Use asm for the tight loops in the sound mixer

agb/build.rs

@@ -4,6 +4,7 @@ fn main() {
     println!("cargo:rerun-if-changed=crt0.s");
     println!("cargo:rerun-if-changed=gba_mb.ld");
     println!("cargo:rerun-if-changed=interrupt_simple.s");
+    println!("cargo:rerun-if-changed=src/sound/mixer/mixer.s");
     println!("cargo:rerun-if-changed=gfx/test_logo.png");
 
     let out_file_name = "crt0.o";

agb/crt0.s

@@ -63,3 +63,4 @@ b .Initialise_mb
 .pool
 
 .include "interrupt_simple.s"
+.include "src/sound/mixer/mixer.s"

agb/examples/mixer_basic.rs

@@ -3,10 +3,10 @@
 
 extern crate agb;
 
+use agb::input::{Button, ButtonController, Tri};
+use agb::number::Num;
 use agb::sound::mixer::SoundChannel;
 use agb::Gba;
-use agb::input::{ButtonController, Tri, Button};
-use agb::number::Num;
 
 // Music - "I will not let you let me down" by Josh Woodward, free download at http://joshwoodward.com
 const I_WILL_NOT_LET_YOU_LET_ME_DOWN: &[u8] = include_bytes!("i-will-not-let-you-let-me-down.raw");

agb/src/number.rs

@@ -91,6 +91,7 @@ fixed_width_unsigned_integer_impl!(usize);
 fixed_width_signed_integer_impl!(i16);
 fixed_width_signed_integer_impl!(i32);
 
+#[repr(C)]
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct Num<I: FixedWidthUnsignedInteger, const N: usize>(I);
 

agb/src/sound/mixer/mixer.s

@@ -0,0 +1,98 @@
+.arm
+.global agb_rs__mixer_add
+.section .iwram, "ax"
+.align
+agb_rs__mixer_add:
+    @ Arguments
+    @ r0 - pointer to the data to be copied (u8 array)
+    @ r1 - pointer to the sound buffer (i16 array)
+    @ r2 - playback speed (usize fixnum with 8 bits)
+    @ r3 - amount to modify the left channel by (u16 fixnum with 4 bits)
+    @ stack position 1 - amount to modify the right channel by (u16 fixnum with 4 bits)
+    @
+    @ The sound buffer must be SOUND_BUFFER_SIZE * 2 in size = 176 * 2
+
+    @ lr = amount to modify right channel by
+
+    push {r4-r10, lr}
+
+    @ load the right channel modification amount into lr
+    ldr lr, [sp, #32]
+
+    @ current write offset into the resulting buffer
+    mov r12, #0
+    mov r8, #352
+
+    @ current index
+    mov r5, #0
+
+1:
+    @ load the current sound buffer location
+    mov r6, r1
+    
+    @ calculate the address of the next read form the sound buffer
+    add r4, r0, r5, asr #8
+    
+    @ calculate the position to read the next step from
+    add r5, r5, r2
+
+    @ load the current buffer value (r6 being the current location, r12 being the offset)
+    @ but pre-increment r6 by r12
+    ldrh r9, [r6, r12]!
+
+    @ load the current value we want to read
+    ldrsb r10, [r4]
+
+    @ increment the current write offset in the resulting buffer
+    add r12, r12, #2
+
+    @ check if we're done
+    cmp r12, #352
+
+    @ r7 = r10 * r3 + r9 = current sound value * left amount + previous buffer value
+    mla r7, r10, r3, r9
+    @ *(r6 + r8) = r7, r8 = 352 = offset for the right hand side
+    strh r7, [r6], r8
+
+    @ same for the left hand side (slightly confused here, but this is what was generated)
+    ldrh r7, [r6]
+    mla r4, r10, lr, r7
+    strh r4, [r6]
+
+    bne 1b
+
+    pop {r4-r10, lr}
+    bx lr
+.pool
+
+.arm
+.global agb_rs__mixer_collapse
+.section .iwram
+.align
+agb_rs__mixer_collapse:
+    @ Arguments:
+    @ r0 = target buffer (i8)
+    @ r1 = input buffer (i16) of fixnums with 4 bits of precision
+
+    mov r2, #0
+
+1:
+    @ r12 = *r1; r1++
+    ldrsh r12, [r1], #2
+    
+    lsr r3, r12, #4     @ r3 = r12 >> 4
+
+    cmn r12, #2048      @ compare r12 against -2048
+    mvnlt r3, #127      @ r3 = -127 if r12 <= 2048
+
+    cmp r12, #2048      @ compare r12 against 2048
+    movge r3, #127      @ r3 = 127 if r12 >= 2048
+
+    strb r3, [r0, -r2]  @ r2 counts down, so need a negative offset
+
+    sub r2, r2, #1      @ r2 -= 1
+    cmn r2, #352        @ compare r2 against -352
+
+    bne 1b              @ loop if not equal
+
+    bx lr

agb/src/sound/mixer/sw_mixer.rs

@@ -3,6 +3,19 @@ use super::hw::LeftOrRight;
 use super::{SoundChannel, SoundPriority};
 use crate::number::Num;
 
+// Defined in mixer.s
+extern "C" {
+    fn agb_rs__mixer_add(
+        sound_data: *const u8,
+        sound_buffer: *mut Num<i16, 4>,
+        playback_speed: Num<usize, 8>,
+        left_amount: Num<i16, 4>,
+        right_amount: Num<i16, 4>,
+    );
+
+    fn agb_rs__mixer_collapse(sound_buffer: *mut i8, input_buffer: *const Num<i16, 4>);
+}
+
 pub struct Mixer {
     buffer: MixerBuffer,
     channels: [Option<SoundChannel>; 16],
@@ -131,18 +144,21 @@ impl MixerBuffer {
                 }
             }
 
-            for i in 0..SOUND_BUFFER_SIZE {
-                let v = (channel.data[channel.pos.floor()] as i8) as i16;
-                channel.pos += channel.playback_speed;
-
-                buffer[i] += left_amount * v;
-                buffer[i + SOUND_BUFFER_SIZE] += right_amount * v;
+            unsafe {
+                agb_rs__mixer_add(
+                    channel.data.as_ptr().offset(channel.pos.floor() as isize),
+                    buffer.as_mut_ptr(),
+                    channel.playback_speed,
+                    left_amount,
+                    right_amount,
+                );
             }
+            channel.pos += channel.playback_speed * SOUND_BUFFER_SIZE;
         }
 
         let write_buffer = self.get_write_buffer();
-        for i in 0..SOUND_BUFFER_SIZE * 2 {
-            write_buffer[i] = buffer[i].floor().clamp(i8::MIN as i16, i8::MAX as i16) as i8
+        unsafe {
+            agb_rs__mixer_collapse(write_buffer.as_mut_ptr(), buffer.as_ptr());
         }
     }