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Special case mixer without multiplication

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This commit is contained in:
Gwilym Kuiper 2021-10-03 23:00:43 +01:00
parent f6f6f4d22e
commit 871d90c889
1 changed files with 50 additions and 0 deletions
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50
agb/src/sound/mixer/mixer.s
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@ -13,11 +13,16 @@ agb_arm_func agb_rs__mixer_add
ldr r7, [sp, #20] @ load the right channel modification amount into r7
cmp r7, r3 @ check if left and right channel need the same modifications
beq same_modification
modifications_fallback:
orr r7, r7, r3, lsl #16 @ r7 now is the left channel followed by the right channel modifications.
mov r5, #0 @ current index we're reading from
mov r8, #SOUND_BUFFER_SIZE @ the number of steps left
1:
.macro mixer_add_loop
add r4, r0, r5, asr #8 @ calculate the address of the next read from the sound buffer
@ -41,6 +46,51 @@ agb_arm_func agb_rs__mixer_add
pop {r4-r8}
bx lr
same_modification:
@ check to see if this is a perfect power of 2
@ r5 is a scratch register, r7 = r3 = amount to modify
sub r5, r7, #1
ands r5, r5, r7
bne modifications_fallback @ not 0 means we need to do the full modification
@ count leading zeros of r7 into r3
mov r3, #0
1:
add r3, r3, #1
lsrs r7, r7, #1
bne 1b
mov r5, #0 @ current index we're reading from
mov r8, #SOUND_BUFFER_SIZE @ the number of steps left
.macro mixer_add_loop_simple
add r4, r0, r5, asr #8 @ calculate the address of the next read from the sound buffer
ldrsb r6, [r4] @ load the current sound sample to r6
add r5, r5, r2 @ calculate the position to read the next sample from
ldr r4, [r1] @ read the current value
lsl r6, r6, #16
orr r6, r6, lsr #16
add r4, r4, r6, lsl r3 @ r4 += r6 << r3 (calculating both the left and right samples together)
str r4, [r1], #4 @ store the new value, and increment the pointer
.endm
1:
mixer_add_loop_simple
mixer_add_loop_simple
mixer_add_loop_simple
mixer_add_loop_simple
subs r8, r8, #4 @ loop counter
bne 1b @ jump back if we're done with the loop
pop {r4-r8}
bx lr
agb_arm_end agb_rs__mixer_add
.macro clamp_s8 reg:req