Re: [PATCH 1/3] x86: Split REX/REX2 old registers judgment.

[Jan Beulich <jbeulich@suse.com>]

On 20.05.2024 08:22, Cui, Lili wrote:
> --- a/gas/config/tc-i386.c
> +++ b/gas/config/tc-i386.c
> @@ -4303,22 +4303,20 @@ static void establish_rex (void)
>    /* Respect a user-specified REX prefix.  */
>    i.rex |= i.prefix[REX_PREFIX] & REX_OPCODE;
>  
> -  /* For 8 bit registers we need an empty rex prefix.  Also if the
> -     instruction already has a prefix, we need to convert old
> -     registers to new ones.  */
> -
> -  if ((i.types[first].bitfield.class == Reg && i.types[first].bitfield.byte
> -       && ((i.op[first].regs->reg_flags & RegRex64) != 0 || i.rex != 0
> -	   || i.rex2 != 0))
> -      || (i.types[last].bitfield.class == Reg && i.types[last].bitfield.byte
> -	  && ((i.op[last].regs->reg_flags & RegRex64) != 0 || i.rex != 0
> -	      || i.rex2 != 0)))
> -    {
> -      unsigned int x;
> -
> -      if (!is_apx_rex2_encoding () && !is_any_vex_encoding(&i.tm))
> -	i.rex |= REX_OPCODE;
> -      for (x = first; x <= last; x++)
> +  /* For 8 bit registers without a prefix, we need an empty rex prefix.  */

As you're touching this comment, can you please also correct it? It's
not all 8-bit registers which are affected here (as expressed by the
RegRex64 checks).

> +  if (((i.types[first].bitfield.class == Reg && i.types[first].bitfield.byte
> +	&& ((i.op[first].regs->reg_flags & RegRex64) != 0 ))

Nit: Please parenthesize this line and ...

> +       || (i.types[last].bitfield.class == Reg && i.types[last].bitfield.byte
> +	   && (i.op[last].regs->reg_flags & RegRex64) != 0 ))

... this one the same way, preferably like the latter. Also please omit
the blanks before the closing parentheses.

> +      && !is_apx_rex2_encoding () && !is_any_vex_encoding (&i.tm) && !i.rex)

Is the !i.rex part needed? It wasn't there before. If it's needed, it
would imo better come ahead of the APX/VEX checks.

> +    i.rex |= REX_OPCODE;
> +
> +  /* For REX/REX2 prefix instructions, we need to convert old registers
> +     (AL, CL, DL and BL) to new ones (AXL, CXL, DXL and BXL) and report bad
> +     for AH, CH, DH and BH.  */

"report bad for" is a little odd; how about simply "reject"?

Okay with respective adjustments.

Jan

> +  if (i.rex || i.rex2)
> +    {
> +      for (unsigned int x = first; x <= last; x++)
>  	{
>  	  /* Look for 8 bit operand that uses old registers.  */
>  	  if (i.types[x].bitfield.class == Reg && i.types[x].bitfield.byte