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

On Wed, Jun 22, 2016 at 9:36 AM, Nirav Davé <llvm-dev at lists.llvm.org>
wrote:
> In any case, the issue appears to be that llvm doesn't realize that the
> target address is resolved and erroneously applies branch relaxation to the
> jump. I don't know why a linker private symbol would make a difference.
>
Relaxation is the process of *shortening* jumps that can be shortened, and
then re-running instruction layout to discover more relaxations until
fixpoint. Removing the -relax-all flag in clang won't help here, it would
hurt.

I'm not exactly sure what the semantics of linker private symbols are, but
using a normal assembler temporary label is probably the way to go anyway.
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I thought jumps start short and relaxation widens them as needed until
fixpoint. So relax-all causes them all to be widened unconditionally.

On Wed, Jun 29, 2016 at 9:27 AM, Reid Kleckner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On Wed, Jun 22, 2016 at 9:36 AM, Nirav Davé <llvm-dev at
lists.llvm.org>
> wrote:
>
>> In any case, the issue appears to be that llvm doesn't realize that
the
>> target address is resolved and erroneously applies branch relaxation to
the
>> jump. I don't know why a linker private symbol would make a
difference.
>>
>
> Relaxation is the process of *shortening* jumps that can be shortened, and
> then re-running instruction layout to discover more relaxations until
> fixpoint. Removing the -relax-all flag in clang won't help here, it
would
> hurt.
>
> I'm not exactly sure what the semantics of linker private symbols are,
but
> using a normal assembler temporary label is probably the way to go anyway.
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>

-- 
~Craig
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On Wed, Jun 29, 2016 at 10:05 AM, Craig Topper <craig.topper at gmail.com>
wrote:
> I thought jumps start short and relaxation widens them as needed until
> fixpoint. So relax-all causes them all to be widened unconditionally.
>
My mistake, you're right.

I've been reading that code for years and assuming that it goes
large-to-small, but I guess the process is the same regardless of which
direction you go. :)
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