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Date: Mon, 30 Jul 2012 17:04:00 -0000
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Subject: Re: [PATCH] x86-64: correct segment override prefix generation
From: "H.J. Lu" <hjl.tools@gmail.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: binutils@sourceware.org
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On Tue, Jul 24, 2012 at 8:28 AM, Jan Beulich <JBeulich@suse.com> wrote:
> Despite them being ignored by the CPU, gas issues segment override
> prefixes for other than FS/GS in 64-bit mode. If doing so at all, it
> should clearly do this correctly. Determining the default segment,
> however, requires to take into consideration RegRex (so far, RSP, RBP,
> R12, and R13 were all treated equally here).
>
> 2012-07-24  Jan Beulich <jbeulich@suse.com>
>
>         * config/tc-i386-intel.c (build_modrm_byte): Split determining
>         default segment from figuring out encoding. Honor RegRex for
>         the former.
>
> --- a/gas/config/tc-i386.c
> +++ b/gas/config/tc-i386.c
> @@ -5733,18 +5733,14 @@ build_modrm_byte (void)
>               i.sib.base = i.base_reg->reg_num;
>               /* x86-64 ignores REX prefix bit here to avoid decoder
>                  complications.  */
> -             if ((i.base_reg->reg_num & 7) == EBP_REG_NUM)
> -               {
> +             if (!(i.base_reg->reg_flags & RegRex)
> +                 && (i.base_reg->reg_num == EBP_REG_NUM
> +                  || i.base_reg->reg_num == ESP_REG_NUM))
>                   default_seg = &ss;
> -                 if (i.disp_operands == 0)
> -                   {
> -                     fake_zero_displacement = 1;
> -                     i.types[op].bitfield.disp8 = 1;
> -                   }
> -               }
> -             else if (i.base_reg->reg_num == ESP_REG_NUM)
> +             if (i.base_reg->reg_num == 5 && i.disp_operands == 0)
>                 {
> -                 default_seg = &ss;
> +                 fake_zero_displacement = 1;
> +                 i.types[op].bitfield.disp8 = 1;
>                 }
>               i.sib.scale = i.log2_scale_factor;
>               if (i.index_reg == 0)
>

Please provide a testcase to show the correct behavior.

Thanks.

-- 
H.J.