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Commit: Gas: Add .nop directive.

Jose E. Marchesi jose.marchesi@oracle.com
Mon Sep 14 15:27:00 GMT 2020 
Hi Nick.

>   I am checking in the attached patch to add a ".nop" directive to the
>   assembler.  This directive creates a single no-op instruction.  It is
>   similar to the already existing ".nops" directive, apart from two
>   important differences:
>
>     1. It is implemented for all architectures, not just x86.
>     2. The instruction it generates counts as a real instruction for the
>        purposes of DWARF line number table generation.
>
>   Fact 1 is important as it allows the directive to be used to generate
>   architecture neutral test cases containing real instructions.
>
>   Tested with a large variety of different targets.

Very nice!

In the same spirit, it would be awesome to be able to generate
architecture-neutral data directives as well, for DWARF and CTF and the
like :)

> diff --git a/gas/config/tc-bpf.h b/gas/config/tc-bpf.h
> index 5765833997..cb02d6c133 100644
> --- a/gas/config/tc-bpf.h
> +++ b/gas/config/tc-bpf.h
> @@ -48,3 +48,4 @@
>  /* Values passed to md_apply_fix don't include the symbol value.  */
>  #define MD_APPLY_SYM_VALUE(FIX) 0
>  
> +#define md_single_noop_insn "mov %r1,%r1"

For BPF we have this in GCC:

(define_insn "nop"
  [(const_int 0)]
  ""
  "mov\t%%r0,%%r0"
  [(set_attr "type" "alu")])

I think it would be good to settle on the same way to encode nops in
both GCC and binutils.
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