linux-kernel - Re: Avoiding unnecessary jump relocations in gas?

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Message-ID: <CALCETrX35KY8M0JKdoF1CB853AiS8wqH2-uQ976EptU3xf3=Dg@mail.gmail.com>
Date:	Thu, 7 May 2015 20:22:42 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H.J. Lu" <hjl.tools@...il.com>
Cc:	Jan Beulich <JBeulich@...e.com>,
	Binutils <binutils@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Avoiding unnecessary jump relocations in gas?

On Thu, May 7, 2015 at 9:21 AM, H.J. Lu <hjl.tools@...il.com> wrote:
> On Thu, May 7, 2015 at 4:52 AM, Jan Beulich <JBeulich@...e.com> wrote:
>>>>> On 07.05.15 at 08:02, <luto@...capital.net> wrote:
>>> AFAICT gas will produce relocations for jumps to global labels in the
>>> same file.  This doesn't seem directly harmful to me, except that, on
>>> x86, it forces five-byte jumps instead of two-byte jumps.
>>>
>>> This seems especially unfortunate, since even hidden and protected
>>> symbols have this problem.
>>>
>>> Given that many users don't want interposition support (especially the
>>> kernel and anyone using .hidden or .protected), it would be nice to
>>> have a command-line option to turn this off and probably also to turn
>>> it off by default for hidden and protected symbols.  Can gas do this?
>>
>> I've been running with the below changes (taken off of a bigger set
>> of changes, so the line numbers may look a little odd) for the last
>> couple of years. I never tried to submit this change because so far
>> I couldn't find the time to check whether this would have any
>> unwanted side effects on cases I don't normally use.
>>
>
> This is the patch I checked in.
>
> Thanks.
>
> --
> H.J.
> ---
> Branches to global non-weak symbols defined in the same segment with
> non-default visibility can be optimized the same way as branches to
> local symbols.

Would it make sense to also add a command line option along the lines
of gcc's -fno-semantic-interposition or some way to override the
default visibility?  AFAICS this patch helps but only if asm code gets
liberally sprinkled with .hidden or .protected directives.

--Andy
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