transcribe-cpp-sys 0.0.4

Native FFI bindings for transcribe.cpp, a C/C++ speech-to-text library built on ggml
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
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
// arch/granite/encoder.cpp - Granite Speech audio encoder (Conformer).
//
// Reference: GraniteSpeechCTCEncoder and GraniteSpeechConformerBlock in
// transformers/models/granite_speech/modeling_granite_speech.py.
//
// The encoder is structurally a NeMo-style Conformer (macaron FFN +
// rel-pos MHSA + GLU conv module + post-block LN) with two granite
// deviations:
//
//   1. Block-local Shaw attention. Each block reshapes the sequence
//      into context_size=200 chunks and attends only within each chunk.
//      The relative-position bias is a learned [2*max_pos_emb+1,
//      head_dim] embedding looked up by (k - q + max_pos_emb).
//   2. Self-conditioned CTC bypass. After the N/2-th block (0-indexed
//      idx N/2 - 1), the running hidden state is projected through
//      a 1024 -> 348 CTC head, softmaxed over the channel axis, and
//      projected back via a 348 -> 1024 head. The result is added as
//      a residual to the running hidden state.
//
// The Conformer macaron FFN and GLU conv module *shape* matches
// `transcribe::conformer::*`, but the helper there hardcodes
// inner_dim == d_model (parakeet / cohere / canary all use
// conv_expansion=1). Granite uses conv_expansion=2 — the pointwise1
// kernel maps d_model → 4*d_model and GLU halves to 2*d_model. We
// inline the conv module here rather than parameterise the shared
// helper. The macaron FF helper is fine to reuse (it only depends on
// the FF expansion ratio implicit in the tensor shapes, not on d_model).

#include "encoder.h"

#include "granite.h"
#include "weights.h"

#include "conformer/conformer.h"
#include "granite_conformer/shaw_attn.h"
#include "transcribe-debug.h"
#include "transcribe-mel.h"
#include "transcribe-log.h"

#include "ggml.h"

#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <limits>
#include <vector>

namespace transcribe::granite {

// ---------------------------------------------------------------------------
// Constants
// ---------------------------------------------------------------------------

namespace {

// LayerNorm epsilon. Granite's encoder uses nn.LayerNorm whose default
// eps is 1e-5, matching the conformer helper's kLayerNormEps.
constexpr float kLayerNormEps = 1e-5f;

ggml_tensor * named(ggml_tensor * t, const char * name) {
    if (t != nullptr && name != nullptr) {
        ggml_set_name(t, name);
    }
    return t;
}

ggml_tensor * layer_norm(ggml_context * ctx, ggml_tensor * x,
                         ggml_tensor * gamma, ggml_tensor * beta)
{
    ggml_tensor * y = ggml_norm(ctx, x, kLayerNormEps);
    y = ggml_mul(ctx, y, gamma);
    if (beta != nullptr) {
        y = ggml_add(ctx, y, beta);
    }
    return y;
}

ggml_tensor * linear(ggml_context * ctx, ggml_tensor * x,
                     ggml_tensor * w, ggml_tensor * b)
{
    ggml_tensor * y = ggml_mul_mat(ctx, w, x);
    if (b != nullptr) {
        y = ggml_add(ctx, y, b);
    }
    return y;
}

} // namespace

// ---------------------------------------------------------------------------
// Host-side mel + 2-frame stack
// ---------------------------------------------------------------------------

transcribe_status compute_mel_encoder_input(
    const transcribe::MelFrontend & mel,
    const float *                   pcm,
    int                             n_samples,
    int                             n_threads,
    std::vector<float> &            out_mel,
    int &                           out_t_enc)
{
    if (pcm == nullptr || n_samples <= 0) {
        return TRANSCRIBE_ERR_INVALID_ARG;
    }

    std::vector<float> raw;
    int n_mels   = 0;
    int n_frames = 0;
    if (const transcribe_status st = mel.compute(pcm,
                                                 static_cast<size_t>(n_samples),
                                                 raw, n_mels, n_frames,
                                                 n_threads);
        st != TRANSCRIBE_OK)
    {
        return st;
    }

    // MelFrontend (whisper-mode) returns [n_mels, n_frames] row-major,
    // with the last (center-pad) frame already dropped. Granite then
    // drops the last frame if the resulting count is still odd:
    if ((n_frames % 2) == 1) {
        --n_frames;
    }
    const int t_enc = n_frames / 2;
    if (t_enc <= 0) {
        return TRANSCRIBE_ERR_INVALID_ARG;
    }

    // Reshape to [t_enc, 2*n_mels] row-major. Output row t is
    // [logmel(t=2t, 0..n_mels-1) || logmel(t=2t+1, 0..n_mels-1)].
    // raw[m * n_frames_original + t] addresses bin m at frame t. Note
    // that whisper-mode's drop reduces n_frames AFTER compute(), so the
    // stride here is the ORIGINAL n_frames it computed against. We can
    // recover that as raw.size() / n_mels.
    const int stride = static_cast<int>(raw.size() / static_cast<size_t>(n_mels));
    if (stride <= 0 || stride < n_frames) {
        log_msg(TRANSCRIBE_LOG_LEVEL_ERROR,
                     "granite: mel buffer stride (%d) < expected frames (%d)",
                     stride, n_frames);
        return TRANSCRIBE_ERR_GGUF;
    }

    const int input_dim = 2 * n_mels;
    out_mel.assign(static_cast<size_t>(t_enc) * input_dim, 0.0f);
    for (int t = 0; t < t_enc; ++t) {
        float * dst = out_mel.data() + static_cast<size_t>(t) * input_dim;
        for (int m = 0; m < n_mels; ++m) {
            dst[m]            = raw[static_cast<size_t>(m) * stride + 2 * t    ];
            dst[m + n_mels]   = raw[static_cast<size_t>(m) * stride + 2 * t + 1];
        }
    }
    out_t_enc = t_enc;
    return TRANSCRIBE_OK;
}

// ---------------------------------------------------------------------------
// Shaw attention_dists + last-block mask
// ---------------------------------------------------------------------------

std::vector<int32_t> precompute_attention_dists(int context_size, int max_pos_emb)
{
    // Reference (modeling_granite_speech.py:295-297):
    //   seq = torch.arange(context_size)
    //   relpos_dist = seq.view(-1, 1) - seq.view(1, -1)
    //   attention_dists = clamp(relpos_dist, ±context_size) + max_pos_emb
    //
    // `seq.view(-1, 1) - seq.view(1, -1)` broadcasts to a [c, r] matrix
    // whose element at (c, r) is `c - r` — i.e., the offset from the
    // KEY position back to the QUERY position. (i in the row index is
    // the query position; j in the column index is the key position.)
    //
    // We were writing `r - c` here — the opposite sign — which mirrored
    // the Shaw bias across the diagonal and corrupted block-local
    // attention from the first encoder layer.
    std::vector<int32_t> dists(static_cast<size_t>(context_size) * context_size);
    for (int c = 0; c < context_size; ++c) {
        for (int r = 0; r < context_size; ++r) {
            int d = c - r;
            if (d < -context_size) d = -context_size;
            if (d >  context_size) d =  context_size;
            // Row-major: dists[c * context_size + r].
            dists[static_cast<size_t>(c) * context_size + r] =
                static_cast<int32_t>(d + max_pos_emb);
        }
    }
    return dists;
}

std::vector<float> precompute_last_block_mask(int context_size, int t_enc_remainder)
{
    // t_enc_remainder is the number of REAL positions in the last
    // block. Positions in [t_enc_remainder, context_size) are pad.
    //
    // We mask ONLY the pad-K columns (force them to -INF so they
    // drop out of every valid-Q row's softmax). Pad-Q rows are NOT
    // masked: doing so would set every score in those rows to -INF
    // and softmax(-INF, -INF, ...) is NaN, which then propagates to
    // neighbouring valid positions via the depthwise conv. Pad-Q
    // row outputs are garbage but bounded (softmax-weighted V averages),
    // and they're sliced off before any caller-visible tensor.
    //
    // Mask shape ggml ne order: [k=context_size, q=context_size].
    // ne[0]=k is the innermost, so mask[q * context_size + k] addresses
    // (q, k). Linearize: -INF if k >= remainder, else 0. (Independent of q.)
    std::vector<float> mask(static_cast<size_t>(context_size) * context_size, 0.0f);
    if (t_enc_remainder <= 0 || t_enc_remainder >= context_size) {
        return mask;  // no pad in last block — all-zero mask is a no-op
    }
    const float neg_inf = -std::numeric_limits<float>::infinity();
    for (int q = 0; q < context_size; ++q) {
        for (int k = t_enc_remainder; k < context_size; ++k) {
            mask[static_cast<size_t>(q) * context_size + k] = neg_inf;
        }
    }
    return mask;
}

// ---------------------------------------------------------------------------
// Per-block builders
// ---------------------------------------------------------------------------

namespace {

// FFN (macaron half): pre-norm -> up_proj -> SiLU -> down_proj.
// Then residual: x = 0.5 * ff(x) + x.
//
// Reuses transcribe::conformer::macaron_ff_residual which has the
// exact same shape (pre-LN with bias, two biased linears, SiLU between,
// 0.5x residual scale).
ggml_tensor * granite_macaron(ggml_context * ctx,
                              ggml_tensor *  x,
                              const GraniteEncBlock & b,
                              bool           is_ff1)
{
    if (is_ff1) {
        return transcribe::conformer::macaron_ff_residual(
            ctx, x,
            b.norm_ff1_w, b.norm_ff1_b,
            b.ff1_up_w,   b.ff1_up_b,
            b.ff1_down_w, b.ff1_down_b);
    } else {
        return transcribe::conformer::macaron_ff_residual(
            ctx, x,
            b.norm_ff2_w, b.norm_ff2_b,
            b.ff2_up_w,   b.ff2_up_b,
            b.ff2_down_w, b.ff2_down_b);
    }
}

// Conv module: LN -> pointwise1 (d_model -> 2*inner_dim) -> GLU split
// (-> inner_dim) -> depthwise (k=15) -> BN -> SiLU -> pointwise2
// (inner_dim -> d_model). Operates on [d_model, T] (ne ordering).
//
// inner_dim = d_model * conv_expansion. For granite conv_expansion=2,
// so inner_dim = 2*d_model and the pw1 kernel maps d_model -> 4*d_model.
//
// The conformer::conv_module helper hardcodes inner_dim == d_model
// (parakeet / cohere / canary all use conv_expansion=1). We inline the
// module here. The shape of operations is otherwise identical.
ggml_tensor * granite_conv_module(ggml_context *          ctx,
                                  ggml_tensor *           x,
                                  const GraniteEncBlock & b,
                                  ggml_tensor *           bn_fused_scale,
                                  ggml_tensor *           bn_fused_bias,
                                  int                     conv_kernel,
                                  int                     inner_dim)
{
    const int64_t d_model = x->ne[0];
    const int64_t T       = x->ne[1];

    // Pre-LayerNorm with bias.
    x = layer_norm(ctx, x, b.norm_conv_w, b.norm_conv_b);

    // Pointwise1: [1, d_model, 2*inner_dim] kernel as a direct mul_mat.
    // After reshape to [d_model, 2*inner_dim] and mul_mat with
    // [d_model, T], output is [2*inner_dim, T].
    {
        ggml_tensor * pw1 = ggml_reshape_2d(ctx, b.conv_pointwise1_w,
                                            d_model, 2 * inner_dim);
        x = ggml_mul_mat(ctx, pw1, x);  // [2*inner_dim, T]
        x = ggml_add(ctx, x, b.conv_pointwise1_b);
    }

    // GLU: split ne[0] in half, gate * sigmoid(value).
    {
        ggml_tensor * gate  = ggml_view_2d(ctx, x, inner_dim, T,
                                           x->nb[1], /*offset=*/0);
        ggml_tensor * value = ggml_view_2d(ctx, x, inner_dim, T,
                                           x->nb[1],
                                           inner_dim * ggml_element_size(x));
        x = ggml_mul(ctx, gate, ggml_sigmoid(ctx, value));
    }
    // x ne = [inner_dim, T]

    // Transpose for depthwise conv: [inner_dim, T] -> [T, inner_dim].
    x = ggml_cont(ctx, ggml_permute(ctx, x, 1, 0, 2, 3));

    // Depthwise conv1d: kernel [k, 1, inner_dim], symmetric pad (k-1)/2
    // (= 7 for k=15). Granite's depthwise has no bias.
    const int padding = (conv_kernel - 1) / 2;
    x = transcribe::conformer::conv_1d_dw_f32(
        ctx, b.conv_depthwise_w, x,
        /*stride=*/1, /*padding=*/padding, /*dilation=*/1);

    // Fused BatchNorm: y = x * fused_scale + fused_bias, with 1-D scale
    // and bias broadcast across the time axis.
    x = transcribe::conformer::fused_batch_norm(ctx, x,
                                                bn_fused_scale,
                                                bn_fused_bias);

    // SiLU.
    x = ggml_silu(ctx, x);

    // Transpose back: [T, inner_dim] -> [inner_dim, T].
    x = ggml_cont(ctx, ggml_permute(ctx, x, 1, 0, 2, 3));

    // Pointwise2: [1, inner_dim, d_model] kernel as a direct mul_mat.
    // After reshape to [inner_dim, d_model] and mul_mat with
    // [inner_dim, T], output is [d_model, T].
    {
        ggml_tensor * pw2 = ggml_reshape_2d(ctx, b.conv_pointwise2_w,
                                            inner_dim, d_model);
        x = ggml_mul_mat(ctx, pw2, x);
        x = ggml_add(ctx, x, b.conv_pointwise2_b);
    }

    return x;
}


} // namespace

// ---------------------------------------------------------------------------
// Encoder graph
// ---------------------------------------------------------------------------

EncoderBuild build_encoder_graph(ggml_context *           ctx,
                                 const GraniteWeights &   weights,
                                 const GraniteHParams &   hp,
                                 int                      T_enc,
                                 bool                     /*use_flash*/)
{
    EncoderBuild eb {};
    eb.n_blocks_local = (T_enc + hp.enc_context_size - 1) / hp.enc_context_size;
    const int T_pad   = eb.n_blocks_local * hp.enc_context_size;
    eb.last_block_rem = T_enc - (eb.n_blocks_local - 1) * hp.enc_context_size;

    const int64_t d_model    = hp.enc_hidden;
    const int64_t input_dim  = hp.enc_input_dim;
    const int64_t inner_dim  = static_cast<int64_t>(hp.enc_hidden) *
                                hp.enc_conv_expansion;
    const int     conv_k     = hp.enc_conv_kernel_size;
    const int     n_heads    = hp.enc_n_heads;
    const int     head_dim   = hp.enc_head_dim;
    const int     ctx_size   = hp.enc_context_size;

    // ----- Graph inputs -----
    // mel_in: [input_dim, T_enc]. Encoder operates at T_enc throughout;
    // the only T_pad expansion happens INSIDE the Shaw block-local
    // attention helper, mirroring the reference's
    // `F.pad(hidden_states, (0, 0, 0, context_size - remainder))`
    // step which sits between pre_norm and the QKV projections.
    eb.mel_in = ggml_new_tensor_2d(ctx, GGML_TYPE_F32, input_dim, T_enc);
    named(eb.mel_in, "enc.mel_in");
    ggml_set_input(eb.mel_in);

    // attention_dists: [ctx_size, ctx_size] int32, row-major over (c, r).
    eb.attention_dists = ggml_new_tensor_1d(ctx, GGML_TYPE_I32,
                                            static_cast<int64_t>(ctx_size) * ctx_size);
    named(eb.attention_dists, "enc.attention_dists");
    ggml_set_input(eb.attention_dists);

    // last_block_mask: [ctx_size, ctx_size, n_blocks_local] additive
    // mask, all zero except last slice which carries -INF in pad cells.
    eb.last_block_mask = ggml_new_tensor_3d(ctx, GGML_TYPE_F32,
                                            ctx_size, ctx_size,
                                            eb.n_blocks_local);
    named(eb.last_block_mask, "enc.last_block_mask");
    ggml_set_input(eb.last_block_mask);

    // zero_pad: only needed when T_enc is not aligned to context_size.
    // [d_model, T_pad - T_enc] f32 zeros, uploaded by the caller. This
    // is the buffer that attention's internal pre-norm output is
    // concatenated against to reach T_pad.
    if (T_pad > T_enc) {
        eb.zero_pad = ggml_new_tensor_2d(ctx, GGML_TYPE_F32,
                                         d_model, T_pad - T_enc);
        named(eb.zero_pad, "enc.zero_pad");
        ggml_set_input(eb.zero_pad);
    }

    // ----- Input linear (160 -> 1024) -----
    ggml_tensor * x = linear(ctx, eb.mel_in,
                             weights.enc_top.input_linear_w,
                             weights.enc_top.input_linear_b);
    // x ne = [d_model, T_enc]
    named(x, "enc.input_linear.out");
    eb.dumps.input_linear_out = x;
    transcribe::debug::mark_tensor_for_dump(x);

    // BN fusion happens at load time and we read scale/bias from the
    // model (filled in model.cpp). For Phase 2 we accept that the
    // fused tensors come from external storage (via build_encoder_graph
    // caller wiring); we look them up off the granite weights struct.
    //
    // Each block in granite has its own [inner_dim] BN gamma/beta plus
    // running stats. The graph references the FUSED scale/bias which
    // model.cpp computes and writes once. They live as separate
    // ggml_tensor*s outside the GraniteWeights struct (in
    // GraniteModel::bn_fused); see model.cpp::fuse_batch_norm().
    //
    // For Phase 2 we accept a callback-style override: the caller must
    // ensure GraniteEncBlock::conv_bn_w / conv_bn_b / conv_bn_mean /
    // conv_bn_var have been "fused" into a pair of [inner_dim] f32
    // tensors and exposed through a side table. To keep build_encoder
    // self-contained we recompute the fusion on the fly here using
    // ggml ops:
    //   scale_i = bn_gamma_i / sqrt(bn_var_i + eps)
    //   bias_i  = bn_beta_i  - bn_mean_i * scale_i
    // These are 1-D fp32 tensors that fold cleanly into the graph.

    const int n_layers = static_cast<int>(weights.enc_blocks.size());
    const int bypass_after = hp.enc_n_layers / 2;  // 1-indexed boundary

    // cat_hidden_layers captures (post-LN, pre-bypass). Empty for 1b/2b;
    // size 1 with idx=3 for granite-speech-4.1-2b-plus. Holds raw ggml
    // tensor pointers; they remain valid for the lifetime of the
    // compute context.
    std::vector<ggml_tensor *> exported_hidden_states;
    exported_hidden_states.reserve(hp.enc_cat_hidden_layers.size());

    for (int i = 0; i < n_layers; ++i) {
        const auto & b = weights.enc_blocks[i];

        // --- FF1 macaron half ---
        x = granite_macaron(ctx, x, b, /*is_ff1=*/true);

        // --- Block-local Shaw self-attention ---
        // attention takes x at T_enc, pads internally to T_pad for the
        // block-local compute, and returns at T_enc — mirroring the
        // reference where to_out runs on the unpadded slice. This is
        // load-bearing: the conv module below cannot see pad-row
        // garbage near the tail (depthwise kernel would spread it).
        const transcribe::granite_conformer::ShawAttnWeights shaw_w {
            b.norm_attn_w, b.norm_attn_b,
            b.attn_q_w, b.attn_kv_w, b.attn_rel_pos_emb,
            b.attn_out_w, b.attn_out_b,
        };
        ggml_tensor * attn_out = transcribe::granite_conformer::shaw_block_attn(
            ctx, x,
            eb.zero_pad, eb.attention_dists, eb.last_block_mask,
            shaw_w,
            n_heads, head_dim, ctx_size, eb.n_blocks_local, T_enc,
            kLayerNormEps);
        if (attn_out == nullptr) {
            return eb;
        }
        x = ggml_add(ctx, x, attn_out);

        // --- Conv module (with pre-fused BN scale/bias) ---
        // model.cpp::fuse_batch_norm() precomputes
        //   scale = bn_w / sqrt(bn_var + eps)
        //   bias  = bn_b - bn_mean * scale
        // into [inner_dim] tensors stashed under conv_bn_fused_*.
        ggml_tensor * conv_out = granite_conv_module(
            ctx, x, b,
            b.conv_bn_fused_scale, b.conv_bn_fused_bias,
            conv_k, static_cast<int>(inner_dim));
        x = ggml_add(ctx, x, conv_out);

        // --- FF2 macaron half ---
        x = granite_macaron(ctx, x, b, /*is_ff1=*/false);

        // --- Post-block LayerNorm ---
        x = layer_norm(ctx, x, b.norm_post_w, b.norm_post_b);

        // --- cat_hidden_layers capture (-plus) ---
        // HF GraniteSpeechPlusCTCEncoder.forward:
        //   for idx, layer in enumerate(self.layers, start=1):
        //       hidden_states = layer(...)            # post_norm included
        //       if idx in cat_layers:
        //           exported_hidden_states.append(hidden_states)
        //       if idx == num_layers // 2:
        //           hidden_states += bypass
        // We are 0-indexed (i = idx - 1); the HF semantic is to capture
        // BEFORE the CTC bypass injection (which fires below).
        for (int32_t k : hp.enc_cat_hidden_layers) {
            if ((i + 1) == k) {
                exported_hidden_states.push_back(x);
                break;
            }
        }

        // --- Self-conditioned CTC bypass at layer N/2 ---
        // Reference: triggered when (i + 1) == num_layers // 2.
        if ((i + 1) == bypass_after) {
            // x: [d_model, T_pad]. ctc_proj: d_model -> output_dim.
            ggml_tensor * ctc_logits = ggml_mul_mat(ctx,
                                                    weights.enc_top.ctc_proj_w,
                                                    x);
            ctc_logits = ggml_add(ctx, ctc_logits, weights.enc_top.ctc_proj_b);
            // softmax over the channel axis (ne[0]).
            ggml_tensor * ctc_soft = ggml_soft_max(ctx, ctc_logits);
            // ctc_bypass: output_dim -> d_model.
            ggml_tensor * bypass = ggml_mul_mat(ctx,
                                                weights.enc_top.ctc_bypass_w,
                                                ctc_soft);
            bypass = ggml_add(ctx, bypass, weights.enc_top.ctc_bypass_b);
            x = ggml_add(ctx, x, bypass);
        }

        // --- Dumps ---
        // x is already [d_model, T_enc] (no pad), matching the
        // reference's hook on the bare layer output.
        if (i == 0) {
            named(x, "enc.block.0.out");
            eb.dumps.block_0_out = x;
            transcribe::debug::mark_tensor_for_dump(x);
        }
        if (i == bypass_after) {
            // i.e., block index N/2 (0-indexed). For n=16, this is
            // block 8 — Stage 2 dumps `enc.block.8.out` after block 8's
            // forward completes (with the bypass residual already
            // injected at block-8 input).
            char bname[64];
            std::snprintf(bname, sizeof(bname), "enc.block.%d.out", i);
            named(x, bname);
            eb.dumps.block_mid_out = x;
            transcribe::debug::mark_tensor_for_dump(x);
        }
        if (i == n_layers - 1) {
            char bname[64];
            std::snprintf(bname, sizeof(bname), "enc.block.%d.out", i);
            named(x, bname);
            eb.dumps.block_last_out = x;
            transcribe::debug::mark_tensor_for_dump(x);
        }
    }

    // ----- cat_hidden_layers channel concat (-plus only) -----
    // HF: hidden_states = torch.cat([*exported_hidden_states, hidden_states], dim=-1)
    // i.e. earlier-layer captures FIRST, final hidden LAST, along the
    // channel axis. ggml dim=0 is the channel axis for [hidden, T].
    // Build right-to-left so the final order matches HF: walk captures
    // in reverse, each prepended via concat(exp, acc).
    for (auto it = exported_hidden_states.rbegin();
         it != exported_hidden_states.rend(); ++it)
    {
        x = ggml_concat(ctx, *it, x, /*dim=*/0);
    }

    // ----- Final output (already at T_enc) -----
    named(x, "enc.out");
    eb.out = x;
    eb.dumps.out_named = x;
    ggml_set_output(eb.out);
    transcribe::debug::mark_tensor_for_dump(eb.out);

    eb.graph = ggml_new_graph_custom(ctx, /*size=*/16384, /*grads=*/false);
    if (eb.graph == nullptr) {
        log_msg(TRANSCRIBE_LOG_LEVEL_ERROR, "granite encoder: ggml_new_graph_custom failed");
        return eb;
    }
    ggml_build_forward_expand(eb.graph, eb.out);
    if (eb.dumps.input_linear_out) ggml_build_forward_expand(eb.graph, eb.dumps.input_linear_out);
    if (eb.dumps.block_0_out)      ggml_build_forward_expand(eb.graph, eb.dumps.block_0_out);
    if (eb.dumps.block_mid_out)    ggml_build_forward_expand(eb.graph, eb.dumps.block_mid_out);
    if (eb.dumps.block_last_out)   ggml_build_forward_expand(eb.graph, eb.dumps.block_last_out);

    return eb;
}

} // namespace transcribe::granite