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z80_unpacker: optimisations: shorter >>4 in probs update

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Peter Helcmanovsky (Ped)
2022-09-13 23:15:18 +02:00
parent a19ec2abb7
commit ea5c0b1b15
1 changed files with 5 additions and 8 deletions
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13
z80_unpacker/unpack.asm
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@@ -239,19 +239,16 @@ decode_bit:
jr c,.bit_is_1
ld d,-16 ; 0xF0
.bit_is_1: ; D:E = -prob_offset:prob, A = prob
;FIXME and + 4x rra will be probably shorter!
srl a
srl a
srl a
srl a
and $F8
rra
rra
rra
rra
adc a,d ; A = -prob_offset + ((prob + 8) >> 4)
neg
add a,e ; A = prob_offset + prob - ((prob + 8) >> 4)
ld (bc),a ; update probs[context_index]
pop af ; restore resulting CF = bit
; TODO: check if it's possible to `cpl` instead of neg, have +1 on original prob,
; and get correct CF=bit from `add a,e` then (without extra push+pop AF)
; !!! I think this will **NOT** work, because clamping of prob ends with +-0 at both ends (cpl 0 -> 255 -> CF=1)
pop de
ret