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improve safety of blit()

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bendn 2024-06-25 21:34:39 +07:00
parent 8649f3c2a2
commit 4082ca3dda
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2 changed files with 17 additions and 27 deletions
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1
src/lib.rs
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@ -58,7 +58,6 @@
iter_array_chunks,
core_intrinsics,
slice_as_chunks,
slice_flatten,
rustc_private,
portable_simd,
array_windows,

43
src/overlay.rs
View file

@ -70,45 +70,36 @@ pub trait ClonerOverlay<const W: usize, const C: usize>: Sealed {
/// # Safety
/// - UB if rgb.len() % 3 != 0
/// - UB if rgba.len() % 4 != 0
unsafe fn blit(rgb: &mut [u8], rgba: &[u8]) {
let mut srci = 0;
let mut dsti = 0;
while dsti + 16 <= rgb.len() {
// SAFETY: i think it ok
let old: Simd<u8, 16> = Simd::from_slice(unsafe { rgb.get_unchecked(dsti..dsti + 16) });
// SAFETY: definitly ok
let new: Simd<u8, 16> = Simd::from_slice(unsafe { rgba.get_unchecked(srci..srci + 16) });
unsafe fn blit(mut rgb: &mut [u8], mut rgba: &[u8]) {
while rgb.len() >= 16 {
let dst = rgb.first_chunk_mut::<16>().unwrap();
let src = rgba.first_chunk::<16>().unwrap();
let old = Simd::from_slice(dst);
let new: u8x16 = Simd::from_slice(src);
let threshold = new.simd_ge(Simd::splat(128)).to_int().cast::<u8>();
let mut mask = simd_swizzle!(
threshold,
// [r, g, b, a (3)] [r, g, b, a(7)]
[3, 3, 3, 7, 7, 7, 11, 11, 11, 15, 15, 15, 0, 0, 0, 0]
);
mask &= Simd::from_array([
255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 0, 0, 0, 0,
]);
// [r(0), g, b] <skip a> [r(4), g, b]
let new_rgb = simd_swizzle!(new, [0, 1, 2, 4, 5, 6, 8, 9, 10, 12, 13, 14, 0, 0, 0, 0]);
let blended = (new_rgb & mask) | (old & !mask);
// SAFETY: 4 * 4 == 16, so in bounds
blended.copy_to_slice(unsafe { rgb.get_unchecked_mut(dsti..dsti + 16) });
srci += 16;
dsti += 12;
blended.copy_to_slice(dst);
rgb = &mut rgb[12..];
rgba = &rgba[16..];
}
while dsti + 3 <= rgb.len() {
// SAFETY: caller guarantees slice is big enough
if unsafe { *rgba.get_unchecked(srci + 3) } >= 128 {
// SAFETY: slice is big enough!
let src = unsafe { rgba.get_unchecked(srci..=srci + 2) };
// SAFETY: i hear it bound
let end = unsafe { rgb.get_unchecked_mut(dsti..=dsti + 2) };
end.copy_from_slice(src);
while rgb.len() >= 3 {
// SAFETY: guaranteed
if unsafe { *rgba.get_unchecked(3) } >= 128 {
rgb[..3].copy_from_slice(&rgba[..3]);
}
srci += 4;
dsti += 3;
rgba = &rgba[4..];
rgb = &mut rgb[3..];
}
}