Object coding: SMMUL where to shift / SSAT when to saturate

[robot_mixeur]

Hi,

I've read the various pages / posts about object coding & fixed point math. There are two things that remained unclear:

First, re ___SMUL:
In https://sebiik.github.io/community.axoloti.com.backup/t/coding-axoloti-objects/2606/11 it is shown that we can do e.g.
___SMUL(a << 3, b << 2) or
___SMUL(a << 5, b) or
___SMUL(a, b) << 5

What is the difference in the the three variants (in terms of numerical result), assuming that a and b are both within the 27 bit range?

Second , re __SSAT:
Are there an "best practices" when one should saturate after certain operations, and when it's not necessary?

Thanks!

[lokki]

What is the difference in the the three variants (in terms of numerical result), assuming that a and b are both within the 27 bit range?

the ___SMMUL instruction uses the higher 15bits to do the multiply and disregards the lower bits. so if you shift up the a and b from the 27bit range before the multiplication you can use those higher bits to get a more precise result. if you shift after the multiply the 5 lowest bits will all be zero, so you loose resolution. better to use the <<3 and <<2.

[robot_mixeur]

okay got it. thanks!

[DrJustice]

Second , re __SSAT:
Are there an "best practices" when one should saturate after certain operations, and when it's not necessary?

E.g. when you add two Q27 numbers and want to clamp the result to Q27, you'd use x = __SSAT( x, 28 ) to make sure the result fits within the allotted 28 bits without overflowing. Or in other words, when a number may be bigger than you want, use __SSAT to clip it to the range that fits in the given number of bits.