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Mojo struct

SoftmaxRDNA

struct SoftmaxRDNA[dtype: DType, num_m_mmas: Int, num_n_mmas: Int, num_warps_m: Int, num_warps_n: Int, use_exp2: Bool = False]

Online softmax accumulator for RDNA Wave32 attention kernels.

Tracks per-row running maximum and sum across online softmax iterations so that attention scores can be normalized incrementally as new key/value tiles arrive. The full() method runs one complete iteration (max → exp → sum → correction → output update) end-to-end.

Parameters​

  • ​dtype (DType): Element type of the score and output fragments.
  • ​num_m_mmas (Int): Number of MMA tiles along the query (row) dimension.
  • ​num_n_mmas (Int): Number of MMA tiles along the key (column) dimension.
  • ​num_warps_m (Int): Number of warps assigned to the row dimension.
  • ​num_warps_n (Int): Number of warps assigned to the column dimension.
  • ​use_exp2 (Bool): Selects exp2 instead of natural exp for the softmax kernel.

Fields​

  • ​rowmax_tensor (SoftmaxRDNA[dtype, num_m_mmas, num_n_mmas, num_warps_m, num_warps_n, use_exp2].RowMaxTensorType):
  • ​rowsum_tensor (SoftmaxRDNA[dtype, num_m_mmas, num_n_mmas, num_warps_m, num_warps_n, use_exp2].RowSumTensorType):
  • ​score_frag_rowmax (SoftmaxRDNA[dtype, num_m_mmas, num_n_mmas, num_warps_m, num_warps_n, use_exp2].ScoreFragTensorType):
  • ​score_frag_rowsum (SoftmaxRDNA[dtype, num_m_mmas, num_n_mmas, num_warps_m, num_warps_n, use_exp2].ScoreFragTensorType):
  • ​correction (SoftmaxRDNA[dtype, num_m_mmas, num_n_mmas, num_warps_m, num_warps_n, use_exp2].ScoreFragTensorType):

Implemented traits​

AnyType, ImplicitlyDeletable

comptime members​

exp_function​

comptime exp_function = _exp2_concrete[_, _] if use_exp2 else _exp_concrete[_, _]

frag_is_row_vector​

comptime frag_is_row_vector = True

frag_num_rows​

comptime frag_num_rows = ComptimeInt[Int(1)].static_value

frag_size​

comptime frag_size = SoftmaxRDNA[dtype, num_m_mmas, num_n_mmas, num_warps_m, num_warps_n, use_exp2].FragmentLayoutT.static_product

FragmentLayoutT​

comptime FragmentLayoutT = Layout[*?, *?]

num_colwise_lanes​

comptime num_colwise_lanes = SIMD(Int(16))

num_colwise_tiles​

comptime num_colwise_tiles = num_m_mmas

num_colwise_warps​

comptime num_colwise_warps = num_warps_m

num_rowwise_lanes​

comptime num_rowwise_lanes = SIMD(Int(2))

num_rowwise_tiles​

comptime num_rowwise_tiles = num_n_mmas

num_rowwise_warps​

comptime num_rowwise_warps = num_warps_n

row_layout​

comptime row_layout = row_major[num_m_mmas, Int(1)]()

RowMaxTensorType​

comptime RowMaxTensorType = TileTensor[dtype, Layout[*?, *?], MutUntrackedOrigin, address_space=AddressSpace.LOCAL]

RowSumTensorType​

comptime RowSumTensorType = SoftmaxRDNA[dtype, num_m_mmas, num_n_mmas, num_warps_m, num_warps_n, use_exp2].RowMaxTensorType

rowwise_lanes_stride​

comptime rowwise_lanes_stride = SIMD(Int(16))

score_frag_layout​

comptime score_frag_layout = row_major[SoftmaxRDNA[dtype, num_m_mmas, num_n_mmas, num_warps_m, num_warps_n, use_exp2].num_colwise_tiles, Int(1)]()

ScoreFragTensorType​

comptime ScoreFragTensorType = TileTensor[dtype, Layout[*?, *?], MutUntrackedOrigin, address_space=AddressSpace.LOCAL]

warp_cols​

comptime warp_cols = ComptimeInt[Int(2)].static_value

warp_rows​

comptime warp_rows = ComptimeInt[Int(16)].static_value

WarpLayoutT​

comptime WarpLayoutT = Layout[*?, *?]

Methods​

__init__​

def __init__(out self)

calculate_qk_max​

def calculate_qk_max(self, score: TileTensor[dtype, Storage=score.Storage, address_space=score.address_space, linear_idx_type=score.linear_idx_type], warp_scratch: TileTensor[dtype, Storage=warp_scratch.Storage, address_space=warp_scratch.address_space, linear_idx_type=warp_scratch.linear_idx_type])

calculate_qk_sum​

def calculate_qk_sum(self, score: TileTensor[dtype, Storage=score.Storage, address_space=score.address_space, linear_idx_type=score.linear_idx_type], warp_scratch: TileTensor[dtype, Storage=warp_scratch.Storage, address_space=warp_scratch.address_space, linear_idx_type=warp_scratch.linear_idx_type])

exp​

def exp(self, score: TileTensor[dtype, Storage=score.Storage, address_space=score.address_space, linear_idx_type=score.linear_idx_type])

calculate_correction​

def calculate_correction(self)

update_output​

def update_output(self, output: TileTensor[dtype, Storage=output.Storage, address_space=output.address_space, linear_idx_type=output.linear_idx_type])

update_sum​

def update_sum(self)

update_max​

def update_max(self)

full​

def full(self, output: TileTensor[dtype, Storage=output.Storage, address_space=output.address_space, linear_idx_type=output.linear_idx_type], score: TileTensor[dtype, Storage=score.Storage, address_space=score.address_space, linear_idx_type=score.linear_idx_type], warp_scratch: TileTensor[dtype, Storage=warp_scratch.Storage, address_space=warp_scratch.address_space, linear_idx_type=warp_scratch.linear_idx_type])

Single-pass online softmax iteration: max -> exp -> sum -> correction -> update output -> update max/sum.

Args:

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