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Re: [PATCH, v2] x86-64: correct segment override prefix generation


On Tue, Aug 7, 2012 at 6:47 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 07.08.12 at 15:44, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>> On Tue, Aug 7, 2012 at 3:32 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>> On 30.07.12 at 19:03, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>>>> Please provide a testcase to show the correct behavior.
>>>
>>> Here you go.
>>>
>>> Jan
>>>
>>> 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).
>>>
>>> gas/
>>> 2012-08-07  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.
>>>
>>> gas/testsuite/
>>> 2012-08-07  Jan Beulich <jbeulich@suse.com>
>>>
>>>         * gas/i386/x86-64-segovr.{s,l}: New.
>>>         * gas/i386/i386.exp: Run new test.
>>>
>>> --- 2012-08-07/gas/config/tc-i386.c     2012-07-31 09:45:03.000000000 +0200
>>> +++ 2012-08-07/gas/config/tc-i386.c     2012-08-07 12:13:39.000000000 +0200
>>> @@ -5729,18 +5729,14 @@ 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)
>>
>> Please use EBP_REG_NUM instead 5 here.
>
> But that change was intentional - we're _not_ looking for EBP here,
> we're looking for "EBP or R13". The previous use of EBP_REG_NUM
> was part of why this was broken imo.
>
>> OK with the EBP_REG_NUM change above if Linux x86-64 kernel
>> compiles and runs.
>
> Sure, that has been the case for many weeks already.
>

OK then.

Thanks.


-- 
H.J.


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