attention_flash_true.c File Reference

Flash-style attention (online softmax, causal, streaming) More...

#include <math.h>
#include <stddef.h>
#include <stdint.h>

Go to the source code of this file.

Macros
#define	CK_FLASH_ATTN_FAST_EXP   0
#define	CK_FLASH_ATTN_TILE_K   32

Functions
void	attention_flash_cleanup (void)
	Clean up flash attention resources.
void	attention_flash_decode (float *out, const float *q, const float *k, const float *v, int T_q, int T_k, int H, int D_h, float scale)
	Main flash attention function with SIMD dispatch.
static void	attention_flash_decode_scalar (float *out, const float *q, const float *k, const float *v, int T_q, int T_k, int H, int D_h, float scale)
	Scalar flash-style attention (online softmax)
void	attention_flash_init (int max_context, int max_heads, int max_head_dim)
	Initialize flash attention buffers.
static float	ck_expf (float x)
static float	ck_fast_expf (float x)
int	ck_flash_attn_choose_tile_k (int D_h)
int	ck_flash_attn_fast_exp_kind (void)
static int	ck_flash_attn_tile_k (int D_h)
static int	max_k_for_query (int t_q, int T_q, int T_k)

Detailed Description

Flash-style attention (online softmax, causal, streaming)

CK-ENGINE KERNEL RULES:

1. NO malloc/free - memory via bump allocator, pointers passed in
2. NO OpenMP - parallelization at orchestrator/codegen layer
3. API must define: inputs, outputs, workspace, and memory layouts
4. Pure computation - deterministic, no side effects

After changes: make test && make llamacpp-parity-full

Layout: Q/K/V/Out: [T, H, D_h] contiguous

Causal alignment: Queries are assumed to correspond to the last T_q positions in the KV cache. This makes T_q == T_k behave like standard causal prefill, and T_q == 1 behave like decode over a full KV cache.

Notes:

• This is O(T_k) per query head; it avoids materializing the score matrix.
• SIMD paths are provided for AVX-512 and AVX.

Definition in file attention_flash_true.c.

Macro Definition Documentation

CK_FLASH_ATTN_FAST_EXP

#define CK_FLASH_ATTN_FAST_EXP   0

Definition at line 44 of file attention_flash_true.c.

CK_FLASH_ATTN_TILE_K

#define CK_FLASH_ATTN_TILE_K   32

Definition at line 40 of file attention_flash_true.c.

Function Documentation

attention_flash_cleanup()

void attention_flash_cleanup

(

void

)

Clean up flash attention resources.

Definition at line 739 of file attention_flash_true.c.

                                   {
    // For future optimization: free pre-allocated buffers
}

attention_flash_decode()

void attention_flash_decode	(	float *	out,
		const float *	q,
		const float *	k,
		const float *	v,
		int	T_q,
		int	T_k,
		int	H,
		int	D_h,
		float	scale
	)

Main flash attention function with SIMD dispatch.

Parameters

out	Output [T_q, H, D_h]
q	Query [T_q, H, D_h]
k	Key [T_k, H, D_h]
v	Value [T_k, H, D_h]
T_q	Number of query tokens (1 for decode)
T_k	Number of key/value tokens (context length)
H	Number of heads
D_h	Head dimension
scale	1/sqrt(D_h)

Definition at line 696 of file attention_flash_true.c.

{
    if (!out || !q || !k || !v) {
        return;
    }
    if (T_q <= 0 || T_k <= 0 || H <= 0 || D_h <= 0) {
        return;
    }
 
    // Dispatch based on CPU features
#if defined(__AVX512F__)
    attention_flash_decode_avx512(out, q, k, v, T_q, T_k, H, D_h, scale);
#elif defined(__AVX__) && !defined(__AVX512F__)
    attention_flash_decode_avx(out, q, k, v, T_q, T_k, H, D_h, scale);
#else
    attention_flash_decode_scalar(out, q, k, v, T_q, T_k, H, D_h, scale);
#endif
}

References attention_flash_decode_scalar().

Referenced by attention_forward_chunk_head_major_gqa_flash_gemma4(), attention_forward_decode_head_major_gqa_flash(), attention_forward_decode_head_major_gqa_flash_f16cache(), attention_forward_decode_head_major_gqa_flash_gemma4(), ck_attention_flash_decode_wrapper(), mega_fused_attention_prefill(), and mega_fused_attention_prefill_q8_0().

attention_flash_decode_scalar()

static void attention_flash_decode_scalar ( float *  out, const float *  q, const float *  k, const float *  v, int  T_q, int  T_k, int  H, int  D_h, float  scale  )

static

Scalar flash-style attention (online softmax)

Definition at line 142 of file attention_flash_true.c.

{
    const int total = T_q * H;
    const size_t stride = (size_t)H * (size_t)D_h;
    const int tile_k = ck_flash_attn_tile_k(D_h);
 
    for (int idx = 0; idx < total; ++idx) {
        const int t_q = idx / H;
        const int h = idx - t_q * H;
        const int max_k = max_k_for_query(t_q, T_q, T_k);
 
        const float *q_head = q + (size_t)t_q * stride + (size_t)h * (size_t)D_h;
        float *out_head = out + (size_t)t_q * stride + (size_t)h * (size_t)D_h;
        const float *k_base = k + (size_t)h * (size_t)D_h;
        const float *v_base = v + (size_t)h * (size_t)D_h;
 
        for (int d = 0; d < D_h; ++d) {
            out_head[d] = 0.0f;
        }
 
        float m = -INFINITY;
        float s = 0.0f;
 
        float scores[CK_FLASH_ATTN_TILE_K];
 
        for (int t_k0 = 0; t_k0 <= max_k; t_k0 += tile_k) {
            int blk_len = max_k - t_k0 + 1;
            if (blk_len > tile_k) {
                blk_len = tile_k;
            }
 
            float m_block = -INFINITY;
            for (int bi = 0; bi < blk_len; ++bi) {
                const int t_k = t_k0 + bi;
                const float *k_head = k_base + (size_t)t_k * stride;
 
                float dot = 0.0f;
                for (int d = 0; d < D_h; ++d) {
                    dot += q_head[d] * k_head[d];
                }
 
                float score = dot * scale;
                scores[bi] = score;
                if (score > m_block) {
                    m_block = score;
                }
            }
 
            if (m_block > m) {
                float scale_old = (m == -INFINITY) ? 0.0f : ck_expf(m - m_block);
                s *= scale_old;
                for (int d = 0; d < D_h; ++d) {
                    out_head[d] *= scale_old;
                }
                m = m_block;
            }
 
            for (int bi = 0; bi < blk_len; ++bi) {
                const int t_k = t_k0 + bi;
                const float *v_head = v_base + (size_t)t_k * stride;
                float w = ck_expf(scores[bi] - m);
                s += w;
                for (int d = 0; d < D_h; ++d) {
                    out_head[d] += w * v_head[d];
                }
            }
        }
 
        if (s > 0.0f) {
            float inv_s = 1.0f / s;
            for (int d = 0; d < D_h; ++d) {
                out_head[d] *= inv_s;
            }
        } else {
            for (int d = 0; d < D_h; ++d) {
                out_head[d] = 0.0f;
            }
        }
    }
}

References ck_expf(), CK_FLASH_ATTN_TILE_K, ck_flash_attn_tile_k(), max_k_for_query(), and score.

Referenced by attention_flash_decode().

attention_flash_init()

void attention_flash_init	(	int	max_context,
		int	max_heads,
		int	max_head_dim
	)

Initialize flash attention buffers.

Definition at line 731 of file attention_flash_true.c.

                                                                            {
    // For future optimization: pre-allocate scratch buffers
    // Currently using stack/heap allocation
}

ck_expf()

static float ck_expf ( float  x )

inlinestatic

Definition at line 80 of file attention_flash_true.c.

                                     {
#if CK_FLASH_ATTN_FAST_EXP
    return ck_fast_expf(x);
#else
    return expf(x);
#endif
}

References ck_fast_expf().

Referenced by attention_flash_decode_scalar().

ck_fast_expf()

static float ck_fast_expf ( float  x )

inlinestatic

Definition at line 47 of file attention_flash_true.c.

                                          {
    const float max_val = 88.0f;
    const float min_val = -88.0f;
    if (x > max_val) {
        x = max_val;
    } else if (x < min_val) {
        x = min_val;
    }
 
    const float log2e = 1.4426950408889634f;
    float z = x * log2e;
    float zf = nearbyintf(z);
    float f = z - zf;
 
    const float c0 = 1.0f;
    const float c1 = 0.6931471805599453f;
    const float c2 = 0.2402265069591007f;
    const float c3 = 0.05550410866482158f;
    const float c4 = 0.009618129107628478f;
 
    float poly = ((c4 * f + c3) * f + c2) * f + c1;
    poly = poly * f + c0;
 
    int32_t zi = (int32_t)zf + 127;
    uint32_t bits = (uint32_t)zi << 23;
    union {
        uint32_t i;
        float f;
    } u;
    u.i = bits;
    return poly * u.f;
}

Referenced by ck_expf().

ck_flash_attn_choose_tile_k()

int ck_flash_attn_choose_tile_k

(

int

D_h

)

Definition at line 108 of file attention_flash_true.c.

                                         {
    return ck_flash_attn_tile_k(D_h);
}

References ck_flash_attn_tile_k().

ck_flash_attn_fast_exp_kind()

int ck_flash_attn_fast_exp_kind

(

void

)

Definition at line 112 of file attention_flash_true.c.

                                      {
#if CK_FLASH_ATTN_FAST_EXP
#if defined(__AVX512F__)
    return 512;
#elif defined(__AVX__)
    return 256;
#else
    return 0;
#endif
#else
    return 0;
#endif
}

ck_flash_attn_tile_k()

static int ck_flash_attn_tile_k ( int  D_h )

inlinestatic

Definition at line 88 of file attention_flash_true.c.

                                                {
    int tile = CK_FLASH_ATTN_TILE_K;
    if (D_h > 128) {
        tile = CK_FLASH_ATTN_TILE_K / 4;
    } else if (D_h > 64) {
        tile = CK_FLASH_ATTN_TILE_K / 2;
    }
 
    if (CK_FLASH_ATTN_TILE_K >= 8 && tile < 8) {
        tile = 8;
    }
    if (tile > CK_FLASH_ATTN_TILE_K) {
        tile = CK_FLASH_ATTN_TILE_K;
    }
    if (tile < 1) {
        tile = 1;
    }
    return tile;
}

References CK_FLASH_ATTN_TILE_K.

Referenced by attention_flash_decode_scalar(), and ck_flash_attn_choose_tile_k().

max_k_for_query()

static int max_k_for_query ( int  t_q, int  T_q, int  T_k  )

inlinestatic

Definition at line 126 of file attention_flash_true.c.

                                                             {
    int q_pos_offset = (T_k > T_q) ? (T_k - T_q) : 0;
    int max_k = q_pos_offset + t_q;
    if (max_k >= T_k) {
        max_k = T_k - 1;
    }
    return max_k;
}

Referenced by attention_flash_decode_scalar().