daedalus-fourier/tests/cdef_ref.c

/*
 * Standalone bit-exact C reference for AV1 CDEF filter, 8x8 luma 8bpc,
 * combined primary + secondary path.
 *
 * Algorithm transcribed from dav1d's `cdef_filter_block_c` in
 * src/cdef_tmpl.c (vendored at external/dav1d-snapshot/, tag 1.4.3).
 *
 * **Layout note (cycle 5 phase 3 finding):** dav1d's NEON expects
 * tmp with stride 16 (uint16 elements), not stride 12 like the C
 * reference uses. The NEON has its own directions table baked at
 * stride 16 in src/arm/64/cdef_tmpl.S `dir_table 8, 16`. The C
 * reference uses stride 12 and the table in src/tables.c.
 *
 * To compare bit-exact against NEON, this standalone C ref uses
 * NEON's stride-16 layout + its embedded directions table. Same
 * algorithm, different stride convention than dav1d's C path.
 *
 * Signature mirrors the dav1d NEON convention:
 *   void(uint8_t *dst, ptrdiff_t dst_stride, const uint16_t *tmp,
 *        int pri_strength, int sec_strength,
 *        int dir, int damping, int h);
 *
 * tmp is a (12 rows × 16 cols × uint16) padded buffer, stride 16.
 * Center 8x8 region at tmp[r=2..9][c=2..9].
 *
 * License: BSD-2-Clause (matches dav1d upstream).
 *
 * Spec: AV1 specification §7.15 (CDEF).
 */
#include <stdint.h>
#include <stddef.h>
#include <stdlib.h>

#define TMP_STRIDE 16

/* dav1d's stride-16 directions table — verbatim from
 * external/dav1d-snapshot/src/arm/64/cdef_tmpl.S `dir_table 8, 16`.
 * 8 directions + 6 wrap-around copies (dir 0..5 repeated) = 14
 * entries × 2 = 28 bytes. The asm needs ≥14 entries because for
 * dir=7 the secondary-2 offset (+12 bytes = +6 entries) reads
 * index 13 (which is wrap = dir 5). */
static const int8_t neon_directions8[14][2] = {
    /* index 0 */ { -1 * TMP_STRIDE + 1, -2 * TMP_STRIDE + 2 },
    /* index 1 */ {  0 * TMP_STRIDE + 1, -1 * TMP_STRIDE + 2 },
    /* index 2 */ {  0 * TMP_STRIDE + 1,  0 * TMP_STRIDE + 2 },
    /* index 3 */ {  0 * TMP_STRIDE + 1,  1 * TMP_STRIDE + 2 },
    /* index 4 */ {  1 * TMP_STRIDE + 1,  2 * TMP_STRIDE + 2 },
    /* index 5 */ {  1 * TMP_STRIDE + 0,  2 * TMP_STRIDE + 1 },
    /* index 6 */ {  1 * TMP_STRIDE + 0,  2 * TMP_STRIDE + 0 },
    /* index 7 */ {  1 * TMP_STRIDE + 0,  2 * TMP_STRIDE - 1 },
    /* wrap 8  = dir 0 */ { -1 * TMP_STRIDE + 1, -2 * TMP_STRIDE + 2 },
    /* wrap 9  = dir 1 */ {  0 * TMP_STRIDE + 1, -1 * TMP_STRIDE + 2 },
    /* wrap 10 = dir 2 */ {  0 * TMP_STRIDE + 1,  0 * TMP_STRIDE + 2 },
    /* wrap 11 = dir 3 */ {  0 * TMP_STRIDE + 1,  1 * TMP_STRIDE + 2 },
    /* wrap 12 = dir 4 */ {  1 * TMP_STRIDE + 1,  2 * TMP_STRIDE + 2 },
    /* wrap 13 = dir 5 */ {  1 * TMP_STRIDE + 0,  2 * TMP_STRIDE + 1 },
};

static inline int abs_i(int x) { return x < 0 ? -x : x; }
static inline int imin(int a, int b) { return a < b ? a : b; }
static inline int imax(int a, int b) { return a > b ? a : b; }
static inline int umin(int a, int b) { return (unsigned)a < (unsigned)b ? a : b; }
static inline int iclip(int v, int lo, int hi) {
    return v < lo ? lo : v > hi ? hi : v;
}
static inline int apply_sign(int v, int s) { return s < 0 ? -v : v; }

static inline int constrain(int diff, int threshold, int shift)
{
    int adiff = abs_i(diff);
    return apply_sign(imin(adiff, imax(0, threshold - (adiff >> shift))),
                      diff);
}

static inline int ulog2(unsigned x)
{
    return 31 - __builtin_clz(x);
}

/* NEON-layout reference: tmp is (12 rows × 16 uint16 cols), center
 * at [r=2..9][c=2..9]. dir is the precomputed direction [0..7].
 * Direction lookups use NEON's table (stride-16-precomputed offsets).
 *
 * Note: dav1d's dispatcher branches dir+2, dir+4, dir+0 (after
 * adjusting for the +2 leading offset in the table). With our 12-entry
 * table indexed without the +2 lead, the equivalent is:
 *   primary:    [dir][k]      (was [dir + 2][k] with +2-prefixed table)
 *   secondary1: [(dir + 2) % 8][k]      (was [dir + 4][k])
 *   secondary2: [(dir - 2 + 8) % 8][k]  (was [dir + 0][k])
 * Our `neon_directions8` includes 4 wrap-around entries (idx 8..11
 * = idx 0..3) so [(dir+2)%8] is safe without explicit modulo.
 */
void daedalus_cdef_filter_8x8_pri_sec_ref(
    uint8_t *dst, ptrdiff_t dst_stride,
    const uint16_t *tmp,
    int pri_strength, int sec_strength,
    int dir, int damping, int h)
{
    const int pri_tap = 4 - (pri_strength & 1);
    const int pri_shift = imax(0, damping - ulog2((unsigned) pri_strength));
    /* Cycle 5 phase 5 RED-2: NEON `uqsub` saturates to 0. Mirror it
     * here so the C ref is bit-exact against NEON for damping-light
     * cases (which the original bench param gen didn't exercise). */
    const int sec_shift = imax(0, damping - ulog2((unsigned) sec_strength));

    /* Walk into the center 8x8 region of the 12×16 padded buffer. */
    tmp = tmp + 2 * TMP_STRIDE + 2;

    /* dav1d's dispatcher uses dir+2, dir+4, dir+0 with the C-side
     * 2-prefixed directions table. Our table starts at index 0 = dir 0,
     * so the equivalent indices are dir, (dir+2)%8, (dir-2+8)%8. */
    const int pri_dir_idx = dir;
    const int sec1_dir_idx = (dir + 2) & 7;
    const int sec2_dir_idx = (dir + 6) & 7;   /* (dir - 2) % 8 */

    do {
        for (int x = 0; x < 8; x++) {
            int px = dst[x];
            int sum = 0;
            int max = px, min = px;
            int pri_tap_k = pri_tap;

            for (int k = 0; k < 2; k++) {
                int off1 = neon_directions8[pri_dir_idx][k];
                int p0 = tmp[x + off1];
                int p1 = tmp[x - off1];
                sum += pri_tap_k * constrain(p0 - px, pri_strength, pri_shift);
                sum += pri_tap_k * constrain(p1 - px, pri_strength, pri_shift);
                pri_tap_k = (pri_tap_k & 3) | 2;
                min = umin(p0, min); max = imax(p0, max);
                min = umin(p1, min); max = imax(p1, max);

                int off2 = neon_directions8[sec1_dir_idx][k];
                int off3 = neon_directions8[sec2_dir_idx][k];
                int s0 = tmp[x + off2];
                int s1 = tmp[x - off2];
                int s2 = tmp[x + off3];
                int s3 = tmp[x - off3];
                int sec_tap = 2 - k;
                sum += sec_tap * constrain(s0 - px, sec_strength, sec_shift);
                sum += sec_tap * constrain(s1 - px, sec_strength, sec_shift);
                sum += sec_tap * constrain(s2 - px, sec_strength, sec_shift);
                sum += sec_tap * constrain(s3 - px, sec_strength, sec_shift);
                min = umin(s0, min); max = imax(s0, max);
                min = umin(s1, min); max = imax(s1, max);
                min = umin(s2, min); max = imax(s2, max);
                min = umin(s3, min); max = imax(s3, max);
            }

            dst[x] = (uint8_t) iclip(px + ((sum - (sum < 0) + 8) >> 4),
                                      min, max);
        }
        dst += dst_stride;
        tmp += TMP_STRIDE;
    } while (--h);
}