[fpc-devel] x86_64 question

J. Gareth Moreton gareth at moreton-family.com
Sun Oct 4 13:01:38 CEST 2020

Hi Nikolay,

I've got some good code to test, but I need to double-check with someone 
to see if the licensing agreements allow (the code is rather complex, 
but showcases the effect of the TEST instructions quite nicely).

Is your platform a Windows or a Unix machine?  I ask because I don't 
want to send you functions that use the wrong calling convention!

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
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