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Re: Avoiding unnecessary jump relocations in gas?
- From: Andy Lutomirski <luto at amacapital dot net>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Jan Beulich <JBeulich at suse dot com>, Binutils <binutils at sourceware dot org>, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>
- Date: Thu, 7 May 2015 20:22:42 -0700
- Subject: Re: Avoiding unnecessary jump relocations in gas?
- Authentication-results: sourceware.org; auth=none
- References: <CALCETrUXnXVARO=KD+r0YV1wi2vvXF-41VS++F877HsJs1qz9Q at mail dot gmail dot com> <554B6E0102000078000778D0 at mail dot emea dot novell dot com> <CAMe9rOri61BL=uUgC8FVCs0peGR=KKRh629rOsRtbzMs8N=MrA at mail dot gmail dot com>
On Thu, May 7, 2015 at 9:21 AM, H.J. Lu <email@example.com> wrote:
> On Thu, May 7, 2015 at 4:52 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>> On 07.05.15 at 08:02, <firstname.lastname@example.org> 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.
> 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.