How To

Source Code Tips and Tricks

nathanielbabiak 2020-12-04 21:50 (Edited)

In this post "small" and "large" refer to the number of tokens required.

Non-zero Evaluations

Non-zero values (int and float) evaluate to true, these are smaller and faster:

• Replace IF X<>0 THEN with IF X THEN
• Replace IF X<>0 AND Y<>0 THEN with IF X*Y THEN
• Replace IF X=0 OR Y=0 THEN with IF X*Y=0 THEN

There's two non-zero evaluation tricks if you're sure X and Y will always have the same sign:

• Replace IF X<>0 OR Y<>0 THEN with IF X+Y THEN
• Replace IF X=0 AND Y=0 THEN with IF X+Y=0 THEN

There's a few more non-zero evaluation tricks when X (and Y) are integers:

• Replace IF X AND %001=%001 THEN with IF X AND %001 THEN
• Replace IF X AND %010=%010 THEN with IF X AND %010 THEN
• Replace IF X AND %100=%100 THEN with IF X AND %100 THEN
• Replace IF X<=-Y OR X>=Y THEN with IF X\Y THEN
• Replace IF X>-Y AND X<Y THEN with IF X\Y=0 THEN

Keywords from C

This switch-case structure is small but slow:

IF X=7 THEN
  '...
ELSE IF X=6 THEN
  '...
ELSE IF X=5 THEN
  '...
ELSE IF X=4 THEN
  '...
ELSE IF X=3 THEN
  '...
ELSE IF X=2 THEN
  '...
ELSE IF X THEN
  'X=1
  '...
ELSE
  'X=0
  '...
END IF

This switch-case structure is fast but large:

IF X AND %100 THEN
  IF X AND %010 THEN
    IF X AND %001 THEN
      'X=7
      '...
    ELSE
      'X=6
      '...
    END IF
  ELSE
    IF X AND %001 THEN
      'X=5
      '...
    ELSE
      'X=4
      '...
    END IF
  END IF
ELSE
  IF X AND %010 THEN
    IF X AND %001 THEN
      'X=3
      '...
    ELSE
      'X=2
      '...
    END IF
  ELSE
    IF X AND %001 THEN
      'X=1
      '...
    ELSE
      'X=0
      '...
    END IF
  END IF
END IF

The continue keyword can also be implemented:

FOR X=0 TO 7
  CALL FOO
  'IF TEST1 THEN "CONTINUE"
  IF NOT TEST1 THEN
    CALL BAR
    CALL ZOT
  END IF
  'NO CODE ALLOWED HERE, "END IF" AND "NEXT" MUST BE ADJACENT.
NEXT X

Small Declaration

To return a value from a subprograms in a simple variable, the variable name must be declared prior. If an algorithm needs more than 3 new variable names, this is smaller than declaring one variable per-line. It's slower only because of overhead, otherwise the same speed O(n).

DECLARE:
DATA 0, 0, 0, 0, 0, 0, 0, 0
LOAD DECLARE
READ Y0, Y1, Y2, Y3, Y4, Y5, Y6, Y7

Single-command if-then statements are smaller (and sometimes faster too) when written on a single line like this:

• IF TEST1 THEN CALL FOO
• IF TEST1 THEN CALL FOO ELSE CALL BAR
• IF TEST1 THEN CALL FOO ELSE IF TEST2 THEN CALL BAR ELSE CALL ZOT

Loops

This loop is small but slow:

FOR X=0 TO 7
  TEXT 0,X,"-"
NEXT X

This unrolled loop is fast but large:

TEXT 0,0,"-"
TEXT 0,1,"-"
TEXT 0,2,"-"
TEXT 0,3,"-"
TEXT 0,4,"-"
TEXT 0,5,"-"
TEXT 0,6,"-"
TEXT 0,7,"-"

This is a sample loop:

DIM LIST(XHI-1)
FOR X=0 TO XHI-1
  CALL FOO(LIST(X),B*X+C)
NEXT X

The sample loop calculates the subtraction twice before entering the loop, then again at each iteration of the loop. This loop only calculates it once, it's faster and larger:

XMAX=XHI-1
DIM LIST(XMAX)
FOR X=0 TO XMAX
  CALL FOO(LIST(X),B*X+C)
NEXT X

The sample loop calculates math inside the loop. This loop uses a separate variable to track the math, it's faster and larger:

XMAX=XHI-1
DIM LIST(XMAX)
Y=C
FOR X=0 TO XMAX
  CALL FOO(LIST(X),Y)
  ADD Y,B
NEXT X

The separate-variable loop uses instructions inside the loop to track the math. This loop uses the for-variable itself, it's faster and larger (and also the code doesn't work, but I hope you get the point):

X=0
XMAX=XHI-1
DIM LIST(XMAX)
YMAX=B*XMAX+C
FOR Y=C TO YMAX STEP B
  CALL FOO(LIST(X),Y)
  INC X
NEXT Y

Note the step variable B, if replaced by a calculation, would be calculated at each iteration of the loop, even if B is negative (e.g. -3 is a calculation of "negation" on the constant 3).

Misc. Stuff

• X = ASC( CHR$( X ) ) will convert an 8-bit value from unsigned to signed.
• Additional ROM space may be mimicked (with sequential access only) using DATA commands, containing strings of hex-values, use VAL("0X"+VALUE$) to retrieve three bytes at a time.

moechofe 2020-12-05 07:29

good stuff

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