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fix mha bwd golden perf issue #1666

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Merged
slippedJim merged 2 commits into from
Dec 18, 2025
Merged

fix mha bwd golden perf issue #1666

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c1a380c
fix mha bwd golden perf issue
JaxChen29 Dec 17, 2025
c88a847
Merge branch 'main' into fix_golden_perf_issue
JaxChen29 Dec 18, 2025
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21 changes: 14 additions & 7 deletions op_tests/cpp/mha/benchmark_mha_bwd.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -953,17 +953,24 @@ bool run(const ck_tile::ArgParser& arg_parser)
}

// dS_i_j = P_i_j .* (dP_i_j - dO_i dot O_i)
ds_hp_host_ref.ForEach([&](auto& self, auto idx_gmn) {
AccDataType do_dot_o = 0;
// Precompute dO_i dot O_i for each (head, seq_q) to avoid redundant computation
// This reduces complexity from O(nhead * seqlen_q * seqlen_k * hdim_v) to
// O(nhead * seqlen_q * hdim_v) + O(nhead * seqlen_q * seqlen_k)
ck_tile::HostTensor<AccDataType> do_dot_o_ref({nhead, real_seqlen_q});
do_dot_o_ref.ForEach([&](auto& self, auto idx_gm) {
AccDataType sum = 0;
for(int o = 0; o < hdim_v; o++)
{
auto idx_gmo = idx_gmn;
idx_gmo[2] = o;
do_dot_o += ck_tile::type_convert<AccDataType>(do_host_ref(idx_gmo)) *
ck_tile::type_convert<AccDataType>(o_host_refs[wb](idx_gmo));
sum += ck_tile::type_convert<AccDataType>(do_host_ref(idx_gm[0], idx_gm[1], o)) *
ck_tile::type_convert<AccDataType>(o_host_refs[wb](idx_gm[0], idx_gm[1], o));
}
self(idx_gm) = sum;
});

ds_hp_host_ref.ForEach([&](auto& self, auto idx_gmn) {
self(idx_gmn) = ck_tile::type_convert<AccDataType>(
p_hp_host_refs[wb](idx_gmn) * (dp_hp_host_ref(idx_gmn) - do_dot_o));
p_hp_host_refs[wb](idx_gmn) *
(dp_hp_host_ref(idx_gmn) - do_dot_o_ref(idx_gmn[0], idx_gmn[1])));
});

if(use_dbias)
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