# Advanced Speed Optimization Techniques for QB

### by Toshi (Toshihiro Horie)

• Optimize your algorithm first before you try these optimizations! That's usually where you get the biggest speed increases.

Before:

' a very slowly converging formula for pi
' This takes a minute or so on a fast Pentium III.
r# = 0
s% = 1
FOR i& = 1 TO 90000000 STEP 2
r# = r# + s% * 1# / i&
s% = -s%
NEXT
pi! = CSNG(r# * 4)
PRINT pi!

After:

' a fast! constant time formula for pi
' This is faster than you can blink.
PI! = ATN(1) * 4
PRINT pi!

Once you've optimized the algorithm all that you can, you can start looking at algebraic and loop optimizations.

• The classic one is to use DEFINT A-Z. This forces you to use as many integer variables as possible.

• Use integer variables to index FOR loops. This may require substitution and algebraic simplification.

• Before:

FOR i! = 0 to 0.3 STEP 0.01
p! = i! * 3
NEXT

After:

FOR i% = 0 to 30
p! = i% * 0.03
NEXT
• If your code has a lot of floating point calculations that need high accuracy, compile with QB 4.0.

e.g. a raytracer

• If your code has a lot of floating point calculations that don't need more than 8 bits of accuracy, then definitely convert it to fixed point. Even if it needs up to 16 bits of accuracy, it might be worth converting to fixed point, if it is being used in the main loop.

e.g. a rotozoomer or voxel terrain.

• Don't use IFs (conditional branches). Some comparison results can be directly be used in a calculation. Note that in QB, a TRUE boolean expression equals -1, and a FALSE one equals 0.

Before:

IF a > 4 THEN
b = 5
ELSE
b = 0
END IF

After:

b = -5 * (a > 4)

Actually, the above example is too simple for the After: version to be faster. But for more complicated expressions involving multiplication and division, it can make a difference.

• Buffer your reads from a file. This is especially useful in a non-disk-cached environment like DOS 4.0.

Before:

DIM c AS STRING * 1
OPEN "file.bin" FOR BINARY AS #1
FOR i = 0 TO 10 * 256
GET #1, , c
NEXT
CLOSE #1

After:

DIM buffer(0) AS STRING * 256
OPEN "file.bin" FOR BINARY AS #1
FOR i = 0 TO 10
GET #1, , buffer(0)
NEXT
CLOSE #1
• Use an assembler keyboard handler or INP(&H60) plus keyboard buffer clearing routines instead of INKEY\$.

e.g. For a user controlled floormapper routine, this made a huge difference in rending fps.

• For a straight QB multikey handler, don't bother to clear the keyboard buffer every vertical retrace. Instead, slow down the keyboard repeat rate, and check every few frames.

e.g. This made a huge difference in QBMKEY.BAS

• Use integer division for integers.

Before:

x% = x% / y%

After:

x% = x% \ y%
• Make an integer division lookup table if there is a division slowing down the inner loop.

• Store the results of complicated expressions in look-up tables.

Before:

pi = ATN(1) * 4
DO
FOR i = 0 to 360
x! = 100 + COS(i * pi / 180!)
y! = 100 + SIN(i * pi / 180!)
PSET (x!, y!), c
NEXT i
LOOP UNTIL LEN(INKEY\$)

After:

pi = ATN(1) * 4
Editor's note: I think some code is missing here...
• If several complicated expressions in a loop has common subexpressions, move the common subexpressions out of the loop.

Before:

FOR x = 1 to 32767
FOR y = 1 to 10
c = sin(x) * 30 + sqr(x) + y
NEXT y
NEXT x

After:

FOR x = 1 to 32767
xc = sin(x) * 30 + sqr(x)
FOR y = 1 to 10
c = xc + y
NEXT
NEXT
• Make constants CONST. Unfortunately, you can't use transcendental functions like ATN on the right side anymore.

Before:

pi = ATN(1) * 4
piover2 = pi / 2

After:

CONST pi = 3.14159265358979#
CONST piover2 = pi/2
• Unroll short loops.

Before:

FOR a = 1 to 8
POKE a, a
NEXT

After:

POKE 1,1
POKE 2,2
POKE 3,3
POKE 4,4
POKE 5,5
POKE 6,6
POKE 7,7
POKE 8,8
• Partially unroll long loops.

Before:

FOR x = 0 TO 319
POKE x,a
NEXT

After:

' this is a silly example, you should be using
' MMX filling or REP STOSB at least.
FOR x = 0 TO 319 STEP 4
POKE x, a
POKE x + 1, a
POKE x + 2, a
POKE x + 3, a
NEXT x
• Move junk outside of the inner loops (code movement).

Before:

FOR y = 0 TO 199
FOR x = 0 TO 319
a = x * 4 + COS(t)
b = y * 3 + SIN(t)
NEXT
NEXT

After:

FOR y = 0 TO 199
b = y * 3 + SIN(t)
FOR x = 0 TO 319
a = x * 4 + COS(t)
NEXT
NEXT
• Use cache sensitive programming. This means, try to access your arrays in a sequential manner if possible. If not, access them in small blocks that are adjacent to each other. For example, QB arrays are usually stored in a column major order, so dimension your arrays as vscreen(xmax,ymax) if you are doing scanline-based algorithms, and only change move in the x (scanline) direction in the inner loop.

Before:

'\$DYNAMIC
xmax = 319: ymax = 199
DIM buf(xmax, ymax)
FOR x = 0 TO xmax
FOR y = 0 TO ymax
buf(x, y) = INT(RND * 256)
NEXT
NEXT
DEF SEG

After:

'\$DYNAMIC
xmax = 319: ymax = 199
DIM buf(xmax, ymax)
FOR y = 0 TO ymax
FOR x = 0 TO xmax
buf(x, y) = INT(RND * 256)
NEXT
NEXT
DEF SEG
• Use a precalculated (canned) pseudo-random number sequence.

Before:

'main loop
FOR i = 1 TO 1000
x = INT(RND * 256)
y = INT(RND * 256)
c = INT(RND * 256)
PSET (x, y), c
NEXT i

After:

'precalculation
DIM rand(8191)
FOR i = 0 TO 8191
rand(i) = INT(RND * 256)
NEXT i

'main loop
count = 0
FOR i = 1 TO 1000
x = rand(count)
y = rand(count + 1)
c = rand(count + 2)
PSET (x, y), c
count = (count + 3) ' AND 8192 (needed in general)
NEXT i
• Prefer array indexing over user defined TYPEs. (1)

Warning: This makes code unreadable unless it is well commented.

• Cache often-used array elements in scalar variables. (2)

• Cache intermediate values into temporary variables. (3)

Example of both optimizations being used.

Before:

TYPE PtType
x AS INTEGER
y AS INTEGER
z AS INTEGER
END TYPE
TYPE TriType
pt1 AS INTEGER 'index of first point in points array
pt2 AS INTEGER 'index of second point in points array
pt3 AS INTEGER 'index of third point in points array
END TYPE
DIM points(numpoints, 1 TO 3) AS PtType
DIM tri(numtriangles) AS TriType
CONST screendist = 200
CONST lightx = 1, lighty = 0, lightz = 0

FOR i = 1 TO numtriangles
V1x = points(tri(i).pt2).x - points(tri(i).pt1).x
V2x = points(tri(i).pt3).x - points(tri(i).pt1).x
V1y = points(tri(i).pt2).y - points(tri(i).pt1).y
V2y = points(tri(i).pt3).y - points(tri(i).pt1).y
V1z = points(tri(i).pt2).z - points(tri(i).pt1).z
V2z = points(tri(i).pt3).z - points(tri(i).pt1).z

length1 = SQR(V1x * V1x + V1y * V1y + V1z + V1z)
length2 = SQR(V2x * V2x + V2y * V2y + V2z + V2z)

vx = V1y * V2z - V2y * V1z
vy = V2x * V1z - V1x * V2z
vz = V1x * V2y - V2x * V1y
CALL normalize(vx, vy, vz)
brightness = vx * lightx + vy * lighty + vz * lightz

xp1 = screendist * x1 / z1
yp1 = screendist * y1 / z1
xp2 = screendist * y1 / z1
yp2 = screendist * y2 / z2

'... and so on...
NEXT i

After:

' index 1 = x coordinate of point
' index 2 = y coordinate of point
' index 3 = z coordinate of point
DIM points(numpoints, 1 TO 3)
DIM tri(numtriangles, 1 TO 3)
CONST screendist = 200
CONST lightx = 1, lighty = 0, lightz = 0

FOR i = 1 TO numtriangles
x1 = points(tri(i, 1), 1) ' example of optimization 1
y1 = points(tri(i, 1), 2) ' and optimization 2
z1 = points(tri(i, 1), 3)
x2 = points(tri(i, 2), 1)
y2 = points(tri(i, 2), 2)
z2 = points(tri(i, 2), 3)
x3 = points(tri(i, 2), 1)
y3 = points(tri(i, 2), 2)
z3 = points(tri(i, 2), 3)
V1x = (x2 - x1): V2x = (x3 - x1)
V1y = (y2 - y1): V2y = (y3 - y1)
V1z = (z2 - z1): V2z = (z3 - z1)

length1 = SQR(V1x * V1x + V1y * V1y + V1z + V1z)
length2 = SQR(V2x * V2x + V2y * V2y + V2z + V2z)

vx = V1y * V2z - V2y * V1z
vy = V2x * V1z - V1x * V2z
vz = V1x * V2y - V2x * V1y
CALL normalize(vx, vy, vz)
brightness = vx * lightx + vy * lighty + vz * lightz

xp1 = screendist * x1 / z1
yp1 = screendist * y1 / z1
xp2 = screendist * y1 / z1
yp2 = screendist * y2 / z2

'... and so on...
NEXT i
• Use REDIM to clear a large array instead of using a FOR loop to set each element to zero.

Before:

DIM x(32000)
DO
FOR i = 0 TO 32000
x(i) = 0 'clear array slowly
NEXT i
x(RND * 32000) = 50
x(RND * 32000) = 93
LOOP UNTIL LEN(INKEY\$)

After:

DIM x(32000)
DO
REDIM x(32000) 'clear array faster
x(RND * 32000) = 50
x(RND * 32000) = 93
LOOP UNTIL LEN(INKEY\$)
• Avoid multidimensional arrays. Use array head lookup tables like in the POKE vs. PSET example for faster access of single dimension arrays as multidimensional ones.

Before:

DIM x(63, 63)

After:

DIM x(4095)
• Don't waste an extra element. Unlike C arrays, the declaration of QB arrays specify the first and last element indicies rather than the size of the array. This matters when you want to make a 64KB array without using '\$DYNAMIC.

Before:

DIM x(256, 256) 'allocate 66049 elements

After:

DIM x(0 TO 255, 0 TO 255) 'allocate 66536 elements

or

'OPTION BASE 0
DIM x(255, 255)
• Use incremental calculation instead of evaluating the entire equation every loop. This usually means multiplies will be replaced by addition. It's very important that you do this in any linear interpolation function you use for Gouraud Shading, Texture Mapping, etc. Most line DDAs (digital difference analyzers) use this method.

Before:

slope! = 0.1
FOR x = 0 TO max
y! = slope! * x
NEXT x

After:

slope! = 0.1
y! = 0
FOR x = 0 TO max
y! = y! + slope!
NEXT x
• Use POKE instead of PSET. This is a simple way to get 2x performance in graphics intensive apps.

Before:

CONST xmax = 319, ymax = 199, scansize& = 320
FOR i = 0 TO 255
PSET (i, 0), i
NEXT i
FOR i = 0 TO 255
PSET (i, 10), i
NEXT i

After:

CONST xmax = 319, ymax = 199, scansize& = 320
DIM ytab&(ymax)
FOR y = 0 to ymax
ytab&(y) = y * scansize&
NEXT y

DEF SEG = &HA000
FOR i = 0 TO 255
POKE i, i
NEXT i
FOR i = 0 TO 255
POKE ytab&(10) + i, i
NEXT i
DEF SEG
• Use INTEGER variables instead of LONGs for unsigned integers in the range 0 to 65535. This will only work when the program is compiled.

• PEEKing from video memory is slower than PEEKing from system memory. Therefore, use double buffering when you need to do feedback effects.

• Use DEF SEG sparingly. You don't need to DEF SEG back to the default segment when you are accessing arrays in the default segment. DEF SEG only applies to PEEK and POKE and SETMEM.

• Don't use '\$DYNAMIC. QB arrays in the default segment are accessed at blazing speed, because there is no segment switching. However, '\$DYNAMIC puts them in different segments, which need extra instructions to accessed, slowing them down. This makes a big difference in programs that use large lookup tables in their inner loop. It seems that huge arrays (allowed using the QB/AH command) are the slowest to access.

Before:

'\$DYNAMIC
DIM hugetable(319, 199)
FOR y = 0 TO 199
FOR x = 0 TO 319
xo = (x - 160) \ 2
yo = (y - 100) \ 2
hugetable(x, y) = xo * xo + yo * yo
NEXT
NEXT

After:

'\$STATIC
DIM hugetable1(319, 99)
DIM hugetable2(319, 99)

FOR y = 0 TO 99
FOR x = 0 TO 319
xo = (x - 160) \ 2
yo = (y - 100) \ 2
hugetable(x, y) = xo * xo + yo * yo
NEXT
NEXT
FOR y = 100 TO 199
FOR x = 0 TO 319
xo = (x - 160) \ 2
yo = (y - 100) \ 2
hugetable(x, y - 100) = xo * xo + yo * yo
NEXT
NEXT
• Use SELECT CASE instead of a bunch of ELSEIFs. The only exception is when one case executes much more often than the others.

Before:

IF i = 1 THEN
CALL DrawSprite
ELSEIF i = 6 THEN
CALL PlaySound
ELSEIF i > 9 AND i < 16 THEN
CALL Calculate(i)
ELSE
PRINT "."
END IF

After:

SELECT CASE i
CASE 1
CALL DrawSprite
CASE 6
CALL PlaySound
CASE 10 TO 15
CALL Calculate(i)
CASE ELSE
PRINT "."
END SELECT
• Use AND instead of MOD for MODing by a power of 2.

Before:

a = b MOD 64

After:

a = b AND 63
• Simplify compares against zero.

Before:

IF a <> 0 THEN
b% = b% - 1
END IF

After:

IF a% THEN  'note <>0 is gone
b% = b% - 1
END IF
• Use -x to find the negative of a number instead of -1*x. This is an obvious optimization if you know that the CPU has a NEG instruction, which is faster than IMUL.

• Don't put the main loop in the main code-- put it in a SUB. This makes a difference in the IDE, probably because the p-code interpreter has less variables to wade through when you are in a SUB.

• Use static storage for non-recursive SUB parameters. This makes very little improvement in speed, unless there are tons of variables passed to a SUB.

Before:

SUB drawcircle(x%,y%,r%)
'routine to draw a circle
END SUB

After:

SUB drawcircle(x%,y%,r%) STATIC
'routine to draw a circle
END SUB
• Pass dummy parameters to functions that take an odd number of arguments in order to improve data alignment. The dummy parameter is not used by the function, but is there to encourage burst memory writes. This only makes a minimal difference in speed.

Before:

CALL drawcircle(x%, y%, r%)

After:

CALL drawcircle(x%, y%, r%, dummy%)
• Don't initialize QB array elements to zero. Warning: this is a dangerous habit to get into, if you plan to use C or C++ later on. This is because C does not initialize variables by default.

Before:

DIM div320(32767)
FOR i = 0 TO 32767
div320(i) = i \ 320
NEXT

After:

DIM div320(32767)
FOR i = 320 TO 32767
div320(i) = i \ 320
NEXT
• For floating point, multiply by the reciprocal of a number instead of dividing by a number.

Before:

SUB normalize (x!, y!, z!)
norm! = SQR(x! * x! + y! * y! + z! * z!)
x! = x! / norm!
y! = y! / norm!
z! = z! / norm!
END SUB

After:

SUB normalize (x!, y!, z!)
recipnorm! = 1 / SQR(x! * x! + y! * y! + z! * z!)
x! = x! * recipnorm!
y! = y! * recipnorm!
z! = z! * recipnorm!
END SUB
• Simplify comparisons using simpler monotonic functions. Monotonic functions are functions that always grow upwards or always grow downwards. For example, x^2 is a monotonic function of x, so is 2*x. In the example, an expensive square root was removed by squaring both sides, since squaring is a monotonic function.

Before:

dist = SQR(x * x + y * y)
'inside circle
END IF

After: