Code optimization challenge

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marste
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Joined: Sun Aug 10, 2014 8:58 pm
Location: Italy
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Code optimization challenge

Post by marste » Wed Oct 04, 2017 9:50 pm

As the last time with "waiting some time" solution (viewtopic.php?t=1448), I hope to receive interesting idea to optimize a piece of code:

The objective is to do some instructions optimizing space (and time also where possible),

In particular DO "dec d" and "ld c',l" (the alternate c register with the standard l, that require to make additional ld),
JUST IF "(b=x1xx1xxx and l=z010xxxx) or (b=x1xx0xxx and l=z101xxxx)" - where x means whatever value, z means that the value there is known a priori and is zero, and 0 and 1 means that that bits should be exactly that values

Register a and flags can be destroied together with hl' (the alternate one), the others should keep the original values.

Note: I can swap the usage of the 6th bit of b with one of the other of the higher nibble (7th-4th) if useful for the solution.

Initial attempt pretty long is:

Code: Select all


    bit 6,b
    jr z,skip
    ld a,l
    and $70
    bit 3,b
    jr nz,check2
    sub $50
    jr z,doit
    jr skip
check2:
    sub $20
    jr nz,skip

doit:
    ld a,l
    exx
    ld c,a
    exx
    dec d

skip:
Happy hacking! :)

User avatar
marste
Posts: 97
Joined: Sun Aug 10, 2014 8:58 pm
Location: Italy
Contact:

Re: Code optimization challenge

Post by marste » Thu Oct 05, 2017 5:22 pm

Some improvements:

1 byte less with

Code: Select all

    ld a,b
    and 01001000b
    xor 01001000b
    jr z,point1
    xor 00001000b
    jr nz,skip
    ld a,01110000b
point1:
    xor 01010000b
    xor l
    and 01110000b
    jr nz,skip
doit:
    ld a,l
    exx
    ld c,a
    exx
    dec d
skip:
Changing the b 6th bit with 7th, 4 byte less:

Code: Select all

    ld a,b
    rla
    jr nc,skip
    and 00010000b
    jr nz,point1
    ld a,01100000b ; a was 0
point1:
    xor 01010000b ; a was 00010000b from jump
    xor l
    and 01110000b
    jr nz,skip
doit:
    ld a,l
    exx
    ld c,a
    exx
    dec d
skip:
I'm sure it can be done better... :)

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