Re: [PATCH] X86: Disassemble primary opcode map's group 2 ModRM.reg == 6 aliases correctly

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

On Tue, May 30, 2017 at 7:48 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 30.05.17 at 16:43, <hjl.tools@gmail.com> wrote:
>> On Tue, May 30, 2017 at 7:41 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>> On 30.05.17 at 16:35, <hjl.tools@gmail.com> wrote:
>>>> On Tue, May 30, 2017 at 1:37 AM, Borislav Petkov <bp@suse.de> wrote:
>>>>> On Tue, May 30, 2017 at 02:34:28AM -0600, Jan Beulich wrote:
>>>>>> which I think should read
>>>>>>
>>>>>> "The CF flag contains the value of the last bit shifted out of the
>>>>>>  destination operand; for instructions other than SAR it is undefined
>>>>>>  when the count is greater than or equal to the size (in bits) of
>>>>>>  the destination operand."
>>>>>
>>>>> Makes sense.
>>>>
>>>> I look another look.  Disassembler always display "shl".  Please
>>>> don't use "sal" now.
>>>
>>> Which disassembler are you talking about, as you can't mean
>>> objdump or anything else that's libopcode-based?
>>
>> [hjl@gnu-6 tmp]$ cat x.s
>> shl %cl, %eax
>> sal %cl, %eax
>> shl $1, %eax
>> sal $1, %eax
>> [hjl@gnu-6 tmp]$ gcc -c x.s
>> [hjl@gnu-6 tmp]$ objdump -dw x.o
>>
>> x.o:     file format elf64-x86-64
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <.text>:
>>    0: d3 e0                 shl    %cl,%eax
>>    2: d3 e0                 shl    %cl,%eax
>>    4: d1 e0                 shl    %eax
>>    6: d1 e0                 shl    %eax
>> [hjl@gnu-6 tmp]$ objdump -dw -Mintel x.o
>>
>> x.o:     file format elf64-x86-64
>>
>>
>> Disassembly of section .text:
>>
>> 0000000000000000 <.text>:
>>    0: d3 e0                 shl    eax,cl
>>    2: d3 e0                 shl    eax,cl
>>    4: d1 e0                 shl    eax,1
>>    6: d1 e0                 shl    eax,1
>> [hjl@gnu-6 tmp]$
>
> Right, but here all instructions use the /4 encoding. We're talking
> about how to display the /6 encoding, though, and I think it would
> be helpful to the user if she could distinguish one from the other
> without having to look at the opcode bytes.

This is adjust an opcode alias.  These aren't the only instructions with
identical opcodes.


-- 
H.J.