Skip to content

Fix stable softmax exp_table indexing sign-bit inefficiency #1480

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
niteshg97 wants to merge 2 commits into from
+4 −3
Closed

Fix stable softmax exp_table indexing sign-bit inefficiency #1480

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
d3d06eb
Fix stable softmax LUT indexing for nonnegative inputs
niteshg97 May 23, 2026
55bd6db
Fix formatting and remove unnecessary line in softmax function
niteshg97 May 23, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
7 changes: 4 additions & 3 deletions hls4ml/templates/vivado/nnet_utils/nnet_activation.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -138,10 +138,11 @@ template <class data_T, unsigned table_size> inline float softmax_real_val_from_
return (float)x;
}

template <class data_T, unsigned table_size> inline unsigned softmax_idx_from_real_val(data_T x) {
template <class data_T, unsigned table_size> inline unsigned softmax_idx_from_real_val_unsigned(data_T x) {
// Slice the top N bits to get an index into the table
static constexpr int N = ceillog2(table_size); // number of address bits for table
ap_uint<N> y = x(x.width - 1, x.width - N); // slice the top N bits of input
// skip sign bit because inputs are guaranteed nonnegative
ap_uint<N> y = x(x.width - 2, x.width - N - 1); // slice the top N bits of input
return (unsigned)y(N - 1, 0);
}

Expand Down Expand Up @@ -254,7 +255,7 @@ void softmax_stable(data_T data[CONFIG_T::n_slice], res_T res[CONFIG_T::n_slice]
typename CONFIG_T::inv_inp_t exp_sum(0);
for (unsigned i = 0; i < CONFIG_T::n_slice; i++) {
#pragma HLS unroll
unsigned x = softmax_idx_from_real_val<typename CONFIG_T::inp_norm_t, CONFIG_T::exp_table_size>(d_xi_xmax[i]);
unsigned x = softmax_idx_from_real_val_unsigned<typename CONFIG_T::inp_norm_t, CONFIG_T::exp_table_size>(d_xi_xmax[i]);
exp_res[i] = exp_table[x];
}

Expand Down