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[Executorch] Use temp allocator for allocating scratch memory #15728

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[Executorch] Use temp allocator for allocating scratch memory #15728

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[Executorch] Use temp allocator for allocating scratch memory
kimishpatel Nov 11, 2025
1d96c89
Update on "[Executorch] Use temp allocator for allocating scratch mem…
kimishpatel Nov 14, 2025
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6 changes: 6 additions & 0 deletions extension/llm/custom_ops/op_sdpa.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -273,6 +273,7 @@ Tensor& flash_attention_kernel_out(
// we might consider another appraoch
if (seq_len >= 768) {
sdpa::impl::cpu_flash_attention<CTYPE, 256, 512>(
ctx,
output,
query,
key,
Expand All @@ -289,6 +290,7 @@ Tensor& flash_attention_kernel_out(
nullopt);
} else if (seq_len >= 192) {
sdpa::impl::cpu_flash_attention<CTYPE, 64, 512>(
ctx,
output,
query,
key,
Expand All @@ -305,6 +307,7 @@ Tensor& flash_attention_kernel_out(
nullopt);
} else {
sdpa::impl::cpu_flash_attention<CTYPE, 32, 512>(
ctx,
output,
query,
key,
Expand Down Expand Up @@ -418,6 +421,7 @@ Tensor& custom_sdpa_out_impl(
// we might consider another appraoch
if (seq_len >= 768) {
sdpa::impl::cpu_flash_attention<CTYPE, 256, 512>(
ctx,
output,
q,
k,
Expand All @@ -437,6 +441,7 @@ Tensor& custom_sdpa_out_impl(
num_keys_for_causal_attention);
} else if (seq_len >= 192) {
sdpa::impl::cpu_flash_attention<CTYPE, 64, 512>(
ctx,
output,
q,
k,
Expand All @@ -456,6 +461,7 @@ Tensor& custom_sdpa_out_impl(
num_keys_for_causal_attention);
} else {
sdpa::impl::cpu_flash_attention<CTYPE, 32, 512>(
ctx,
output,
q,
k,
Expand Down
39 changes: 26 additions & 13 deletions extension/llm/custom_ops/op_sdpa_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -35,6 +35,7 @@ enum class SeqDim { ONE = 1, TWO };

namespace sdpa::impl {

static std::vector<char> scratch_for_quant_dequant_vec;
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Copilot AI Nov 17, 2025

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This static vector scratch_for_quant_dequant_vec is declared but never used in the code. It appears to be a leftover from the refactoring where the local vector was replaced with the temp allocator approach. This should be removed.

Suggested change
static std::vector<char> scratch_for_quant_dequant_vec;

Copilot uses AI. Check for mistakes.
struct MaybeQuantizedMatrixData {
const void* data{nullptr};
const int8_t* zero_points{nullptr};
Expand Down Expand Up @@ -543,6 +544,7 @@ TODO: Just handle conversion of bool mask to float
*/
template <typename scalar_t, int64_t q_split_size, int64_t kv_split_size>
void cpu_flash_attention(
RuntimeContext& ctx,
Tensor& output,
const Tensor& query,
const Tensor& key,
Expand Down Expand Up @@ -766,26 +768,37 @@ void cpu_flash_attention(
int64_t size_of_intermediate_precision = sizeof(accum_t);
int64_t size_bytes = size_per_thread * num_thread * query.element_size() *
size_of_intermediate_precision;
std::vector<char> buf_vec(size_bytes);
void* buf = reinterpret_cast<void*>(buf_vec.data());
// Need to double check the following
size_bytes = num_thread * qSplitSize * kvSplitSize * query.element_size();
std::vector<char> buf_reduced_vec(size_bytes);
void* buf_reduced = reinterpret_cast<void*>(buf_reduced_vec.data());
// at::Tensor buf_reduced = at::empty(
// {num_thread, qSplitSize, is_reduced_type ? kvSplitSize : 0},
// query.options());
Result<void*> buff_res = ctx.allocate_temp(size_bytes);
std::unique_ptr<char[]> allocated_buf;
void* buf;
if (!buff_res.ok()) {
allocated_buf = std::make_unique<char[]>(size_bytes);
buf = reinterpret_cast<void*>(allocated_buf.get());
} else {
buf = buff_res.get();
}
void* buf_reduced = nullptr;
int64_t size_per_thread_qdq_vec = qSplitSize * kvSplitSize * headSize;
// Lets align size_per_thread_qdq_vec to 64 bytes, for coalesced cache reads,
// by padding with right number of per thread elements
constexpr int64_t kAlignment = 32;
size_per_thread_qdq_vec =
(size_per_thread_qdq_vec + kAlignment - 1) & (-(kAlignment - 1));
int64_t size_per_thread_qdq_bytes = size_per_thread_qdq_vec * sizeof(accum_t);
int64_t size_per_thread_qdq_bytes =
size_per_thread_qdq_vec * size_of_intermediate_precision;
int64_t size_qdq_bytes = size_per_thread_qdq_bytes * num_thread;
std::vector<char> scratch_for_quant_dequant_vec(size_qdq_bytes);
accum_t* scratch_for_quant_dequant =
reinterpret_cast<accum_t*>(scratch_for_quant_dequant_vec.data());
std::unique_ptr<char[]> allocated_buf_for_qdq;
Result<void*> scratch_for_quant_dequant_res =
ctx.allocate_temp(size_qdq_bytes);
accum_t* scratch_for_quant_dequant;
if (!scratch_for_quant_dequant_res.ok()) {
allocated_buf_for_qdq = std::make_unique<char[]>(size_qdq_bytes);
scratch_for_quant_dequant =
reinterpret_cast<accum_t*>(allocated_buf_for_qdq.get());
} else {
scratch_for_quant_dequant =
reinterpret_cast<accum_t*>(scratch_for_quant_dequant_res.get());
}

// Data ptrs
const scalar_t* q_data = query.const_data_ptr<scalar_t>();
Expand Down
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