Re: [PATCH] x86: tighten condition for emitting LOCK on control register accesses

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, May 30, 2018 at 12:42 AM, Jan Beulich <JBeulich@suse.com> wrote:
> The control register is never expressed by REX.B; this bit only affects
> the involved GPR. Also only one of the operands can have its "control"
> flag set, so only check the correct operand.
>
> gas/
> 2018-05-30  Jan Beulich  <jbeulich@suse.com>
>
>         * config/tc-i386.c (build_modrm_byte): Drop REX_B from condition
>         checking for the need of emitting LOCK. Check "control" bit just
>         once.
>
> --- a/gas/config/tc-i386.c
> +++ b/gas/config/tc-i386.c
> @@ -6894,12 +6894,11 @@ build_modrm_byte (void)
>           if ((i.op[source].regs->reg_flags & RegVRex) != 0)
>             i.vrex |= REX_R;
>         }
> -      if (flag_code != CODE_64BIT && (i.rex & (REX_R | REX_B)))
> +      if (flag_code != CODE_64BIT && (i.rex & REX_R))
>         {
> -         if (!i.types[0].bitfield.control
> -             && !i.types[1].bitfield.control)
> +         if (!i.types[i.tm.operand_types[0].bitfield.regmem].bitfield.control)
>             abort ();
> -         i.rex &= ~(REX_R | REX_B);
> +         i.rex &= ~REX_R;
>           add_prefix (LOCK_PREFIX_OPCODE);
>         }
>      }
>

Does it have any impact on assembly code?


-- 
H.J.