Function arm_nn_size_mul

Function Documentation

static int64_t arm_nn_size_mul(const int64_t acc, const int64_t factor)

Fold one dimension into a running buffer-size product, reporting overflow as -1.

Buffer-size queries return an int32_t byte count, so the product of the dimensions they multiply has to be rejected as soon as it cannot fit. Folding one factor at a time keeps the accumulator bounded: an accumulator already known to be <= INT32_MAX times a factor <= INT32_MAX cannot exceed about 2^62, so the int64_t accumulator itself never wraps. Chaining raw (int64_t) casts across three or more int32_t dims does not have that property - 65536 * 65536 * 65536 * 65536 is exactly 2^64 and folds back to 0, which would sail through a trailing “> INT32_MAX” test.

Note

This is the -1 sentinel family, used by the s8/s16 integer buffer-size queries, by the eight SVDF staging queries (arm_svdf_{s8,state_s16_s8,f32,f16}_{input,output}_ctx_get_buffer_size) and by the s8/s16 LSTM temp-buffer queries and the GRU temp queries (arm_lstm_unidirectional_{s8,s16}_temp{1,2}_get_buffer_size, arm_gru_unidirectional_{f32,f16}_temp1_get_buffer_size). The four f32/f16 LSTM temp queries have no dimensions to fold (the buffers are unused) and answer -1 only for NULL params, 0 otherwise. It is not interchangeable with the arm_nn_checked_size_mul() / arm_nn_size_to_i32_or_zero() helpers in Source/NNSupportFunctions (shared header for the float sizers), which most f32 and f16 buffer-size queries use and which report an out-of-range size as 0. Mixing the two silently flips a sizer’s out-of-range contract from “must never be used to size a buffer” to “you may pass { NULL, 0 }”, so pick the one the surrounding family already uses.

Note

The split is per sizer, not per datatype. The four SVDF f32/f16 staging queries deliberately use this -1 family rather than the 0 one their neighbours use, because their kernels read ctx->size and a size of 0 opts out of the scratch-size check - so a 0-on-overflow answer fed back as { alloc(0), 0 } would disable the very check meant to catch it. Do not infer a sizer’s sentinel from its datatype suffix.

Parameters:

  • acc – [in] Running product, or -1 if an earlier fold already overflowed.

  • factor – [in] Next factor to fold in.

Returns:

acc * factor, or -1 if acc is already -1, factor is negative or out of int32_t range, or the product exceeds INT32_MAX.