Re: FW: [PATCH 1/2] [PATCH 1/2] Enable Intel AVX512_FP16 instructions

[Jan Beulich <jbeulich@suse.com>]

On 20.07.2021 13:26, Cui, Lili wrote:
> 
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: Tuesday, July 20, 2021 4:46 PM
>> To: Cui, Lili <lili.cui@intel.com>
>> Cc: hjl.tools@gmail.com; binutils@sourceware.org
>> Subject: Re: FW: [PATCH 1/2] [PATCH 1/2] Enable Intel AVX512_FP16
>> instructions
>>
>> On 20.07.2021 09:08, Cui, Lili wrote:
>>>
>>>> -----Original Message-----
>>>> From: Jan Beulich <jbeulich@suse.com>
>>>> Sent: Wednesday, July 14, 2021 11:21 PM
>>>> To: Cui, Lili <lili.cui@intel.com>
>>>> Cc: hjl.tools@gmail.com; binutils@sourceware.org
>>>> Subject: Re: [PATCH 1/2] [PATCH 1/2] Enable Intel AVX512_FP16
>>>> instructions
>>>>
>>>> On 13.07.2021 08:58, Cui, Lili wrote:
>>>>
>>>> Disassembler:
>>>>
>>>> d_scalar_mode looks to be unused.
>>>>
>>>> This
>>>>
>>>>   /* EVEX_W_MAP5_2A_P_1 */
>>>>   {
>>>>     { "vcvtsi2sh{%LQ|}",	{ XMScalar, VexScalar, EXxEVexR, Ed }, 0 },
>>>>     { "vcvtsi2sh{%LQ|}",	{ XMScalar, VexScalar, EXxEVexR, Eq }, 0 },
>>>>   },
>>>>
>>>> can imo be expressed without decoding EVEX.W, by using Edq instead of
>>>> (separately) Ed and Eq. There's at least one similar case elsewhere.
>>>> Interestingly in the 2si/2usi conversions you do use Gdq already,
>>>> which I think handles the EVEX.W=1 case correctly outside of 64-bit
>>>> mode (unlike Eq, which will unconditionally produce 64-bit register names
>> afaict).
>>>>
>>>> As to a broader question on decoding EVEX.W: Did you consider
>>>> introducing e.g. %XH (paralleling %XW, just that EVEX.W=1 is not a
>>>> valid encoding), to avoid this decode step for perhaps almost all
>>>> entries? And if that's not an option, decoding EVEX.W first for all
>>>> the opcodes which previously had no meaning at all would, in some
>>>> cases, reduce the overall number of table entries (and in all other
>>>> cases this would then merely be for consistency, as it also wouldn't
>> increase the number of table entries). To give an example:
>>>>
>>>>     { PREFIX_TABLE (PREFIX_EVEX_0F3AC2) },
>>>>
>>>> =>
>>>>
>>>>   /* PREFIX_EVEX_0F3AC2 */
>>>>   {
>>>>     { VEX_W_TABLE (EVEX_W_0F3AC2_P_0) },
>>>>     { VEX_W_TABLE (EVEX_W_0F3AC2_P_1) },
>>>>   },
>>>>
>>>> =>
>>>>
>>>>   /* EVEX_W_0F3AC2_P_0 */
>>>>   {
>>>>     { "vcmpph",	{ XMask, Vex, EXxh, EXxEVexS, Ib }, 0 },
>>>>   },
>>>>   /* EVEX_W_0F3AC2_P_1 */
>>>>   {
>>>>     { "vcmpsh",	{ XMask, VexScalar, EXxmm_mw, EXxEVexS, Ib }, 0 },
>>>>   },
>>>>
>>>> i.e. a total of 1 + 4 + 2 * 2 entries. Whereas decoding W first would
>>>> yield 1
>>>> (evex) + 2 (evex_w) + 4 (prefix) entries.
>>>
>>> Hi Jan,
>>>
>>> Do you want me to change it like this?
>>>      { PREFIX_TABLE (PREFIX_EVEX_0F3AC2) },
>>>
>>>  =>
>>>
>>>    /* PREFIX_EVEX_0F3AC2 */
>>>    {
>>>      { "vcmp%XH",	{ XMask, Vex, EXxh, EXxEVexS, Ib }, 0 },
>>>      { "vcmp%XH",	{ XMask, VexScalar, EXxmm_mw, EXxEVexS, Ib }, 0 },
>>>    },
>>>
>>> "XH" => print 'ph', 'sh' depending on the EVEX.ll bit, if EVEX.W==W1 report
>> bad code.
>>> if  (EVEX.LL== EVEX.LLIG)
>>>       print 'sh'
>>> else
>>>       print 'ph'
>>
>> Not exactly, no. %XH was meant to parallel %XW, which prints 's' or 'd'
>> depending on VEX.W. %XH would print 'h' if EVEX.W is clear and produce an
>> appropriate indication of the encoding being bad if EVEX.W is set.
>> IOW something like
>>
>>    /* PREFIX_EVEX_0F3AC2 */
>>    {
>>      { "vcmpp%XH",	{ XMask, Vex, EXxh, EXxEVexS, Ib }, 0 },
>>      { "vcmps%XH",	{ XMask, VexScalar, EXxmm_mw, EXxEVexS, Ib }, 0 },
>>    },
>>
>>>> The delta is even larger for something like MAP5_7D: 1 + 4 + 4 * 2
>>>> vs. 1 + 2 + 4. This also results in more related entries ending up
>>>> closer to one another.
>>>>
>>> I don't quite understand here,  should I let all FP16 disassembler go
>> through W_TABLE fist? or just add something like %XH instead of going
>> through W_TABLE? Thanks.
>>
>> Where beneficial you will want to decode EVEX.W first, yes. Unless, as per
>> above, you can avoid that decoding step altogether by using %XH.
>>
> I prefer to decode EVEX.W first instead of using %XH.

Well, I can't stop you from avoiding %XH, but I did intentionally say
"Where beneficial you will want to ...". That is, I think that where
possible you should use %XH, and only where that's not suitable decode
EVEX.W (typically earlier than EVEX.pp).

Jan