[PATCH v2] x86: Add -munaligned-vector-move to assembler

[Jan Beulich]

On 21.10.2021 19:50, H.J. Lu wrote:
> Unaligned load/store instructions on aligned memory or register are as
> fast as aligned load/store instructions on modern Intel processors.  Add
> a command-line option, -munaligned-vector-move, to x86 assembler to
> encode encode aligned vector load/store instructions as unaligned
> vector load/store instructions.

As said I'm still lacking the spelling out here of some form of
motivation for the change. The resulting code, afaict, isn't going
to be better than what we have now, yet there's the price of extra
new code that you introduce.

> @@ -4080,6 +4083,26 @@ optimize_encoding (void)
>  {
>    unsigned int j;
>  
> +  /* Encode aligned vector move as unaligned vector move if asked.  */
> +  if (unaligned_vector_move)
> +    switch (i.tm.base_opcode)
> +      {
> +      case 0x28:
> +	/* movaps/movapd/vmovaps/vmovapd.  */
> +	if (i.tm.opcode_modifier.opcodespace == 1
> +	    && i.tm.opcode_modifier.opcodeprefix <= 1)

I don't think the prefix needs checking here? F3 and F2 encodings
don't exist, so maybe at most gas_assert() this?

> +	  i.tm.base_opcode = 0x10;
> +	break;
> +      case 0x6f:
> +	/* movdqa/vmovdqa/vmovdqa64/vmovdqa32 */
> +	if (i.tm.opcode_modifier.opcodespace == 1
> +	    && i.tm.opcode_modifier.opcodeprefix == 1)
> +	  i.tm.opcode_modifier.opcodeprefix = 2;

Please can you avoid using literal numbers here? This not only makes it
needlessly harder to potentially change the SPACE_* and PREFIX_* values
(despite me hoping / assuming that we would never have to do so), but
also makes the code not sufficiently self-documenting.

> @@ -5053,7 +5076,8 @@ md_assemble (char *line)
>        i.disp_operands = 0;
>      }
>  
> -  if (optimize && !i.no_optimize && i.tm.opcode_modifier.optimize)
> +  if (unaligned_vector_move
> +      || (optimize && !i.no_optimize && i.tm.opcode_modifier.optimize))
>      optimize_encoding ();

Isn't this fragile? You now depend on optimize_encoding() to do nothing
optimization-wise if unaligned_vector_move is set. I was rather hoping
you would introduce a separate helper.

Jan