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<< CHAPTER 7 : EXTREME OPTIMISING >>
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This chapter is dedicated to 680x0 instruction optimising. In chapter 5 is
some information about higher level optimisations. This chapter is more like
a big appendix with a huge amount of small optimisation tips gathered from
here and there.
Now there are two rules when it comes to optimising instructions:
1) Shorter is "mostly" faster. If an instruction takes up 10 bytes and can
be replaced with one of only 2 bytes, the latter will be faster most of the
time.
2) Simple instructions are often the fastest. Instructions like multiplies,
subroutine jumps, traps, etc. have loads of functionality in them. Simple
instructions like add, move, etc (much used stuff) are optimised to run
in only a small number of cycles. (sometimes only 1 on a 68060).
3) On the 680x0 series, optimisation is mostly the case of getting the most
used constant and variable data in the registers instead of ram or
immediate data. This is a rule you must always stick to! The trick is to
store all values in registers when initialising a loop.
4) A good strategy has always been precalculating or precalcing as coders
say. Basicly the same as the preshifted sprites things, but a more
general term. Precalcing all kinds of bitmaps is fast because the heavy
calculations are done only once and the CPU is faster at moving bitmaps
then calculating bitmaps realtime.
Addressing:
When accessing the highmemory registers (videocontrol/colorpalette,
interrupt control) or lowmemory vars (the exceptionvectors or cookies). You
can use the word-based addressingmode instead of the long-based
$xxxx.l -> $xxxx.w
$ffffxxxx.l -> $xxxx.w
move.l a0,$00000000.l
=
move.l a0,$0000.w
move.l d0,$ffff8200.l
=
move.l d0,$ffff8200.w
Using programcounter-relative addressingmodes (pc-modes) can also be a bit
faster. Sometimes data can be nearby the programcounter. If it isn't more
than 32KB away you can use it. (I hate the word "pc")
move.w localvar(pc),d0
localvar:
DC.W 0
Branching:
All branch-type instructions (bsr, bra, bcc) offer short versions. This
means they are only as big as a two bytes instead of four bytes. Ofcourse
this label you branch to mustn't be more than 128 bytes away! AND a
shortbranch can never branch 0 bytes away!
bra.s locallabel
move.w #56,d0
locallabel:
move.w d0,d1
Moving:
Moving small immediate values into a register longword, can sometimes be
replaced by "moveq" or "move quickly". Values can only range from -128 to
127.
move.l #-128,d0
=
moveq #128,d0
A really nice trick to save up some bytes is:
move.l #$10000000,d0
=
moveq #1,d0
ror.l #4,d0
Saves 2 bytes =). Not much, but every byte counts when making 128bytro's.
When moving from memory into registers, the "momem" instruction is always
nice. If you're doing 3 or more word/long moves this is an ideal solution.
move.l (a0)+,d0
move.l (a0)+,d1
move.l (a0)+,d2
=
movem.l (a0)+,d0-d2
Ofcourse this can also be done for word-sizes:
move.w (a0)+,d0
move.w (a0)+,d1
move.w (a0)+,d2
ext.l d0
ext.l d1
ext.l d2
=
movem.w (a0)+,d0-d2
This has the added advantage of automaticly extending the words to longs!!
Can be very handy in some cases!
Add's:
Add's and sub's with small immediate data numbers can often be replaced with
"addq" and "subq". These mean "add quickly" and "subtract quickly" and
they're called that for a reason =)
add.l #1,d0
=
addq.l #1,d0
add.l #7,d0
=
addq.l #7,d0
sub.l #1,d0
=
subq.l #1,d0
sub.l #7,d0
=
subq.l #7,d0
Note, "subq.w" and "addq.w" are just a bit faster than "subq.l" and "addq.l"
on a standard 68000.
When using "addq" and "subq" with address registers there is no
speeddifference. Only note that there is a difference between adding with a
long and a word!!
adda.l d0,a0
is faster than:
adda.w d0,a0
Also, when doing an address increment/decrement with immediate data it's the
best idea to use "lea" for this. Ofcourse this again faster.
adda.w #3126,a0
=
lea 3126(a0),a0
But this isn't the only good use of lea..
movea.l a0,a1
adda.w #3126,a1
=
lea 3126(a0),a1
Cool, eh? A very fast and compact solution.
Anding:
When only having to modify the low word of a dataregister it might worth
considering only using a word for it. Byte sizes won't do any good, since
the instruction will be as big as the wordwise "and".
andi.l #$fffffff0,d0 * Slow.
=
andi.w #$fff0,d0 * Faster.
=
andi.b #$f0,d0 * Not faster or smaller.
Clearing:
clr.l/clr.w/clr.b on a register is dead stupid. Motorola made a serious
error in these instructions, so that they aren't that fast anymore. They
work, but should be avoided, because they are slow.
clr.l d0
=
moveq #0,d0
clr.l a0 * Slow.
=
suba.l a0,a0 * Faster.
=
movea.l d0,a0 * Fastest, when d0 already contains 0.
clr.w d0
=
sub.w d0,d0
clr.b d0
=
sub.b d0,d0
For clearing linear parts of memory the clr instruction can also be used.
It's fast enough for doing medium sized blocks and the big advantage is it
doesn't use up data registers. But, please note one thing..
clr.l (a0)+
is actually slower than:
clr.l -(a0)
When clearing big amounts of memory the movem.l command is unmissable.
Simply clear some data registers and movem.l them to the ram.
* Clear all free registers.. Yes.. addressegisters too.
moveq #10-1,d7 * Prepare to loop 100 times.
adda.l #100*14*4,a0 * Set a0 to top of block.
moveq #0,d0
moveq #0,d1
moveq #0,d2
moveq #0,d3
moveq #0,d4
moveq #0,d5
moveq #0,d6
movea.l d0,a1
movea.l d0,a2
movea.l d0,a3
movea.l d0,a4
movea.l d0,a5
movea.l d0,a6
loop: movem.l d0-d6/a1-a6,-(a0) * Move 13 longs to mem.
movem.l d0-d6/a1-a6,-(a0) * etc.
movem.l d0-d6/a1-a6,-(a0) * etc.
movem.l d0-d6/a1-a6,-(a0)
movem.l d0-d6/a1-a6,-(a0)
movem.l d0-d6/a1-a6,-(a0)
movem.l d0-d6/a1-a6,-(a0)
movem.l d0-d6/a1-a6,-(a0)
movem.l d0-d6/a1-a6,-(a0)
movem.l d0-d6/a1-a6,-(a0)
dbra d7,loop
Why "movem.l ...,-(a0)" instead of "movem.l ...,(a0)+"? Well.. because the
(a0)+ doesn't exist! =)
Testing:
After almost every move-/calculation- instruction the statusregister (sr)
takes on a conditioncode. The result of the operation (what is written in
the destination-operand) is tested and bits are set in the sr.
This means you don't always have to perform a test after an operation:
move.w d0,d1 * Copy d0.w to d1.w.
tst.w d1 * This is redundant!
=
move.w d0,d1 * Automaticly tests d1.w!
The same goes for all operations to memory. However, when operating with
address-registers as the destination, automatic testing is not done! If you
want to test values, you HAVE TO TEST with a seperate instruction.
move.l a0,a1 * Copy a0.l in a1.l!
cmpa.l #0,a1 * Test most be done!
Also note that the basic 68000 has no special "tst" instruction for the
address-registers. You have to do this with a compare to zero. The 68020 and
above however, do have a special "tst".
The most important bit is that with dataregister tests a copy-operation to
itself is faster than a "tst"!!
move.l d0,d0
is faster than:
tst.l d0
Alignment:
This memory moving-stuff brings us to alignment of longwords. With the
coming of the 68030 and it's burstcache, it is advisable to put big longword
buffers on a longword egde or a 16 byte egde (16 bytes = size of one 68030
cacheline). A longword operation on an address on a word boundary is 30%
slower than on a longword edge.
So when using large amounts of longs in ram, be sure to allocate all buffers
on 16 byte edges by ANDing addresses with #$fff0.
Swaps:
Swap is used to swap the highword and lowword of a dataregister. It is a
quite fast instruction. Or at least much faster than this:
moveq #16,d0
rol.l d0,d1
=
swap d1
Multiplies:
mulu/muls are costly instructions. Mostly ranging from 20 to 50 cycles (!)
mulu.w #2,d0
=
add.w d0,d0
mulu.w #4,d0
=
lsl.w #2,d0
mulu.w #3,d0
=
move.w d0,d1
add.w d0,d0
add.w d1,d0
mulu.w #7,d0
=
move.w d0,d1
lsl.w #3,d1
sub.w d0,d1
But beware.. Don't overdo this!! Trying to change for a multipication with a
too complex number you'll get too much moves, add's or sub's. and this will
especially on 030 and above cost more cycles than an actual multiply
instruction. The Pure C compiler mostly does make this mistake and the code
generated will be very big and also slower.
Multiply instructions do however have one major advantage.. They automaticly
extend words to longwords. Doing this in a move,add,sub combination costs
you an extra ext.l command.
Divides:
Divides are even more expensive than multiplies. They can sometimes be
replaced by simple shifts!! Whenever you divide with a power of 2
(2,4,8,16,32...) you can do this for instance.
divu.w #8,d1
=
lsr.w #3,d1
When this is possible, please do so.. It saves up a tremendous amount of
cycles. When you really need an awful amount of divisions it's best to
prepare your data for shifting instead of divides.
Divide instructions do also have one advantage. They automaticly perform a
modulo function as well! The modulo from a division is stored in the
highword of the destination register.
divu.w #10,d0 * Perform division and modulo.
swap d0 * Get highword in lowword.
Sometimes fixed-point divides can be replaced by multiplies. Instead of
first adjusting values for fixedpoint divisions and then doing expensive
divides, you can also use only one multiply!
swap d0 * Shift d0.w up 16 bits.
divu.w #3,d0 * Divide it.
=
mulu.w #$5555,d0 * Mulitply d0.w with 1/3.
Modulo:
Which brings us to our next topic. When trying to get the modulo of a
number, you can again make this a modulo function with a power of 2.
andi.w #4-1,d0 * = d0 MOD 4
andi.w #8-1,d0 * = d0 MOD 8
An even better option is making this a modulo function with 256 or 65536.
Why? This is exactly the range of the byte/word unit.
* Perform calc on d0 here...
move.b d0,d1 * = d0 MOD 256
* Perform calc on d0 here
move.w d0,d1 * = d0 MOD 65536
Shifting:
Now you understand that a combination of shifts and moves is actually faster
than a multiply (except on 68060, which has razorfast multiplying). Shifting
left by one can be replaced with a simple "add".
lsl.l #1,d0
=
add.l d0,d0
asl.l #1,d0
=
add.l d0,d0
On a 68000 upto 68030 the shift instruction takes up 8 cycles and an "add"
takes up 4 on 68000 and only 2 on 68030.
With 680x0 coding you're often confronted with having to shift right or left
8 bits. This because you need a byte in the lowest part of the register for
instance. Sometimes this can be avoided with "addx" loops as shown in
chapter 6.
In some other situations, where you need to spilt up a word into two
seperate bytes spread over a longword (i.e. 0000xxyy -> 00xx00yy), you can
use the "movep"-instruction. A movep must have a data-register as one
operand and a memory-address as the other.
For instance:
movep.w d0,(a0) * 0000xxyy -> 00xx00yy
or:
movep.w (a0),d0 * 00xx00yy -> 0000xxyy
The weird thing about the instruction was, that it was originally meant as
an easier way to access the hardware-registers (often using bytes instead of
words), but you really don't need it for that so often as you'd need it for
optimising bitplane copying.
Offsets:
The 680x0 offers the lovely (an,dn) addressingmode.
adda.l d0,a0
move.w d1,(a0)
suba.l d0,a0
=
move.w d1,0(a0,d0.l)
Ofcourse this is only useful in particular cases. When you need a different
offset for every move to an address, this offset addressingmode is
unbeatable.
From the 68020 and on the scaled offset addressingmode is at your disposal.
This is absolutely gorgeous.
move.l d0,d2
lsl.l #3,d0
adda.l d0,a0
move.w d1,(a0)
suba.l d0,a0
move.l d2,d0
=
move.w d1,0(a0,d0.l*8)
There is another big advantage to these addressingmodes.. They can be used
by "lea" as well!
move.l d0,d1
lsl.l #3,d0
movea.l a0,a1
adda.l d0,a1
move.l d1,d0
=
lea (a0,d0.l*8),a1
Stack moves:
As most demo-coders.. I hate the stack. But sometimes when moving immediate-
data onto it you can optimise the lot.
move.w #1,-(sp)
move.w #2,-(sp)
=
move.l #$00020001,-(sp)
move.l #$00001000,-(sp)
=
pea $1000.w
Selfmodifying code:
Please people, avoid this technique. It will only work on simple 68000 or on
>68020 with the caches turned off. Still, on a basic 8MHz 68000 it might be
considering if you absolutely want to get everything out of your machine.
Selfmodifying code is mostly used in loops where you don't have enough
registers left (i.e. used up all 8 data and 7 address registers). Using
variables in RAM is a poor solution since this slows down, quite alot
compared to using registers.
The technique relies on immediate data instructions and modifying the
immediate data everytime you need to change the variable. This means loading
the address of the immediate data field of the instruction and changing the
value.
But selfmodifying code hasn't been used by most coders for quite a while now
and that's a good thing. It makes code horribly unreadable as well as
totally unpredictable and incompatible with newer CPU's.
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