multiplication vs shifting

[Kamikaze47]

ive got to multiply a variable by 25 and i was wondering which of these 2 would be quicker

x = x*25

or

x = x<<4 + x<<3 + x

I have a suspicion that the latter will be quicker, but take up more code space.

[Kamikaze47]

ok, i just did some tests and i was half right and half wrong.

x=x<<4+x<<3+x was quicker (taking 80% of the time that x=x*25 took to execute)

However, what surprised me was that x=x*25 used MORE code space than x=x<<4+x<<3+x. Replacing x=x<<4+x<<3+x with x=x*25 in my program adds 338 bytes.

It appears that x=x<<4+x<<3+x wins on both counts.

[tenaja]

PBP uses a subroutine for performing the multiplication. (For all 16F * calculations, and for 18F * calcs involving more than 8 bits.) If you perform the calculation multiple times, an individual line may be shorter than shifting. However, on just a one-time multiplication, the PBP subroutine will add significant code.

[Ioannis]

No other code, just Variable declaration and END,for a F877:

For BYTE variable:

x=x*25            '40 bytes
x=x<<4+x<<3+x     '43 bytes

For WORD variable:

x=x*25             '43 bytes
x=x<<4+x<<3+x      '49 bytes
x=x<<25            '27 bytes!!!

Ioannis

[Kamikaze47]

whats the point of the last one? shift a word variable 25 times either way and you will just get 0.

i was multiplying a word var and saving into a long var

[Ioannis]

I know 25 shift will not give the result you want.

Just tested to see how much less program space and how faster a shift will be. Useless for you but quite impressive!

Which PIC are you using? Is there any chance to use one with hardware multiplier?

Ioannis