Re: optimal encoding of SIMD insns

["Jan Beulich" <JBeulich at suse dot com>]

>>> On 22.11.17 at 14:47, <hjl.tools@gmail.com> wrote:
> On Wed, Nov 22, 2017 at 5:11 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>> On 22.11.17 at 13:43, <hjl.tools@gmail.com> wrote:
>>> On Tue, Nov 21, 2017 at 11:44 PM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>>> On 21.11.17 at 23:14, <hjl.tools@gmail.com> wrote:
>>>>> On Mon, Nov 20, 2017 at 6:18 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>>>>> On 20.11.17 at 14:51, <hjl.tools@gmail.com> wrote:
>>>>>>> On Mon, Nov 20, 2017 at 5:41 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>> And then - does your reply mean you don't think we could derive
>>>>>> the encoding selection from the most recent -march/.arch (maybe
>>>>>> also -mtune) setting?
>>>>>
>>>>> No, I don't think so.
>>>>
>>>> Please would you mind being a little less terse here: I'd really
>>>> like to understand the "why" behind your answer.
>>>
>>>  -march/.arch shouldn't change encoding.
>>
>> I'm sorry to say that, but this still leaves me asking: Why? Aren't
>> there already cases where they do (for example for shifts with an
>> immediate of 1 for the i486)? By stating the target architecture
>> explicitly (rather than running in the default, everything enabled
>> mode) the user tells us what processor the produced code is
>> intended to run.
> 
> This doesn't apply to SSE/VEX/EVEX encoding.  We don't use
> EVEX encoding to encode AVX instructions just because it is available.

This looks to become a never ending story: You again only state a
fact, without giving any reason. Why should we not do so if it
helps improve code density? The more that the original example I
gave

	vaddps (%rax),xmm0,xmm0

does not in any way state that this is meant to be a VEX-encoded
insn. It is instead a courtesy of the assembler to use the encoding
compatible with the widest range of hardware if it selects the VEX
variant here. -march/.arch are precisely to tell the assembler that
it doesn't need to consider too old hardware.

And considering the example I gave in my previous reply, where
would you draw the boundary?

Jan