ferrox-quant 0.20.0

Quantized weight formats and fused dequant kernels for Ferrox
Documentation
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
//! Q4_K packed 8 rows deep into `block_q4_Kx8` (llama.cpp
//! `make_block_q4_Kx8`), with the GEMV and GEMM that read it.

#[cfg(target_arch = "x86_64")]
use super::avx2;
use super::common::*;
#[cfg(target_arch = "aarch64")]
use super::neon;
use crate::{Q8KActivations, Q4_K_BLOCK_BYTES, Q4_K_BLOCK_ELEMS};

/// Bytes per interleaved `block_q4_Kx8` (8 × f16 d + 8 × f16 dmin + 96 scales + 1024 qs).
pub const Q4_KX8_BLOCK_BYTES: usize = 1152;
/// Number of Q4_K rows packed into one interleaved block.
pub const Q4_KX8_NROWS: usize = 8;

/// Preferred qs interleave width for this CPU: 8 where a `×4` GEMM
/// kernel exists (`ggml_gemm_q4_K_8x8_q8_K` on x86 AVX2 and ARM i8mm),
/// 4 on DotProd-only NEON (`ggml_gemm_q4_K_8x4_q8_K`). One answer for
/// every kind — see [`preferred_interleave`].
#[inline]
pub fn q4_kx8_interleave() -> usize {
    preferred_interleave()
}

/// Pack eight canonical Q4_K super-blocks (same column-block index) into
/// one `block_q4_Kx8`. `interleave` is 4 (ARM DotProd) or 8 (x86 / ARM i8mm).
pub fn make_block_q4_kx8(
    rows: [&[u8]; Q4_KX8_NROWS],
    interleave: usize,
) -> [u8; Q4_KX8_BLOCK_BYTES] {
    debug_assert!(interleave == 4 || interleave == 8);
    for r in &rows {
        debug_assert_eq!(r.len(), Q4_K_BLOCK_BYTES);
    }
    let mut out = [0u8; Q4_KX8_BLOCK_BYTES];
    // d[8] at 0, dmin[8] at 16, scales[96] at 32, qs[1024] at 128.
    for (i, row) in rows.iter().enumerate() {
        out[i * 2] = row[0];
        out[i * 2 + 1] = row[1];
        out[16 + i * 2] = row[2];
        out[16 + i * 2 + 1] = row[3];
    }

    let end = (Q4_K_BLOCK_ELEMS * 4) / interleave; // qs bytes * 8 rows / interleave
    let qs_out = &mut out[128..];
    for i in 0..end {
        let src_id = i % Q4_KX8_NROWS;
        let src_offset = (i / Q4_KX8_NROWS) * interleave;
        let dst_offset = i * interleave;
        let src_qs = &rows[src_id][16..144];
        qs_out[dst_offset..dst_offset + interleave]
            .copy_from_slice(&src_qs[src_offset..src_offset + interleave]);
    }

    // Rearrange 6-bit scales/mins across 8 rows into 96 packed bytes
    // (llama.cpp `make_block_q4_Kx8`).
    let mut s = [0u8; 8];
    let mut m = [0u8; 8];
    let scales_out = &mut out[32..128];

    for i in 0..4 {
        for j in 0..8 {
            let sc = &rows[j][4..16];
            s[j] = sc[i] & 63;
            m[j] = sc[i + 4] & 63;
        }
        let base = i * 12;
        scales_out[base] = (s[0] & 63) + ((s[4] & 48) << 2);
        scales_out[base + 1] = (s[1] & 63) + ((s[5] & 48) << 2);
        scales_out[base + 2] = (s[2] & 63) + ((s[6] & 48) << 2);
        scales_out[base + 3] = (s[3] & 63) + ((s[7] & 48) << 2);
        scales_out[base + 4] = (m[0] & 63) + ((m[4] & 48) << 2);
        scales_out[base + 5] = (m[1] & 63) + ((m[5] & 48) << 2);
        scales_out[base + 6] = (m[2] & 63) + ((m[6] & 48) << 2);
        scales_out[base + 7] = (m[3] & 63) + ((m[7] & 48) << 2);
        scales_out[base + 8] = (s[4] & 15) + ((m[4] & 15) << 4);
        scales_out[base + 9] = (s[5] & 15) + ((m[5] & 15) << 4);
        scales_out[base + 10] = (s[6] & 15) + ((m[6] & 15) << 4);
        scales_out[base + 11] = (s[7] & 15) + ((m[7] & 15) << 4);
    }

    for i in 0..4 {
        for j in 0..8 {
            let sc = &rows[j][4..16];
            s[j] = ((sc[i] & 192) >> 2) | (sc[i + 8] & 15);
            m[j] = ((sc[i + 4] & 192) >> 2) | ((sc[i + 8] & 240) >> 4);
        }
        let base = 48 + i * 12;
        scales_out[base] = (s[0] & 63) + ((s[4] & 48) << 2);
        scales_out[base + 1] = (s[1] & 63) + ((s[5] & 48) << 2);
        scales_out[base + 2] = (s[2] & 63) + ((s[6] & 48) << 2);
        scales_out[base + 3] = (s[3] & 63) + ((s[7] & 48) << 2);
        scales_out[base + 4] = (m[0] & 63) + ((m[4] & 48) << 2);
        scales_out[base + 5] = (m[1] & 63) + ((m[5] & 48) << 2);
        scales_out[base + 6] = (m[2] & 63) + ((m[6] & 48) << 2);
        scales_out[base + 7] = (m[3] & 63) + ((m[7] & 48) << 2);
        scales_out[base + 8] = (s[4] & 15) + ((m[4] & 15) << 4);
        scales_out[base + 9] = (s[5] & 15) + ((m[5] & 15) << 4);
        scales_out[base + 10] = (s[6] & 15) + ((m[6] & 15) << 4);
        scales_out[base + 11] = (s[7] & 15) + ((m[7] & 15) << 4);
    }

    out
}

/// Repack a full Q4_K matrix (row-major canonical blocks) into interleaved
/// `block_q4_Kx8` groups. Rows not divisible by 8 are left out (caller
/// handles the tail with per-row dots). `interleave` defaults via
/// [`q4_kx8_interleave`].
pub fn pack_q4_k_matrix_x8(data: &[u8], rows: usize, cols: usize, interleave: usize) -> Vec<u8> {
    assert!(cols.is_multiple_of(Q4_K_BLOCK_ELEMS));
    let n_blocks = cols / Q4_K_BLOCK_ELEMS;
    let row_bytes = n_blocks * Q4_K_BLOCK_BYTES;
    assert_eq!(data.len(), rows * row_bytes);
    let n_groups = rows / Q4_KX8_NROWS;
    let mut out = Vec::with_capacity(n_groups * n_blocks * Q4_KX8_BLOCK_BYTES);
    for g in 0..n_groups {
        for b in 0..n_blocks {
            let mut row_refs: [&[u8]; Q4_KX8_NROWS] = [&[]; Q4_KX8_NROWS];
            for (r, slot) in row_refs.iter_mut().enumerate() {
                let base = (g * Q4_KX8_NROWS + r) * row_bytes + b * Q4_K_BLOCK_BYTES;
                *slot = &data[base..base + Q4_K_BLOCK_BYTES];
            }
            out.extend_from_slice(&make_block_q4_kx8(row_refs, interleave));
        }
    }
    out
}

/// Scalar GEMV for interleave=4 (`ggml_gemv_q4_K_8x4_q8_K_generic`).
pub(crate) fn gemv_q4_kx8_q8_k_scalar_4(
    packed: &[u8],
    act: &Q8KActivations,
    n_cols: usize,
    n_row_groups: usize,
    out: &mut [f32],
) {
    let nb = n_cols / Q4_K_BLOCK_ELEMS;
    let blocklen = 4;
    let ncols_interleaved = Q4_KX8_NROWS;
    debug_assert_eq!(act.n_blocks(), nb);
    debug_assert_eq!(out.len(), n_row_groups * ncols_interleaved);
    debug_assert_eq!(packed.len(), n_row_groups * nb * Q4_KX8_BLOCK_BYTES);

    for x in 0..n_row_groups {
        let mut sumf = [0f32; 8];
        let mut sum_minf = [0f32; 8];
        let group_off = x * nb * Q4_KX8_BLOCK_BYTES;
        for l in 0..nb {
            let blk = &packed[group_off + l * Q4_KX8_BLOCK_BYTES..][..Q4_KX8_BLOCK_BYTES];
            let d = &blk[0..16];
            let dmin = &blk[16..32];
            let scales = &blk[32..128];
            let qs = &blk[128..];
            let da = act.d[l];
            let q8 = &act.q[l * Q4_K_BLOCK_ELEMS..(l + 1) * Q4_K_BLOCK_ELEMS];
            let bsums = &act.bsums[l * 16..(l + 1) * 16];

            let mut all_scales = [[0u8; 8]; 8];
            let mut all_mins = [[0u8; 8]; 8];
            for sb in 0..8 {
                decode_scales_mins(&scales[sb * 12..], &mut all_scales[sb], &mut all_mins[sb]);
            }

            let n_k = Q4_K_BLOCK_ELEMS / (2 * blocklen); // 32
            for k in 0..n_k {
                let sb_pair = k / 8;
                let sc0 = &all_scales[sb_pair * 2];
                let sc1 = &all_scales[sb_pair * 2 + 1];
                for j in 0..ncols_interleaved {
                    let mut sumi = 0i32;
                    for i in 0..blocklen {
                        let qbyte = qs[k * ncols_interleaved * blocklen + j * blocklen + i];
                        let v0 = (qbyte & 0x0F) as i32;
                        let v1 = (qbyte >> 4) as i32;
                        let a0 = q8[(k / 8) * 64 + (k % 8) * blocklen + i] as i32;
                        let a1 = q8[(k / 8) * 64 + (k % 8) * blocklen + i + 32] as i32;
                        sumi += v0 * a0 * sc0[j] as i32 + v1 * a1 * sc1[j] as i32;
                    }
                    sumf[j] += sumi as f32 * f16_from_bytes(&d[j * 2..]) * da;
                }
            }
            for sb in 0..8 {
                let mins = &all_mins[sb];
                let bsum = bsums[sb * 2] as i32 + bsums[sb * 2 + 1] as i32;
                for j in 0..ncols_interleaved {
                    sum_minf[j] +=
                        mins[j] as f32 * bsum as f32 * f16_from_bytes(&dmin[j * 2..]) * da;
                }
            }
        }
        let base = x * ncols_interleaved;
        for j in 0..ncols_interleaved {
            out[base + j] = sumf[j] - sum_minf[j];
        }
    }
}

/// Scalar GEMV for interleave=8 (`ggml_gemv_q4_K_8x8_q8_K_generic`).
pub(crate) fn gemv_q4_kx8_q8_k_scalar_8(
    packed: &[u8],
    act: &Q8KActivations,
    n_cols: usize,
    n_row_groups: usize,
    out: &mut [f32],
) {
    let nb = n_cols / Q4_K_BLOCK_ELEMS;
    let blocklen = 8;
    let ncols_interleaved = Q4_KX8_NROWS;
    debug_assert_eq!(act.n_blocks(), nb);
    debug_assert_eq!(out.len(), n_row_groups * ncols_interleaved);

    for x in 0..n_row_groups {
        let mut sumf = [0f32; 8];
        let mut sum_minf = [0f32; 8];
        let group_off = x * nb * Q4_KX8_BLOCK_BYTES;
        for l in 0..nb {
            let blk = &packed[group_off + l * Q4_KX8_BLOCK_BYTES..][..Q4_KX8_BLOCK_BYTES];
            let d = &blk[0..16];
            let dmin = &blk[16..32];
            let scales = &blk[32..128];
            let qs = &blk[128..];
            let da = act.d[l];
            let q8 = &act.q[l * Q4_K_BLOCK_ELEMS..(l + 1) * Q4_K_BLOCK_ELEMS];
            let bsums = &act.bsums[l * 16..(l + 1) * 16];

            let mut all_scales = [[0u8; 8]; 8];
            let mut all_mins = [[0u8; 8]; 8];
            for sb in 0..8 {
                decode_scales_mins(&scales[sb * 12..], &mut all_scales[sb], &mut all_mins[sb]);
            }

            let n_k = Q4_K_BLOCK_ELEMS / (2 * blocklen); // 16
            for k in 0..n_k {
                let sb_pair = k / 4;
                let sc0 = &all_scales[sb_pair * 2];
                let sc1 = &all_scales[sb_pair * 2 + 1];
                for j in 0..ncols_interleaved {
                    let mut sumi = 0i32;
                    for i in 0..blocklen {
                        let qbyte = qs[k * ncols_interleaved * blocklen + j * blocklen + i];
                        let v0 = (qbyte & 0x0F) as i32;
                        let v1 = (qbyte >> 4) as i32;
                        let a0 = q8[(k >> 2) * 64 + (k % 4) * blocklen + i] as i32;
                        let a1 = q8[(k >> 2) * 64 + (k % 4) * blocklen + i + 32] as i32;
                        sumi += v0 * a0 * sc0[j] as i32 + v1 * a1 * sc1[j] as i32;
                    }
                    sumf[j] += sumi as f32 * f16_from_bytes(&d[j * 2..]) * da;
                }
            }
            for sb in 0..8 {
                let mins = &all_mins[sb];
                let bsum = bsums[sb * 2] as i32 + bsums[sb * 2 + 1] as i32;
                for j in 0..ncols_interleaved {
                    sum_minf[j] +=
                        mins[j] as f32 * bsum as f32 * f16_from_bytes(&dmin[j * 2..]) * da;
                }
            }
        }
        let base = x * ncols_interleaved;
        for j in 0..ncols_interleaved {
            out[base + j] = sumf[j] - sum_minf[j];
        }
    }
}

/// GEMV: interleaved Q4_K weights × Q8_K activation → `n_row_groups * 8` f32s.
/// Dispatches to NEON (both interleaves) / AVX2 (interleave 8) when available.
pub fn gemv_q4_kx8_q8_k(
    packed: &[u8],
    act: &Q8KActivations,
    n_cols: usize,
    n_row_groups: usize,
    interleave: usize,
    out: &mut [f32],
) {
    assert!(n_cols.is_multiple_of(Q4_K_BLOCK_ELEMS));
    assert_eq!(out.len(), n_row_groups * Q4_KX8_NROWS);
    match interleave {
        4 => {
            #[cfg(target_arch = "aarch64")]
            {
                if std::arch::is_aarch64_feature_detected!("dotprod") {
                    unsafe {
                        neon::gemv_q4_kx8_q8_k_neon_sdot(packed, act, n_cols, n_row_groups, out);
                    }
                    return;
                }
            }
            gemv_q4_kx8_q8_k_scalar_4(packed, act, n_cols, n_row_groups, out);
        }
        8 => {
            #[cfg(target_arch = "x86_64")]
            {
                if is_x86_feature_detected!("avx2") && is_x86_feature_detected!("fma") {
                    unsafe {
                        avx2::gemv_q4_kx8_q8_k_avx2(packed, act, n_cols, n_row_groups, out);
                    }
                    return;
                }
            }
            #[cfg(target_arch = "aarch64")]
            {
                if std::arch::is_aarch64_feature_detected!("dotprod") {
                    unsafe {
                        neon::gemv_q4_kx8_q8_k_neon_8x8(packed, act, n_cols, n_row_groups, out);
                    }
                    return;
                }
            }
            gemv_q4_kx8_q8_k_scalar_8(packed, act, n_cols, n_row_groups, out);
        }
        _ => panic!("q4_kx8 interleave must be 4 or 8, got {interleave}"),
    }
}

/// One row-group (8 outputs) starting at `group` within a packed matrix.
#[inline]
pub fn gemv_q4_kx8_group(
    packed: &[u8],
    group: usize,
    act: &Q8KActivations,
    n_cols: usize,
    interleave: usize,
    out8: &mut [f32],
) {
    debug_assert_eq!(out8.len(), Q4_KX8_NROWS);
    let nb = n_cols / Q4_K_BLOCK_ELEMS;
    let off = group * nb * Q4_KX8_BLOCK_BYTES;
    let slice = &packed[off..off + nb * Q4_KX8_BLOCK_BYTES];
    gemv_q4_kx8_q8_k(slice, act, n_cols, 1, interleave, out8);
}

/// How many activations one [`gemm_q4_kx8_group`] pass keeps in flight.
///
/// Four is llama's shape for `ggml_gemm_q4_K_8x4_q8_K` (`q8_k_blocklen`),
/// and it is what the register file allows here: eight `uint8x16` weight
/// columns plus one activation's four `int8x16` and its accumulator pair
/// stay resident while the batch loop turns.
pub const Q4_KX8_GEMM_NC: usize = 4;

/// GEMM counterpart of [`gemv_q4_kx8_group`]: one row-group (8 rows)
/// against `acts.len()` activations at once.
///
/// Q4_K was the expensive omission. The GEMV repeats, *per activation*,
/// work that depends only on the weights: 16 f16 scale conversions, 8
/// `decode_scales_mins` calls and 16 `q4_cols` loads per 256-element
/// super-block. At batch 512 that is the same 6-bit scale decode run 512
/// times. Q8_0 already had `gemm_q8_0x4_group`; this is the same idea for
/// the format that carries every `*_Q4_K_M` checkpoint's FFN.
///
/// `out` is `[row][act]`: `out[r * acts.len() + j]`, matching
/// [`gemm_q8_0x4_group`] and what `WeightMatrix::apply_batch` writes.
pub fn gemm_q4_kx8_group(
    packed: &[u8],
    group: usize,
    acts: &[Q8KActivations],
    n_cols: usize,
    interleave: usize,
    out: &mut [f32],
) {
    assert_eq!(out.len(), Q4_KX8_NROWS * acts.len());
    assert!(n_cols.is_multiple_of(Q4_K_BLOCK_ELEMS));
    if acts.is_empty() {
        return;
    }
    let nb = n_cols / Q4_K_BLOCK_ELEMS;
    let off = group * nb * Q4_KX8_BLOCK_BYTES;
    let slice = &packed[off..off + nb * Q4_KX8_BLOCK_BYTES];

    #[cfg(target_arch = "aarch64")]
    if acts.len() <= Q4_KX8_GEMM_NC {
        if interleave == 8 && std::arch::is_aarch64_feature_detected!("i8mm") {
            // Compatibility entry: interleaves the quad here, once per call.
            // Batch callers should prepare the quad once per matmul and use
            // [`gemm_q4_kx8_group_x4`] for every row-group instead.
            let tile = prepare_q8_k_acts_x4(acts, n_cols);
            unsafe {
                neon::gemm_q4_kx8_q8_k_neon_i8mm(slice, &tile, n_cols, out);
            }
            return;
        }
        if interleave == 4 && std::arch::is_aarch64_feature_detected!("dotprod") {
            unsafe {
                neon::gemm_q4_kx8_q8_k_neon_sdot(slice, acts, n_cols, out);
            }
            return;
        }
    }
    // Portable fallback: the GEMV, once per activation. Same results,
    // none of the reuse.
    let mut tmp = [0f32; Q4_KX8_NROWS];
    for (j, act) in acts.iter().enumerate() {
        gemv_q4_kx8_q8_k(slice, act, n_cols, 1, interleave, &mut tmp);
        for (r, v) in tmp.iter().enumerate() {
            out[r * acts.len() + j] = *v;
        }
    }
}

/// Whether [`gemm_q4_kx8_group_x4`] is the fast Q4_K batch path on this CPU:
/// ARM i8mm with the interleave-8 layout. Everywhere else preparing the quad
/// buys nothing — x86 dispatches to AVX2 inside the per-activation GEMV — so
/// callers should keep using [`gemm_q4_kx8_group`] and skip the tiles.
#[inline]
pub fn q4_kx8_gemm_uses_acts_x4(interleave: usize) -> bool {
    interleaved_gemm_is_accelerated(interleave)
}

/// [`gemm_q4_kx8_group`] against a pre-interleaved activation quad.
///
/// Callers build the quad once per matmul with [`prepare_q8_k_acts_x4`] and
/// pass it to every row-group, hoisting what the i8mm kernel used to redo
/// `rows/8` times. Only the interleave-8 layout has this kernel; gate call
/// sites with [`q4_kx8_gemm_uses_acts_x4`].
///
/// `out` is `[row][act]` over the `tile.na` real activations:
/// `out[r * tile.na + a]`, matching [`gemm_q4_kx8_group`].
pub fn gemm_q4_kx8_group_x4(
    packed: &[u8],
    group: usize,
    tile: &Q8KActsX4,
    n_cols: usize,
    interleave: usize,
    out: &mut [f32],
) {
    gemm_q4_kx8_group_x4_on(
        packed,
        group,
        tile,
        n_cols,
        interleave,
        AccelX4::detect(),
        out,
    );
}

/// [`gemm_q4_kx8_group_x4`] with the kernel choice already made. Hoist
/// [`AccelX4::detect`] out of the (row-group × quad) loop and pass it here;
/// see [`AccelX4`].
#[inline]
pub fn gemm_q4_kx8_group_x4_on(
    packed: &[u8],
    group: usize,
    tile: &Q8KActsX4,
    n_cols: usize,
    interleave: usize,
    accel: AccelX4,
    out: &mut [f32],
) {
    assert_eq!(
        interleave, 8,
        "the x4 GEMM only exists for interleave-8 packing"
    );
    assert_eq!(out.len(), Q4_KX8_NROWS * tile.na);
    assert!(n_cols.is_multiple_of(Q4_K_BLOCK_ELEMS));
    debug_assert_eq!(tile.n_blocks, n_cols / Q4_K_BLOCK_ELEMS);
    if tile.na == 0 {
        return;
    }
    let nb = n_cols / Q4_K_BLOCK_ELEMS;
    let off = group * nb * Q4_KX8_BLOCK_BYTES;
    let slice = &packed[off..off + nb * Q4_KX8_BLOCK_BYTES];

    #[cfg(target_arch = "aarch64")]
    if accel == AccelX4::NeonI8mm {
        unsafe {
            neon::gemm_q4_kx8_q8_k_neon_i8mm(slice, tile, n_cols, out);
        }
        return;
    }
    #[cfg(target_arch = "x86_64")]
    if accel == AccelX4::Avx2 {
        unsafe {
            avx2::gemm_q4_kx8_q8_k_avx2(slice, tile, n_cols, out);
        }
        return;
    }
    let _ = accel;
    gemm_q4_kx8_acts_x4_scalar_8(slice, tile, n_cols, out);
}

/// Portable reference for the ×4 GEMM: the same math as
/// [`gemv_q4_kx8_q8_k_scalar_8`], per quad row, reading qs / folded bsums /
/// d straight out of the pre-interleaved [`Q8KActsX4`]. Bit-identical to
/// running that GEMV per activation, which is what the tests assert.
pub(crate) fn gemm_q4_kx8_acts_x4_scalar_8(
    packed: &[u8],
    tile: &Q8KActsX4,
    n_cols: usize,
    out: &mut [f32],
) {
    let nb = n_cols / Q4_K_BLOCK_ELEMS;
    let blocklen = 8;
    let ncols_interleaved = Q4_KX8_NROWS;
    let na = tile.na;
    let mut sumf = [[0f32; Q4_KX8_NROWS]; Q4_KX8_GEMM_NC];
    let mut sum_minf = [[0f32; Q4_KX8_NROWS]; Q4_KX8_GEMM_NC];
    for l in 0..nb {
        let blk = &packed[l * Q4_KX8_BLOCK_BYTES..][..Q4_KX8_BLOCK_BYTES];
        let d = &blk[0..16];
        let dmin = &blk[16..32];
        let scales = &blk[32..128];
        let qs = &blk[128..];
        let q8 = &tile.qs[l * Q4_K_BLOCK_ELEMS * 4..][..Q4_K_BLOCK_ELEMS * 4];

        let mut all_scales = [[0u8; 8]; 8];
        let mut all_mins = [[0u8; 8]; 8];
        for sb in 0..8 {
            decode_scales_mins(&scales[sb * 12..], &mut all_scales[sb], &mut all_mins[sb]);
        }

        for a in 0..na {
            let da = tile.d[l * 4 + a];
            let n_k = Q4_K_BLOCK_ELEMS / (2 * blocklen); // 16
            for k in 0..n_k {
                let sb_pair = k / 4;
                let sc0 = &all_scales[sb_pair * 2];
                let sc1 = &all_scales[sb_pair * 2 + 1];
                for j in 0..ncols_interleaved {
                    let mut sumi = 0i32;
                    for i in 0..blocklen {
                        let qbyte = qs[k * ncols_interleaved * blocklen + j * blocklen + i];
                        let v0 = (qbyte & 0x0F) as i32;
                        let v1 = (qbyte >> 4) as i32;
                        // Canonical q8 element `e` lives at run `e/8`, row
                        // `a`, lane `e%8` of the interleaved block.
                        let e0 = (k >> 2) * 64 + (k % 4) * blocklen + i;
                        let e1 = e0 + 32;
                        let a0 = q8[(e0 / 8) * 32 + a * 8 + (e0 % 8)] as i32;
                        let a1 = q8[(e1 / 8) * 32 + a * 8 + (e1 % 8)] as i32;
                        sumi += v0 * a0 * sc0[j] as i32 + v1 * a1 * sc1[j] as i32;
                    }
                    sumf[a][j] += sumi as f32 * f16_from_bytes(&d[j * 2..]) * da;
                }
            }
            for (sb, mins) in all_mins.iter().enumerate() {
                let bsum = tile.bsums[(l * 4 + a) * 8 + sb] as i32;
                for j in 0..ncols_interleaved {
                    sum_minf[a][j] +=
                        mins[j] as f32 * bsum as f32 * f16_from_bytes(&dmin[j * 2..]) * da;
                }
            }
        }
    }
    for j in 0..ncols_interleaved {
        for (a, row) in sumf.iter().take(na).enumerate() {
            out[j * na + a] = row[j] - sum_minf[a][j];
        }
    }
}