perf: fn rav1d_create_lf_mask_{intra,inter}: store as u16s

[kkysen]

Previously, two adjacent 8-bit stores were done. By doing them as u16s instead of [u8]s/[u8; 2]s, we can get a single 16-bit store to be emitted instead.

This gets strhs emitted instead of the pair of strbs previously.

Based on what @iximeow found in iximeow@e133a89 and #1401 (comment).

Before:

❯ cargo asm -p rav1d --lib --rust rav1d_create_lf_mask_intra --color | rg filter_level -C 10
                if start <= end && end <= len {
        cmp x13, x12
        b.hi .LBB440_79
                // /home/khyber/work/rav1d/src/lf_mask.rs : 458
                let lvl = &mut *level_cache.index_mut((idx + 0.., ..2));
        ldr x12, [x1, #8]
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        subs x7, x7, #1
                // /home/khyber/work/rav1d/src/lf_mask.rs : 459
                lvl[0] = filter_level[0][0][0];
        add x12, x12, x11
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        add x11, x11, #4
                // /home/khyber/work/rav1d/src/lf_mask.rs : 459
                lvl[0] = filter_level[0][0][0];
        strb w15, [x12]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 460
                lvl[1] = filter_level[1][0][0];
        strb w16, [x12, #1]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 455
                for x in 0..bw4 {
        b.ne .LBB440_4
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        cmp x6, x20
                // /home/khyber/work/rav1d/src/lf_mask.rs : 454
                for _y in 0..bh4 {
        add x17, x17, x18
--
                // /home/khyber/work/rav1d/src/disjoint_mut.rs : 729
                if start <= end && end <= len {
        add x20, x3, x19
        add x21, x20, #3
        cmp x21, x2
        b.hs .LBB440_80
                // /home/khyber/work/rav1d/src/lf_mask.rs : 496
                let lvl = &mut *level_cache.index_mut((idx + 2.., ..2));
        ldr x2, [x1, #8]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 497
                lvl[0] = filter_level[2][0][0];
        add x2, x2, x3
        add x2, x2, x19
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        add x19, x19, #4
                // /home/khyber/work/rav1d/src/lf_mask.rs : 493
                for x in 0..cbw4 {
        cmp x18, x19
                // /home/khyber/work/rav1d/src/lf_mask.rs : 497
                lvl[0] = filter_level[2][0][0];
        strb w16, [x2, #2]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 498
                lvl[1] = filter_level[3][0][0];
        strb w17, [x2, #3]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 493
                for x in 0..cbw4 {
        b.ne .LBB440_72
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        cmp x6, x4
                // /home/khyber/work/rav1d/src/lf_mask.rs : 492
                for _y in 0..cbh4 {
        add x3, x3, x5

After:

❯ cargo asm -p rav1d --lib --rust rav1d_create_lf_mask_intra --color | rg filter_level -C 10
        ldr x24, [sp, #224]
        ldrb w10, [sp, #184]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 447
                let b_dim = b_dim.map(|it| it as usize);
        ldr w8, [x9, x8, lsl #2]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 448
                let bw4 = cmp::min(iw - bx, b_dim[0]);
        sub x9, x25, x28
        ldrb w6, [sp, #176]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 453
                let filter_level_yuv = filter_level.0.map(|a| a[0][0]);
        ldrb w23, [x3, #32]
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/array/mod.rs : 160
                }
        ldrb w14, [x3, #48]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 450
                let bx4 = bx & 31;
        and x30, x28, #0x1f
                // /home/khyber/work/rav1d/src/lf_mask.rs : 447
                let b_dim = b_dim.map(|it| it as usize);
        and x26, x8, #0xff
--
                let lvl = &mut *level_cache.index_mut((idx + 0.., ..2));
        ldr x12, [x1, #8]
        add w13, w11, w12
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/option.rs : 1001
                match self {
        tbnz w13, #0, .LBB440_21
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        subs x3, x3, #1
                // /home/khyber/work/rav1d/src/lf_mask.rs : 463
                *u16::mut_from(lvl).unwrap() = filter_level_y;
        strh w15, [x12, x11]
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        add x11, x11, #4
                // /home/khyber/work/rav1d/src/lf_mask.rs : 459
                for x in 0..bw4 {
        b.ne .LBB440_4
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        cmp x18, x20
--
        tbnz w19, #0, .LBB440_82
        add x2, x2, x17
        add x2, x2, x21
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        add x21, x21, #4
                // /home/khyber/work/rav1d/src/lf_mask.rs : 496
                for x in 0..cbw4 {
        cmp x16, x21
                // /home/khyber/work/rav1d/src/lf_mask.rs : 500
                *u16::mut_from(lvl).unwrap() = filter_level_uv;
        strh w15, [x2, #2]
                // /home/khyber/work/rav1d/src/lf_mask.rs : 496
                for x in 0..cbw4 {
        b.ne .LBB440_76
                // /home/khyber/.rustup/toolchains/nightly-2025-05-01-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/cmp.rs : 1849
                fn lt(&self, other: &Self) -> bool { *self <  *other }
        cmp x5, x4
                // /home/khyber/work/rav1d/src/lf_mask.rs : 495
                for _y in 0..cbh4 {
        add x17, x17, x18

[kkysen]

@iximeow, @coderkalyan, if you want to review.

[coderkalyan]

Pending the open comments, LGTM.

[coderkalyan]

Will try to take a look at this tomorrow!

[coderkalyan]

Oops, forgot about this - but with the AlignedVec2, LGTM!