Optimizing 0-displacement jumps on i386

[Jan Beulich]

On 08.08.2025 01:26, Petr Skočík wrote:
> Hi. I'm new here. I noticed (1) gas isn't literal and is capable of various
> optimizations & (2) jumps are effectively variable-length due to how gas
> iteratively does jmp relaxations and I was wondering why not take (2) all
> the way and delete sequences like 0eb 00, i.e., jumps to the next
> instruction?
> 
> These come up quite a lot (tailcalls to a neighboring function) & might be
> difficult for compilers to eliminate them.
> (It might be better if the assembler did it for other reasons anyway--like
> I'd like to a bit more efficiently tailcall to some asm without the
> compiler getting too nosy). It might be difficult for gas too, but since
> the framework for length-adjusting jumps is already there ...?
> 
> Alternatively, and this seems rather easy, 0-displacement jumps could be
> turned into 2-byte nops, which are quite a bit faster.
> 
> I tried doing it by modifying the tail of md_convert_frag like so:
> 
>   if(extension==1 && 0==(valueT)(displacement_from_opcode_start -
> extension)){ //new
>       opcode[0]=0x66; //new
>       opcode[1]=0x90; //new
>   }else  //new
>   {
>   /* Now put displacement after opcode.  */
>       md_number_to_chars ((char *) where_to_put_displacement,
>                   (valueT) (displacement_from_opcode_start - extension),
>                   DISP_SIZE_FROM_RELAX_STATE (fragP->fr_subtype));
>   }
>   fragP->fr_fix += extension;
> }
> 
> and it seems to do the job.
> Just a thought. Thanks for reading my message.

Hmm, yes, why not? Would you mind transforming the above into a proper patch?

Jan