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matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10646

Posted: 09:58am 23 Nov 2025

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To optimise performance do not use the same name for local variables in nested subroutines.
Variables are created in a hash table. So by definition if you use i as an index in a subroutine called by a subroutine that also uses i. There will have been a hash table collision that the firmware resolves by looking for the next free slot. This is completely benign but does take extra time: not a lot but some.

Also, it is very slightly quicker if variables are typed at declaration rather than being typed with a suffix

Demo 1

fred
'
Sub fred
Local i%=22,ii%=23
bert
Print i%,ii%
End Sub
'
Sub bert
Local i%,j%,ii%,jj%
Timer =0
For i%=1 To 100000
ii%=i%
Next
Print Timer
Timer =0
For j%=1 To 100000
jj%=j%
Next
Print Timer
End Sub

Demo2

Option explicit
Option default integer
fred
'
Sub fred
Local i=22,ii=23
bert
Print i,ii
End Sub
'
Sub bert
Local i,j,ii,jj
Timer =0
For i=1 To 100000
ii=i
Next
Print Timer
Timer =0
For j=1 To 100000
jj=j
Next
Print Timer
End Sub

LeoNicolas

Guru

Joined: 07/10/2020
Location: Canada
Posts: 542

Posted: 10:37am 23 Nov 2025

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Thank you for the tip

That makes sense. Hash tables have an O(1) time access for the best scenario, and O(N) for the worst case, when keys collide. It is still possible to have a key collision even with keys with different names, if both strings have the same hash, but the chance to happen is very low.

Is it possible to store the local variable in the hash table using a composite key consisting of the method name + variable name? This should solve the issue.

Bleep
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Joined: 09/01/2022
Location: United Kingdom
Posts: 693

Posted: 10:26am 24 Nov 2025

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Hi Peter,
While I agree with what you have said, there are exceptions/problems.
I was optimising

Sub mm.user_bitmap(x%,y%,w%,h%,s%,Fc%,Bc%,bitmap%)
to
Sub mm.user_bitmap(x As Integer,y As Integer,w As Integer,h As Integer, s As Integer,Fc As Integer,Bc As Integer,bitmap As Integer)

and removing all the % in the subroutine, but it actually got slower, presumably because the parser had to parse all those 'As Integer' statements, I got round it by using 'Option Default Integer' but this can't work if there are a mixture of types. Assuming it is the length of the declaration, which slows it down, is a short form possible, relativly easy for you? maybe:-

xyz As %, pqr As %, abc As !, ghi As $
Or even
xyz %, pqy %, abc !, ghi $ 'note space

This is of course assuming it is the length of the decleration that is causing the slow down, it may not be.?
Regards Kevin.

PhenixRising
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Joined: 07/11/2023
Location: United Kingdom
Posts: 1649

Posted: 10:48am 24 Nov 2025

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MM.REPLACE: I haven't seen any mention of it. Could this be used to replace CONSTants with literals for improved performance?

matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10646

Posted: 12:07pm 24 Nov 2025

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Kevin

You can use OPTION DEFAULT for your primary variable type and then just type anything else with the appropriate suffix. I would never declare a subroutine using the "as" syntax

Quote I haven't seen any mention of it. Could this be used to replace CONSTants with literals for improved performance?

No because is then modifies the source. This is how #define works on the CMM2 because it is totally a disk based system and the user never sees the garbled version that actually runs. The PicoMite is like all other variants where the source is de-tokenised for editing/listing/saving and then tokenised into flash for running. If you then edited a file with #defines they would have disappeared and all the literals would be in the source defeating the object

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