perf: document zero-initialized scratch buffers in Rust fallback functions

src/itx.rs

@@ -95,6 +95,10 @@ fn inv_txfm_add<BD: BitDepth>(
     let row_clip_max = !row_clip_min;
     let col_clip_max = !col_clip_min;
 
+    // TODO(perf): This buffer is zero-initialized but only needs to be written
+    // before read, matching the uninitialized C counterpart in dav1d's itx_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut tmp = [0; 64 * 64];
     let mut c = &mut tmp[..];
     for y in 0..sh {
@@ -304,6 +308,10 @@ fn inv_txfm_add_wht_wht_4x4_rust<BD: BitDepth>(
 
     let coeff = &mut coeff[..W * H];
 
+    // TODO(perf): This buffer is zero-initialized but only needs to be written
+    // before read, matching the uninitialized C counterpart in dav1d's itx_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut tmp = [0; W * H];
     let mut c = &mut tmp[..];
     for y in 0..H {

src/looprestoration.rs

@@ -391,6 +391,10 @@ fn wiener_rust<BD: BitDepth>(
 ) {
     // Wiener filtering is applied to a maximum stripe height of 64 + 3 pixels
     // of padding above and below
+    // TODO(perf): These buffers are zero-initialized but only need to be written
+    // before read, matching the uninitialized C counterparts in dav1d's looprestoration_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut tmp = [0.into(); (64 + 3 + 3) * REST_UNIT_STRIDE];
 
     padding::<BD>(&mut tmp, p, left, lpf, lpf_off, w, h, edges);
@@ -650,6 +654,10 @@ fn selfguided_filter<BD: BitDepth>(
 
     // Selfguided filter is applied to a maximum stripe height of 64 + 3 pixels
     // of padding above and below
+    // TODO(perf): These buffers are zero-initialized but only need to be written
+    // before read, matching the uninitialized C counterparts in dav1d's looprestoration_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut sumsq = [0; (64 + 2 + 2) * REST_UNIT_STRIDE];
     // By inverting `a` and `b` after the boxsums, `b` can be of `BD::Coef` instead of `i32`.
     let mut sum = [0.as_::<BD::Coef>(); (64 + 2 + 2) * REST_UNIT_STRIDE];
@@ -803,6 +811,10 @@ fn sgr_5x5_rust<BD: BitDepth>(
 ) {
     // Selfguided filter is applied to a maximum stripe height of 64 + 3 pixels
     // of padding above and below
+    // TODO(perf): These buffers are zero-initialized but only need to be written
+    // before read, matching the uninitialized C counterparts in dav1d's looprestoration_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut tmp = [0.as_(); (64 + 3 + 3) * REST_UNIT_STRIDE];
 
     // Selfguided filter outputs to a maximum stripe height of 64 and a
@@ -867,6 +879,10 @@ fn sgr_3x3_rust<BD: BitDepth>(
     edges: LrEdgeFlags,
     bd: BD,
 ) {
+    // TODO(perf): These buffers are zero-initialized but only need to be written
+    // before read, matching the uninitialized C counterparts in dav1d's looprestoration_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut tmp = [0.as_(); (64 + 3 + 3) * REST_UNIT_STRIDE];
     let mut dst = [0.as_(); 64 * 384];
 
@@ -928,6 +944,10 @@ fn sgr_mix_rust<BD: BitDepth>(
     edges: LrEdgeFlags,
     bd: BD,
 ) {
+    // TODO(perf): These buffers are zero-initialized but only need to be written
+    // before read, matching the uninitialized C counterparts in dav1d's looprestoration_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut tmp = [0.as_(); (64 + 3 + 3) * REST_UNIT_STRIDE];
     let mut dst0 = [0.as_(); 64 * 384];
     let mut dst1 = [0.as_(); 64 * 384];

src/mc.rs

@@ -159,6 +159,10 @@ fn put_8tap_rust<BD: BitDepth>(
     if let Some(fh) = fh {
         if let Some(fv) = fv {
             let tmp_h = h + 7;
+            // TODO(perf): This buffer is zero-initialized but only needs to be written
+            // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+            // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+            // (only used when ASM is unavailable). See PR #1397 and #1399.
             let mut mid = [[0i16; MID_STRIDE]; 135]; // Default::default()
 
             for y in 0..tmp_h {
@@ -223,6 +227,10 @@ fn put_8tap_scaled_rust<BD: BitDepth>(
     let intermediate_bits = bd.get_intermediate_bits();
     let intermediate_rnd = (1 << intermediate_bits) >> 1;
     let tmp_h = ((h - 1) * dy + my >> 10) + 8;
+    // TODO(perf): This buffer is zero-initialized but only needs to be written
+    // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut mid = [[0i16; MID_STRIDE]; 256 + 7]; // Default::default()
 
     for y in 0..tmp_h {
@@ -284,6 +292,10 @@ fn prep_8tap_rust<BD: BitDepth>(
     if let Some(fh) = fh {
         if let Some(fv) = fv {
             let tmp_h = h + 7;
+            // TODO(perf): This buffer is zero-initialized but only needs to be written
+            // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+            // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+            // (only used when ASM is unavailable). See PR #1397 and #1399.
             let mut mid = [[0i16; MID_STRIDE]; 135]; // Default::default()
 
             for y in 0..tmp_h {
@@ -344,6 +356,10 @@ fn prep_8tap_scaled_rust<BD: BitDepth>(
 ) {
     let intermediate_bits = bd.get_intermediate_bits();
     let tmp_h = ((h - 1) * dy + my >> 10) + 8;
+    // TODO(perf): This buffer is zero-initialized but only needs to be written
+    // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut mid = [[0i16; MID_STRIDE]; 256 + 7]; // Default::default()
 
     for y in 0..tmp_h {
@@ -417,6 +433,10 @@ fn put_bilin_rust<BD: BitDepth>(
 
     if mx != 0 {
         if my != 0 {
+            // TODO(perf): This buffer is zero-initialized but only needs to be written
+            // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+            // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+            // (only used when ASM is unavailable). See PR #1397 and #1399.
             let mut mid = [[0i16; MID_STRIDE]; 129]; // Default::default()
             let tmp_h = h + 1;
 
@@ -479,6 +499,10 @@ fn put_bilin_scaled_rust<BD: BitDepth>(
 ) {
     let intermediate_bits = bd.get_intermediate_bits();
     let tmp_h = ((h - 1) * dy + my >> 10) + 2;
+    // TODO(perf): This buffer is zero-initialized but only needs to be written
+    // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut mid = [[0i16; MID_STRIDE]; 256 + 1];
 
     for y in 0..tmp_h {
@@ -523,6 +547,10 @@ fn prep_bilin_rust<BD: BitDepth>(
     let intermediate_bits = bd.get_intermediate_bits();
     if mx != 0 {
         if my != 0 {
+            // TODO(perf): This buffer is zero-initialized but only needs to be written
+            // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+            // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+            // (only used when ASM is unavailable). See PR #1397 and #1399.
             let mut mid = [[0i16; MID_STRIDE]; 129];
             let tmp_h = h + 1;
 
@@ -581,6 +609,10 @@ fn prep_bilin_scaled_rust<BD: BitDepth>(
 ) {
     let intermediate_bits = bd.get_intermediate_bits();
     let tmp_h = ((h - 1) * dy + my >> 10) + 2;
+    // TODO(perf): This buffer is zero-initialized but only needs to be written
+    // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut mid = [[0i16; MID_STRIDE]; 256 + 1];
 
     for y in 0..tmp_h {
@@ -828,6 +860,10 @@ fn warp_affine_8x8_rust<BD: BitDepth>(
     const H: usize = 15;
 
     let intermediate_bits = bd.get_intermediate_bits();
+    // TODO(perf): This buffer is zero-initialized but only needs to be written
+    // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut mid = [[0; W]; H];
 
     for y in 0..H {
@@ -879,6 +915,10 @@ fn warp_affine_8x8t_rust<BD: BitDepth>(
     const H: usize = 15;
 
     let intermediate_bits = bd.get_intermediate_bits();
+    // TODO(perf): This buffer is zero-initialized but only needs to be written
+    // before read, matching the uninitialized C counterpart in dav1d's mc_tmpl.c.
+    // Could use MaybeUninit to avoid zeroing, but this is a Rust fallback function
+    // (only used when ASM is unavailable). See PR #1397 and #1399.
     let mut mid = [[0; W]; H];
 
     for y in 0..H {