ggml : ggml_flash_attn_ext() support ALiBi (CPU)

Author: ggerganov

ggml.c

@@ -6436,7 +6436,8 @@ struct ggml_tensor * ggml_flash_attn_ext(
         struct ggml_tensor  * k,
         struct ggml_tensor  * v,
         struct ggml_tensor  * mask,
-        float                 scale) {
+        float                 scale,
+        float                 max_bias) {
     GGML_ASSERT(ggml_can_mul_mat(k, q));
     // TODO: check if vT can be multiplied by (k*qT)
     if (mask) {
@@ -6458,7 +6459,7 @@ struct ggml_tensor * ggml_flash_attn_ext(
     int64_t ne[4] = { q->ne[0], q->ne[2], q->ne[1], q->ne[3] };
     struct ggml_tensor * result = ggml_new_tensor(ctx, GGML_TYPE_F32, 4, ne);
 
-    float params[] = { scale };
+    float params[] = { scale, max_bias };
     ggml_set_op_params(result, params, sizeof(params));
 
     result->op   = GGML_OP_FLASH_ATTN_EXT;
@@ -6478,7 +6479,7 @@ void ggml_flash_attn_ext_set_prec(
 
     const int32_t prec_i32 = (int32_t) prec;
 
-    ggml_set_op_params_i32(a, 1, prec_i32); // scale is on first pos
+    ggml_set_op_params_i32(a, 2, prec_i32); // scale is on first pos, max_bias on second
 }
 
 // ggml_flash_ff
@@ -15524,8 +15525,17 @@ static void ggml_compute_forward_flash_attn_ext_f16(
     const int ir0 = dr*ith;
     const int ir1 = MIN(ir0 + dr, nr);
 
-    float scale = 1.0f;
-    memcpy(&scale, (float *) dst->op_params + 0, sizeof(float));
+    float scale    = 1.0f;
+    float max_bias = 0.0f;
+
+    memcpy(&scale,    (float *) dst->op_params + 0, sizeof(float));
+    memcpy(&max_bias, (float *) dst->op_params + 1, sizeof(float));
+
+    const uint32_t n_head_kv   = neq2;
+    const uint32_t n_head_log2 = 1u << (uint32_t) floor(log2(n_head_kv));
+
+    const float m0 = powf(2.0f, -(max_bias       ) / n_head_log2);
+    const float m1 = powf(2.0f, -(max_bias / 2.0f) / n_head_log2);
 
     // loop over n_batch and n_head
     for (int ir = ir0; ir < ir1; ++ir) {
@@ -15534,6 +15544,9 @@ static void ggml_compute_forward_flash_attn_ext_f16(
         const int iq2 = (ir - iq3*neq2*neq1)/neq1;
         const int iq1 = (ir - iq3*neq2*neq1 - iq2*neq1);
 
+        const int h = iq2; // head
+        const float slope = (max_bias > 0.0f) ? h < n_head_log2 ? powf(m0, h + 1) : powf(m1, 2*(h - n_head_log2) + 1) : 1.0f;
+
         float S = 0.0f;
         float M = -INFINITY;
 
@@ -15557,7 +15570,7 @@ static void ggml_compute_forward_flash_attn_ext_f16(
         // loop over n_kv and n_head_kv
         // ref: https://arxiv.org/pdf/2112.05682.pdf
         for (int64_t ic = 0; ic < nek1; ++ic) {
-            const float mv = mp ? GGML_FP16_TO_FP32(mp[ic]) : 0.0f;
+            const float mv = mp ? slope*GGML_FP16_TO_FP32(mp[ic]) : 0.0f;
             if (mv == -INFINITY) {
                 continue;
             }
@@ -15628,7 +15641,7 @@ static void ggml_compute_forward_flash_attn_ext(
         const struct ggml_tensor * v,
         const struct ggml_tensor * mask,
         struct ggml_tensor * dst) {
-    switch (dst->op_params[1]) {
+    switch (dst->op_params[2]) {
         case GGML_PREC_DEFAULT:
         case GGML_PREC_F32:
             {

ggml.h

@@ -1731,7 +1731,8 @@ extern "C" {
             struct ggml_tensor  * k,
             struct ggml_tensor  * v,
             struct ggml_tensor  * mask,
-            float                 scale);
+            float                 scale,
+            float                 max_bias);
 
     GGML_API void ggml_flash_attn_ext_set_prec(
             struct ggml_tensor * a,

llama.cpp

@@ -6537,7 +6537,7 @@ static struct ggml_tensor * llm_build_kqv(
                     0);
         cb(v, "v", il);
 
-        cur = ggml_flash_attn_ext(ctx, q, k, v, kq_mask, kq_scale);
+        cur = ggml_flash_attn_ext(ctx, q, k, v, kq_mask, kq_scale, hparams.f_max_alibi_bias);
 
         if (model.arch == LLM_ARCH_PHI2 || model.arch == LLM_ARCH_PHI3) {
             ggml_flash_attn_ext_set_prec(cur, GGML_PREC_F32);