[fpc-devel] x86_64 question

[J. Gareth Moreton]

Sure, I can send you something.  It might have to be to a personal 
e-mail though depending on how big the attachments are. Watch this space.

I may be a bit of a mad scientist when it comes to my testing and 
research (and sometimes I make a stupid mistake like with the recent 
nested function proposal, completely forgetting about the return 
address), but I hope I find the occasional useful gem that can be used 
and polished by others!

Gareth aka. Kit

On 02/10/2020 14:13, Nikolay Nikolov via fpc-devel wrote:
>
> On 10/2/20 2:13 PM, J. Gareth Moreton via fpc-devel wrote:
>> Confirmed my suspicions.  if I zero the upper bits of the register (I 
>> used something akin to "AND RCX, $F"), there is no speed loss.
>>
>> Therefore, I can make the hypothesis, on my Intel(R) Core(TM) 
>> i7-10750H, that using TEST on a sub-register causes a false 
>> dependency if the bits outside of the subset are not zero, even 
>> though the register isn't being modified.
>
> If you send me a test program, I can run it on my Ryzen 5 2500U to see 
> how AMD behaves. We don't specifically optimize for AMD (yet), but 
> it's interesting to know.
>
> Nikolay
>
>>
>> Gareth aka. Kit
>>
>> On 02/10/2020 11:57, J. Gareth Moreton via fpc-devel wrote:
>>> So... I've done some tests, replacing TEST RCX, $4 with TEST CL, $4 
>>> and the like in a number-crunching function, and it seems to cause a 
>>> notable penalty, even though none of the instructions are in my 
>>> critical loop.  So I think it's something that needs to be avoided 
>>> in most cases.  I think the reason why it worked in my Int and Frac 
>>> functions is because the processor knows the upper 48 bits of the 
>>> register are zero.
>>>
>>> Long story short... best not to do it unless you have some 
>>> additional insight into what the registers contain.
>>>
>>> Gareth aka. Kit
>>>
>>>
> _______________________________________________
> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
> https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
>

-- 
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus