Use hand written fast fill to save a few more cycles

agb/src/sound/mixer/mixer.s

@@ -136,12 +136,34 @@ agb_arm_func agb_rs__mixer_add_stereo
 
 agb_arm_end agb_rs__mixer_add_stereo
 
+.section .iwram
+    .balign 4
+constant_zero:
+.rept 4
+    .word 0
+.endr
+
 agb_arm_func agb_rs__mixer_collapse
     @ Arguments:
     @ r0 = target buffer (i8)
     @ r1 = input buffer (i16) of fixnums with 4 bits of precision (read in sets of i16 in an i32)
     push {r4-r11}
 
+    @ zero registers r4-r7 (4 of them)
+    mov r8, #constant_zero
+    ldm r8, {r4-r7}
+
+    @ get the size of the buffer
+    ldr r9, agb_rs__buffer_size
+    @ make a copy of the output buffer pointer
+    mov r10, r0
+1:
+    @ zero 4 words worth of the output buffer
+    stmia r10, {r4-r7}
+    subs r9, r9, #16
+    @ loop if we haven't zeroed everything
+    bne 1b
+
 CONST_0   .req r7
 CONST_FF  .req r8
 CONST_127 .req r9

agb/src/sound/mixer/sw_mixer.rs

@@ -5,7 +5,7 @@ use bare_metal::{CriticalSection, Mutex};
 use super::hw;
 use super::hw::LeftOrRight;
 use super::{SoundChannel, SoundPriority};
-use crate::syscall::cpu_fast_fill_i8;
+
 use crate::{
     fixnum::Num,
     interrupt::free,
@@ -245,9 +245,6 @@ impl MixerBuffer {
                 channel.playback_speed
             };
 
-            let right_amount = ((channel.panning + 1) / 2) * channel.volume;
-            let left_amount = ((-channel.panning + 1) / 2) * channel.volume;
-
             if (channel.pos + playback_speed * constants::SOUND_BUFFER_SIZE).floor()
                 >= channel.data.len()
             {
@@ -268,6 +265,9 @@ impl MixerBuffer {
                     );
                 }
             } else {
+                let right_amount = ((channel.panning + 1) / 2) * channel.volume;
+                let left_amount = ((-channel.panning + 1) / 2) * channel.volume;
+
                 unsafe {
                     agb_rs__mixer_add(
                         channel.data.as_ptr().add(channel.pos.floor()),
@@ -285,7 +285,6 @@ impl MixerBuffer {
         let write_buffer_index = free(|cs| self.state.borrow(cs).borrow_mut().active_advanced());
 
         let write_buffer = &mut self.buffers[write_buffer_index].0;
-        cpu_fast_fill_i8(write_buffer, 0);
 
         unsafe {
             agb_rs__mixer_collapse(write_buffer.as_mut_ptr(), buffer.as_ptr());

agb/src/syscall.rs

@@ -120,29 +120,6 @@ pub fn arc_tan2(x: i16, y: i32) -> i16 {
     result
 }
 
-pub(crate) fn cpu_fast_fill_i8(input: &mut [i8], new_content: i32) {
-    assert_eq!(
-        input.len() % (4 * 8),
-        0,
-        "Input length must be divisible by 32"
-    );
-
-    let input_ptr = [new_content].as_ptr();
-    let output_ptr = input.as_mut_ptr();
-    let length_mode = (1 << 24) | // copy
-        (input.len() / 4);
-
-    unsafe {
-        asm!(
-            "swi 0x0c",
-            in("r0") input_ptr,
-            in("r1") output_ptr,
-            in("r2") length_mode,
-            lateout("r3") _,
-        );
-    }
-}
-
 // pub fn affine_matrix(
 //     x_scale: Num<i16, 8>,
 //     y_scale: Num<i16, 8>,