl.tools@gmail.com>, David Li <davidxl@google.com>
Subject: Re: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt=
Date: Fri, 01 May 2015 17:50:00 -0000 [thread overview]
Message-ID: <CAAs8HmzC0gQOm8t4-XYzVUW-Npe8zZD1h+D1aCwrqkCxa63GQQ@mail.gmail.com> (raw)
In-Reply-To: <87r3r0mjec.fsf@tassilo.jf.intel.com>
On Fri, May 1, 2015 at 8:01 AM, Andi Kleen <andi@firstfloor.org> wrote:
> Sriraman Tallam <tmsriram@google.com> writes:
>>
>> This comes with caveats. This cannot be generally done for all
>> functions marked extern as it is impossible for the compiler to say if
>> a function is "truly extern" (defined in a shared library). If a
>> function is not truly extern(ends up defined in the final executable),
>> then calling it indirectly is a performance penalty as it could have
>> been a direct call. Further, the newly created GOT entries are fixed
>> up at start-up and do not get lazily bound.
>
> This means you need to make it depend on -fno-semantic-interposition ?
Please correct me if I am wrong but I do not see any dependency on
semantic-interposition. The GOT entry created for the function
pointer (whose PLT has been eliminated) has a dynamic relocation
against it to fixup the address at run-time and the dynamic linker
fills it with the right address. This is not a new mechanism. The
same mechanism is used when we access function pointers with PIE for
instance.
Thanks
Sri
>
>> Given this, I propose adding a new option called
>> -fno-plt=<function-name> to the compiler. This tells the compiler
>> that we know that the function is truly extern and we want the
>> indirect call only for these call-sites. I have attached a patch that
>> adds -fno-plt= to GCC. Any number of "-fno-plt=" can be specified and
>> all call-sites corresponding to these named functions will be done
>> indirectly using the mechanism described above without the use of a
>> PLT stub.
>
> The argument seems awkward. The command line may get very long.
> Better an attribute?
>
> Longer term it would be probably better to support it properly
> in the linker.
>
> -Andi
>
> --
> ak@linux.intel.com -- Speaking for myself only
next prev parent reply other threads:[~2015-05-01 17:50 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
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 [this message]
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
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
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
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