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fix: allow flash attention in encoder when DTW is enabled #3704

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fix: allow flash attention in encoder when DTW is enabled #3704

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15 changes: 11 additions & 4 deletions src/whisper.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2317,7 +2317,11 @@ static struct ggml_cgraph * whisper_build_graph_cross(
struct ggml_tensor * k;
struct ggml_tensor * v;

if (wctx.params.flash_attn) {
// Use non-flash layout for cross-attention KV when DTW is enabled,
// since DTW needs the explicit cross-attention weights (KQ_soft_max)
const bool flash_cross = wctx.params.flash_attn && !wctx.params.dtw_token_timestamps;

if (flash_cross) {
k = ggml_view_1d(ctx0, wstate.kv_cross.k, n_state*n_ctx,
(ggml_element_size(wstate.kv_cross.k)*n_state)*(il*n_ctx_pad));

Expand Down Expand Up @@ -2677,7 +2681,11 @@ static struct ggml_cgraph * whisper_build_graph_decoder(
ggml_reshape_3d(ctx0, Qcur, n_state_head, n_head, n_tokens),
0, 2, 1, 3);

if (wctx.params.flash_attn) {
// Use non-flash path for cross-attention when DTW is enabled,
// since DTW needs the explicit cross-attention weights (KQ_soft_max)
const bool flash_cross = wctx.params.flash_attn && !wctx.params.dtw_token_timestamps;

if (flash_cross) {
struct ggml_tensor * Kcross =
ggml_view_3d(ctx0, wstate.kv_cross.k,
n_state_head, n_audio_ctx_pad, n_head,
Expand Down Expand Up @@ -3706,8 +3714,7 @@ struct whisper_context * whisper_init_with_params_no_state(struct whisper_model_
ggml_time_init();

if (params.flash_attn && params.dtw_token_timestamps) {
WHISPER_LOG_WARN("%s: dtw_token_timestamps is not supported with flash_attn - disabling\n", __func__);
params.dtw_token_timestamps = false;
WHISPER_LOG_INFO("%s: flash_attn with dtw - disabling flash attention for cross-attention only\n", __func__);
}

WHISPER_LOG_INFO("%s: use gpu = %d\n", __func__, params.use_gpu);
Expand Down