llvm dev - Jun 2016 - x86: How to Force 2-byte `jmp` instruction in lowering

On 28 June 2016 at 22:14, Dean Michael Berris <dberris at google.com>
wrote:
> On Wed, Jun 29, 2016 at 12:06 PM Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
>>
>> On 22 June 2016 at 16:36, Dean Michael Berris via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > Peter suggested just writing out '.byte 0xeb, 0x09' and
that allowed the
>> > jump instruction to bypass the relaxation, so that fixes my
immediate
>> > problem. The question still stands though whether it should be
possible
>> > to
>> > do through the instruction builder interface.
>> >
>>
>> I don't think so. When the relax-all flag is on MC will relax all
>> instructions.
>>
>
> I see.
>
> So the question becomes what's the advantage (if any) of Clang passing
the
> 'relax-all' flag down to MC? Is there a good reason for this
behaviour at
> all?
Speed, but it has probably been years since anyone benchmarked that.

Cheers,
Rafael

On Wed, Jun 29, 2016 at 12:17 PM Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
> On 28 June 2016 at 22:14, Dean Michael Berris <dberris at google.com>
wrote:
> > On Wed, Jun 29, 2016 at 12:06 PM Rafael Espíndola
> > <rafael.espindola at gmail.com> wrote:
> >>
> >> On 22 June 2016 at 16:36, Dean Michael Berris via llvm-dev
> >> <llvm-dev at lists.llvm.org> wrote:
> >> > Peter suggested just writing out '.byte 0xeb, 0x09'
and that allowed
> the
> >> > jump instruction to bypass the relaxation, so that fixes my
immediate
> >> > problem. The question still stands though whether it should
be
> possible
> >> > to
> >> > do through the instruction builder interface.
> >> >
> >>
> >> I don't think so. When the relax-all flag is on MC will relax
all
> >> instructions.
> >>
> >
> > I see.
> >
> > So the question becomes what's the advantage (if any) of Clang
passing
> the
> > 'relax-all' flag down to MC? Is there a good reason for this
behaviour at
> > all?
>
> Speed, but it has probably been years since anyone benchmarked that.
>
>
Okay. Any objections to just removing that from the clang side?
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On 29 June 2016 at 02:49, Dean Michael Berris <dberris at google.com>
wrote:
> On Wed, Jun 29, 2016 at 12:17 PM Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
>>
>> On 28 June 2016 at 22:14, Dean Michael Berris <dberris at
google.com> wrote:
>> > On Wed, Jun 29, 2016 at 12:06 PM Rafael Espíndola
>> > <rafael.espindola at gmail.com> wrote:
>> >>
>> >> On 22 June 2016 at 16:36, Dean Michael Berris via llvm-dev
>> >> <llvm-dev at lists.llvm.org> wrote:
>> >> > Peter suggested just writing out '.byte 0xeb,
0x09' and that allowed
>> >> > the
>> >> > jump instruction to bypass the relaxation, so that fixes
my immediate
>> >> > problem. The question still stands though whether it
should be
>> >> > possible
>> >> > to
>> >> > do through the instruction builder interface.
>> >> >
>> >>
>> >> I don't think so. When the relax-all flag is on MC will
relax all
>> >> instructions.
>> >>
>> >
>> > I see.
>> >
>> > So the question becomes what's the advantage (if any) of Clang
passing
>> > the
>> > 'relax-all' flag down to MC? Is there a good reason for
this behaviour
>> > at
>> > all?
>>
>> Speed, but it has probably been years since anyone benchmarked that.
>>
>
> Okay. Any objections to just removing that from the clang side?

Depends on what the benchmarks show.

Cheers,
Rafael