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Reduce stereo mixing by 1 instruction

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This commit is contained in:
Gwilym Kuiper 2021-10-26 23:09:50 +01:00
parent 9524b4b3c3
commit 1d38b75289
1 changed files with 4 additions and 4 deletions
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8
agb/src/sound/mixer/mixer.s
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@ -109,11 +109,11 @@ agb_arm_func agb_rs__mixer_add_stereo
ldr r4, [r1] @ read the current value
@ This is slightly convoluted, but is mainly done for performance reasons. It is better
@ to hit ROM just once and then do 4 really simple instructions then do 2 ldrsbs however annoying
@ to hit ROM just once and then do 3 really simple instructions then do 2 ldrsbs however annoying
@ this is. Also, since all this code is in IWRAM and we never hit ROM otherwise, all accesses
@ are sequential and exactly the size of the bus to ROM (16 bits), so hopefully this will be super fast.
@
@ The next 4 instructions set up the current value in r6 to be in the expected format
@ The next 3 instructions set up the current value in r6 to be in the expected format
@ 1 = 2s complement marks (so if negative, these are all 1s, if positive these are 0s)
@ L = the left sample
@ R = the right sample
@ -122,8 +122,8 @@ agb_arm_func agb_rs__mixer_add_stereo
@
@ At this point
@ r6 = | 1 | 1 | L | R | where the upper bytes are 1s if L is negative. No care about R
mov r7, r6, asr #8 @ r7 = | 1 | 1 | 1 | L | drop R off the right hand side
and r7, r7, r5 @ r7 = | 0 | 0 | 1 | L | exactly what we want this to be. The mask puts the 1 as 00001111 ready for the shift later
@ asr #8 | 1 | 1 | 1 | L | drop R off the right hand side
and r7, r5, r6, asr #8 @ r7 = | 0 | 0 | 1 | L | exactly what we want this to be. The mask puts the 1 as 00001111 ready for the shift later
lsl r6, r6, #24 @ r6 = | R | 0 | 0 | 0 | drop everything except the right sample
orr r6, r7, r6, asr #8 @ r6 = | 1 | R | 1 | L | now we have it perfectly set up