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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