[llvm-dev] x86: How to Force 2-byte `jmp` instruction in lowering

Rafael EspĂ­ndola via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 28 19:17:08 PDT 2016

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 EspĂ­ndola
> <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.


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