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

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.

Cheers,
Rafael

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?

Thanks Rafael!
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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