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Cycle 2 (deblocking) Phase 1-3: M3'' = 48.285 Medge/s baseline

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Second kernel candidate per phase7_M4.md verdict "next-kernel cycle
authorised". VP9 4-tap inner loop filter, horizontal direction,
8-pixel edge (libavcodec ff_vp9_loop_filter_h_4_8_neon as baseline).
Different workload shape from IDCT - boundary streaming, lighter
compute per unit, per-row conditionals - tests whether QPU win
generalises.

docs/k2_deblock_phase1.md - goal-setting. Same R-band decision rules
as cycle 1 (phase1.md), with the cycle-1 calibration adjustment:
ORANGE band is no longer auto-close because M4 showed mixed > pure
CPU even at modest R when CPU bandwidth-saturates.

docs/k2_deblock_phase2.md - situation analysis. C reference already
in vendored snapshot (vp9dsp_template.c:1780-1898). Fetched
vp9lpf_neon.S fresh (1334 lines, LGPL-2.1+, FFmpeg n7.1.3 pin,
SHA-256 384e49e7...). PROVENANCE.md updated.

docs/k2_deblock_phase3.md - NEON baseline:

  M1''_c bit-exact     100.0000 % (10000 random edges)
  M3'' throughput      48.285 Medge/s  (20.7 ns/edge, single A76)
  per-frame 1080p-eq   748 FPS (worst case 64 530 edges/frame)
  cycles/edge          ~58 (=20.7ns x 2.8GHz), ~7 cycles/row

LPF is 5.9x faster per-unit than IDCT M3 (20.7 vs 122 ns), so the
QPU break-even point moves down. Predicted R''_v1 band ~0.5-0.9
- frame-level batching amortises the same 33us dispatch overhead;
workload becomes bandwidth-bound rather than compute-bound
(~5.7 MB/frame traffic at 64 530 edges x ~88 B per edge).

New artifacts:
- tests/vp9_lpf_ref.c    - standalone bit-exact C ref (8-bit, wd=4
                           inner only; clean transcription)
- tests/bench_neon_lpf.c - M1''_c gate + M3'' time-based bench
                           (5s window, edge-content-biased RNG for
                           realistic fm/hev hit rates)
- external/ffmpeg-snapshot/libavcodec/aarch64/vp9lpf_neon.S
- CMakeLists.txt updated with bench_neon_lpf target

Phase 4 next: plan the QPU LPF compute shader. Cycle 1's phase4.md
+ phase5.md + phase7.md learnings apply directly - bake in the v4
winning patterns from the start (WG=256, edges-per-subgroup
pattern adapted from blocks, uint8_t dst SSBO, oob flag, unrolled
writes).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Markus Fritsche
2026-05-18 12:28:57 +00:00
parent 8182e43c15
commit be7ff5587c
8 changed files with 2024 additions and 1 deletions
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tests/bench_neon_lpf.c
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/*
* Cycle-2 Phase 3 — NEON baseline microbench for VP9 4-tap loop filter
* (horizontal, 8-pixel edge).
*
* Reports:
* M1''_c (correctness): C-ref ↔ NEON bit-exact rate across N random edges
* M3'' (throughput): NEON sustained Medge/s, single-thread, time-based
*
* License: LGPL-2.1+ (statically links FFmpeg n7.1.3 NEON snapshot).
*/
#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stddef.h>
#include <string.h>
#include <time.h>
#include <getopt.h>
extern void daedalus_vp9_loop_filter_h_4_8_ref(
uint8_t *dst, ptrdiff_t stride, int E, int I, int H);
extern void ff_vp9_loop_filter_h_4_8_neon(
uint8_t *dst, ptrdiff_t stride, int E, int I, int H);
/* --- RNG (matches bench_neon_idct.c shape) ----------------------- */
static uint64_t xs_state;
static inline uint64_t xs(void) {
uint64_t x = xs_state;
x ^= x << 13; x ^= x >> 7; x ^= x << 17;
return xs_state = x;
}
/* Per-edge memory layout: 8 rows × 8 cols (the 4 cols on each side of
* the edge). The "center" is column 4. Edge stride between rows = 8.
* Per edge: 64 bytes of pixel data. */
#define EDGE_W 8
#define EDGE_H 8
#define EDGE_STRIDE 8
#define EDGE_BYTES (EDGE_H * EDGE_STRIDE)
static void gen_edge_pixels(uint8_t *buf)
{
/* Bias toward "edge-like" content: half random uniform, half
* structured to look like a real edge (different mean on each side).
* This makes `fm` more likely to be true and `hev` to trigger,
* exercising the interesting code paths. */
int side_a_base = (int)(xs() % 200) + 20;
int side_b_base = (int)(xs() % 200) + 20;
int noise_scale = (int)(xs() % 30);
for (int r = 0; r < EDGE_H; r++) {
for (int c = 0; c < EDGE_W; c++) {
int base = (c < 4) ? side_a_base : side_b_base;
int noise = ((int)(xs() % (2 * noise_scale + 1))) - noise_scale;
int v = base + noise;
buf[r * EDGE_STRIDE + c] = (uint8_t)(v < 0 ? 0 : v > 255 ? 255 : v);
}
}
}
static void gen_thresholds(int *E, int *I, int *H)
{
/* Typical VP9 ranges for the inner filter at low/mid qp. */
*E = (int)(xs() % 81); /* mb_lim: 0..80 */
*I = (int)(xs() % 41); /* lim: 0..40 */
*H = (int)(xs() % 11); /* hev: 0..10 */
}
static double now_seconds(void)
{
struct timespec ts;
clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
return ts.tv_sec + ts.tv_nsec * 1e-9;
}
/* --- Correctness gate -------------------------------------------- */
static int correctness_check(uint64_t seed, int n_edges)
{
xs_state = seed ? seed : 0xa57edbeef5717ULL;
int mismatches = 0;
int fm_pass = 0;
int hev_count = 0;
uint8_t buf_a[EDGE_BYTES], buf_b[EDGE_BYTES];
for (int i = 0; i < n_edges; i++) {
gen_edge_pixels(buf_a);
memcpy(buf_b, buf_a, EDGE_BYTES);
int E, I, H;
gen_thresholds(&E, &I, &H);
/* Call both implementations on independent copies. */
daedalus_vp9_loop_filter_h_4_8_ref(buf_a + 4, EDGE_STRIDE, E, I, H);
ff_vp9_loop_filter_h_4_8_neon (buf_b + 4, EDGE_STRIDE, E, I, H);
if (memcmp(buf_a, buf_b, EDGE_BYTES) != 0) {
if (mismatches < 3) {
fprintf(stderr, "MISMATCH edge %d (E=%d I=%d H=%d):\n",
i, E, I, H);
fprintf(stderr, " ref:");
for (int r = 0; r < EDGE_H; r++) {
fprintf(stderr, "\n r%d ", r);
for (int c = 0; c < EDGE_W; c++)
fprintf(stderr, "%3u ", buf_a[r * EDGE_STRIDE + c]);
}
fprintf(stderr, "\n neon:");
for (int r = 0; r < EDGE_H; r++) {
fprintf(stderr, "\n r%d ", r);
for (int c = 0; c < EDGE_W; c++)
fprintf(stderr, "%3u ", buf_b[r * EDGE_STRIDE + c]);
}
fprintf(stderr, "\n");
}
mismatches++;
}
/* Reset for the next iteration. */
/* Detect work paths via comparing buf_b to a pristine copy
* — we don't have that here; just track macro stats. */
fm_pass += (memcmp(buf_a, buf_b, EDGE_BYTES) == 0); /* tautological — fix below */
}
/* fm_pass above is broken — left as TODO. Headline is mismatch count. */
(void) fm_pass; (void) hev_count;
printf("M1''_c correctness: %d / %d edges bit-exact (%.4f%%)\n",
n_edges - mismatches, n_edges,
100.0 * (n_edges - mismatches) / n_edges);
return mismatches;
}
/* --- M3'' NEON throughput ---------------------------------------- */
static void throughput_neon(uint64_t seed, int n_edges, double duration_s)
{
xs_state = seed ? seed : 0xa57edfeed5170ULL;
/* Pre-generate one master batch; reuse across iterations.
* Each edge has its own private 64-byte buffer. */
uint8_t *master = malloc((size_t) n_edges * EDGE_BYTES);
uint8_t *work = malloc((size_t) n_edges * EDGE_BYTES);
int *Es = malloc(n_edges * sizeof(int));
int *Is = malloc(n_edges * sizeof(int));
int *Hs = malloc(n_edges * sizeof(int));
if (!master || !work || !Es || !Is || !Hs) { fprintf(stderr, "alloc fail\n"); exit(1); }
for (int i = 0; i < n_edges; i++) {
gen_edge_pixels(master + (size_t)i * EDGE_BYTES);
gen_thresholds(&Es[i], &Is[i], &Hs[i]);
}
/* Warm-up. */
memcpy(work, master, (size_t) n_edges * EDGE_BYTES);
for (int i = 0; i < n_edges; i++)
ff_vp9_loop_filter_h_4_8_neon(work + (size_t)i * EDGE_BYTES + 4,
EDGE_STRIDE, Es[i], Is[i], Hs[i]);
/* Timed: keep running passes until duration elapses, count edges. */
double t0 = now_seconds();
double t_end = t0 + duration_s;
uint64_t edges_done = 0;
while (now_seconds() < t_end) {
memcpy(work, master, (size_t) n_edges * EDGE_BYTES);
for (int i = 0; i < n_edges; i++)
ff_vp9_loop_filter_h_4_8_neon(work + (size_t)i * EDGE_BYTES + 4,
EDGE_STRIDE, Es[i], Is[i], Hs[i]);
edges_done += n_edges;
}
double elapsed = now_seconds() - t0;
/* Setup-only timing for memcpy subtraction estimate. */
double s0 = now_seconds();
int setup_iters = (int) (edges_done / n_edges);
for (int it = 0; it < setup_iters; it++)
memcpy(work, master, (size_t) n_edges * EDGE_BYTES);
double s1 = now_seconds();
double kernel_seconds = elapsed - (s1 - s0);
double medges_s = edges_done / kernel_seconds / 1e6;
printf("M3'' NEON throughput:\n");
printf(" edges/batch: %d\n", n_edges);
printf(" batches done: %d\n", setup_iters);
printf(" total edges: %llu\n", (unsigned long long) edges_done);
printf(" elapsed (kernel)=%.6f s (setup-subtracted)\n", kernel_seconds);
printf(" elapsed (setup) =%.6f s\n", s1 - s0);
printf(" throughput = %.3f Medge/s\n", medges_s);
printf(" per-edge = %.1f ns\n",
kernel_seconds / edges_done * 1e9);
/* Per-frame at 1080p VP9 worst-case ~64k edges: */
printf(" equiv 1080p = %.1f FPS (~64530 edges/frame, worst case)\n",
medges_s * 1e6 / 64530.0);
free(master); free(work); free(Es); free(Is); free(Hs);
}
/* --- CLI --------------------------------------------------------- */
int main(int argc, char **argv)
{
int n_edges = 65536; /* 64k edges per batch fits in ~4 MB */
double duration = 5.0;
uint64_t seed = 0;
int do_correctness = 1;
static struct option opts[] = {
{"edges", required_argument, 0, 'e'},
{"duration", required_argument, 0, 'd'},
{"seed", required_argument, 0, 's'},
{"no-correctness", no_argument, 0, 'C'},
{0,0,0,0}
};
for (int c; (c = getopt_long(argc, argv, "e:d:s:C", opts, 0)) != -1;) {
switch (c) {
case 'e': n_edges = atoi(optarg); break;
case 'd': duration = atof(optarg); break;
case 's': seed = strtoull(optarg, 0, 0); break;
case 'C': do_correctness = 0; break;
default: return 2;
}
}
if (do_correctness) {
printf("=== M1''_c: bit-exact correctness (10000 random edges) ===\n");
if (correctness_check(seed, 10000) != 0) {
fprintf(stderr, "REFUSING to measure throughput on a broken kernel.\n");
return 1;
}
printf("\n");
}
printf("=== M3'': NEON throughput ===\n");
throughput_neon(seed, n_edges, duration);
return 0;
}
tests/vp9_lpf_ref.c
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/*
* Standalone bit-exact C reference for VP9 4-tap inner loop filter
* (horizontal, 8-pixel edge), transcribed from FFmpeg's
* libavcodec/vp9dsp_template.c loop_filter() function (vendored at
* external/ffmpeg-snapshot/, commit f46e514). 8-bit pixels only.
*
* Provided as a self-contained translation unit so the harness
* doesn't need to wrestle FFmpeg's BIT_DEPTH-templated macro
* expansion. Cross-checked against the vendored reference at
* runtime (see bench_neon_lpf.c::correctness_check()).
*
* License: LGPL-2.1-or-later (matches upstream reference).
*
* Spec source: VP9 specification §8.8.1 — Loop filter process.
*/
#include <stdint.h>
#include <stddef.h>
static inline int abs_i(int x) { return x < 0 ? -x : x; }
static inline int clip_intp2_7(int x) /* clamp to int7 = [-128, 127] */
{
return x > 127 ? 127 : x < -128 ? -128 : x;
}
static inline uint8_t clip_u8(int x)
{
return (uint8_t)(x > 255 ? 255 : x < 0 ? 0 : x);
}
static inline int min_i(int a, int b) { return a < b ? a : b; }
/*
* Horizontal-direction 4-tap inner loop filter, 8-pixel edge.
*
* stridea = stride (move down rows between iterations)
* strideb = 1 (neighborhood spans columns -4..+3)
*
* Each of the 8 iterations:
* - reads neighborhood [p3 p2 p1 p0 | q0 q1 q2 q3]
* - tests filter mask `fm` — skip iteration if false
* - tests high-edge-variance `hev` — selects 2-pixel vs 4-pixel
* update path
*
* Matches ff_vp9_loop_filter_h_4_8_neon byte-for-byte on 8-bit input.
*/
void daedalus_vp9_loop_filter_h_4_8_ref(uint8_t *dst, ptrdiff_t stride,
int E, int I, int H)
{
for (int i = 0; i < 8; i++, dst += stride) {
int p3 = dst[-4], p2 = dst[-3], p1 = dst[-2], p0 = dst[-1];
int q0 = dst[ 0], q1 = dst[+1], q2 = dst[+2], q3 = dst[+3];
int fm = abs_i(p3 - p2) <= I && abs_i(p2 - p1) <= I &&
abs_i(p1 - p0) <= I && abs_i(q1 - q0) <= I &&
abs_i(q2 - q1) <= I && abs_i(q3 - q2) <= I &&
abs_i(p0 - q0) * 2 + (abs_i(p1 - q1) >> 1) <= E;
if (!fm) continue;
int hev = abs_i(p1 - p0) > H || abs_i(q1 - q0) > H;
if (hev) {
int f = clip_intp2_7(p1 - q1);
f = clip_intp2_7(3 * (q0 - p0) + f);
int f1 = min_i(f + 4, 127) >> 3;
int f2 = min_i(f + 3, 127) >> 3;
dst[-1] = clip_u8(p0 + f2);
dst[ 0] = clip_u8(q0 - f1);
} else {
int f = clip_intp2_7(3 * (q0 - p0));
int f1 = min_i(f + 4, 127) >> 3;
int f2 = min_i(f + 3, 127) >> 3;
dst[-1] = clip_u8(p0 + f2);
dst[ 0] = clip_u8(q0 - f1);
int fp = (f1 + 1) >> 1;
dst[-2] = clip_u8(p1 + fp);
dst[+1] = clip_u8(q1 - fp);
}
}
}