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Fold the sigmoid gate into the causal-attention reduce epilogue: operator -8.3% where it folds, +0.31% decode #225

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Fold the sigmoid gate into the causal-attention reduce epilogue: operator -8.3% where it folds, +0.31% decode #225

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9fe228a
perf(ops): let causal attention finish with the sigmoid gate
ARVLab Sep 9, 2026
2b7fb9e
test(attention): qualify the gated output against the FP64 oracle, an…
MichaelDementii Sep 9, 2026
d63848e
bench(attention): name the gate mode in the profile header too
MichaelDementii Sep 9, 2026
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88 changes: 68 additions & 20 deletions bench/ops/causal_softmax_attention_bench.cu
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@
#include "core/device.h"
#include "core/paged_kv_cache.h"
#include "core/paged_kv_storage.h"
#include "ninfer/ops/sigmoid_mul.h"
#include "ninfer_bench_common.h"

#include <cuda_profiler_api.h>
Expand Down Expand Up @@ -45,6 +46,12 @@ enum class Execution : std::uint8_t { Eager, Graph, Both };
enum class CacheMode : std::uint8_t { Cold, Warm, Both };
enum class CacheState : std::uint8_t { Cold, Warm };
enum class PageMapping : std::uint8_t { Identity, Fragmented };
// off: no gate at all. standalone: the Op runs ungated and the standalone sigmoid_mul follows,
// which is exactly what a full-attention layer used to issue. fused: the gate is handed to the Op.
// standalone and fused compute the same bytes, so their medians compare directly; off is the
// control that neither form changes. The cached entry takes no gate parameter, so it applies the
// standalone multiply in both gated modes and is a second control inside the same table.
enum class GateMode : std::uint8_t { Off, Standalone, Fused };

struct Geometry {
const char* name;
Expand All @@ -62,6 +69,7 @@ struct Options {
Execution execution = Execution::Graph;
CacheMode cache = CacheMode::Cold;
PageMapping mapping = PageMapping::Identity;
GateMode gate = GateMode::Off;
std::vector<std::int32_t> batches{1};
std::vector<std::int32_t> tokens{1, 2, 4, 6, 8, 12, 16, 1024};
std::vector<std::int32_t> contexts{0, 128, 2048, 8192};
Expand Down Expand Up @@ -100,8 +108,22 @@ struct Result {
bench::ColdTiming timing;
std::size_t graph_nodes = 0, workspace_peak = 0;
int graph_calls = 1;
// The mode this row was actually measured in, not the one asked for on the command line: the
// cached entry takes no gate parameter, so it is standalone whenever a gate is requested.
GateMode gate = GateMode::Off;
};

const char* gate_name(GateMode gate) noexcept {
return gate == GateMode::Off ? "off" : gate == GateMode::Standalone ? "standalone" : "fused";
}

// The cached entry takes no gate parameter, so a requested fused gate is a standalone multiply
// there. Every line that names a gate names this one.
GateMode effective_gate(GateMode requested, Entry entry) noexcept {
return entry == Entry::Cached && requested == GateMode::Fused ? GateMode::Standalone
: requested;
}

[[noreturn]] void usage(const char* message) {
std::fprintf(stderr,
"error: %s\n"
Expand All @@ -112,7 +134,7 @@ struct Result {
"[--context L,...] [--row-contexts L0,...] [--valid-columns V0,...] "
"[--table-rows R0,...] "
"[--execution eager|graph|both] [--cache cold|warm|both] "
"[--mapping identity|fragmented] "
"[--mapping identity|fragmented] [--gate off|standalone|fused] "
"[--warmup N] [--repeat N] [--graph-calls N] [--profile] [--csv-out PATH]\n",
message);
std::exit(2);
Expand Down Expand Up @@ -235,6 +257,16 @@ Options parse_options(int argc, char** argv) {
options.mapping = PageMapping::Fragmented;
else
usage("--mapping expects identity or fragmented");
} else if (argument == "--gate") {
const std::string_view value(next("--gate requires a value"));
if (value == "off")
options.gate = GateMode::Off;
else if (value == "standalone")
options.gate = GateMode::Standalone;
else if (value == "fused")
options.gate = GateMode::Fused;
else
usage("--gate expects off, standalone, or fused");
} else if (argument == "--graph-calls") {
options.graph_calls =
parse_i32(next("--graph-calls requires a value"), 1, 128, "--graph-calls");
Expand Down Expand Up @@ -417,8 +449,9 @@ class Case {
public:
Case(Geometry geometry, KvCacheStorage storage, std::int32_t tokens,
std::span<const std::int32_t> contexts, std::span<const std::int32_t> valid_columns,
std::span<const std::int32_t> table_rows, PageMapping mapping)
: storage_layout_(paged_kv_storage_layout(storage, kHeadDim)),
std::span<const std::int32_t> table_rows, PageMapping mapping,
GateMode gate = GateMode::Off)
: storage_layout_(paged_kv_storage_layout(storage, kHeadDim)), gate_(gate),
batch_(static_cast<std::int32_t>(contexts.size())),
masked_(std::any_of(valid_columns.begin(), valid_columns.end(),
[tokens](std::int32_t valid) { return valid != tokens; })),
Expand Down Expand Up @@ -451,6 +484,8 @@ public:
block_table_(static_cast<std::size_t>(logical_pages_) * batch_ * sizeof(std::int32_t)),
output_(bench::make_zeros(static_cast<std::size_t>(kHeadDim) * geometry.query_heads *
tokens * batch_ * 2)),
gate_buffer_(bench::make_bf16(static_cast<std::size_t>(kHeadDim) * geometry.query_heads *
tokens * batch_)),
workspace_bytes_(workspace_capacity(geometry, storage, tokens, batch_, visible_)),
workspace_(std::max<std::size_t>(workspace_bytes_, 1)),
q_tensor_(q_.p, DType::BF16, {kHeadDim, geometry.query_heads, tokens, batch_}),
Expand All @@ -460,6 +495,8 @@ public:
valid_columns_tensor_(valid_columns_.p, DType::I32, {batch_}),
table_rows_tensor_(table_rows_.p, DType::I32, {batch_}),
output_tensor_(output_.p, DType::BF16, {kHeadDim, geometry.query_heads, tokens, batch_}),
gate_tensor_(gate_buffer_.p, DType::BF16,
{kHeadDim, geometry.query_heads, tokens, batch_}),
cache_view_(make_cache_view(cache_k_, cache_v_, cache_k_scale_, cache_v_scale_,
block_table_, geometry, storage, padded_, physical_pages_)),
batch_cache_view_(make_batch_cache_view(cache_k_, cache_v_, cache_k_scale_,
Expand Down Expand Up @@ -524,12 +561,16 @@ public:
ops::causal_softmax_attention(
q_tensor_, k_tensor_, v_tensor_, positions_tensor_, validity, table_rows_tensor_,
{kHeadDim, q_tensor_.ne[1], k_tensor_.ne[1]}, kScale, batch_cache_view_, envelope_,
workspace_, output_tensor_, stream);
workspace_, output_tensor_, stream,
gate_ == GateMode::Fused ? &gate_tensor_ : nullptr);
} else {
ops::causal_softmax_attention_cached(
q_tensor_, positions_tensor_, {kHeadDim, q_tensor_.ne[1], cache_view_.num_kv_heads},
kScale, cache_view_, envelope_, workspace_, output_tensor_, stream);
}
if (gate_ == GateMode::Standalone || (gate_ == GateMode::Fused && entry == Entry::Cached)) {
ops::sigmoid_mul(gate_tensor_, output_tensor_, stream);
}
}

[[nodiscard]] std::size_t workspace_bytes() const noexcept { return workspace_bytes_; }
Expand All @@ -542,6 +583,7 @@ public:

private:
PagedKVStorageLayout storage_layout_;
GateMode gate_;
std::int32_t batch_;
bool masked_;
std::int32_t visible_;
Expand All @@ -561,6 +603,7 @@ private:
DeviceBuffer cache_v_scale_;
DeviceBuffer block_table_;
DeviceBuffer output_;
DeviceBuffer gate_buffer_;
std::size_t workspace_bytes_;
WorkspaceArena workspace_;
Tensor q_tensor_;
Expand All @@ -570,6 +613,7 @@ private:
Tensor valid_columns_tensor_;
Tensor table_rows_tensor_;
Tensor output_tensor_;
Tensor gate_tensor_;
PagedKVLayerView cache_view_;
PagedKVBatchLayerView batch_cache_view_;
ops::CausalAttentionExecutionEnvelope envelope_;
Expand Down Expand Up @@ -706,15 +750,15 @@ void report(const Result& result) {
const double pv_tflops = result.pv_flops / seconds / 1.0e12;
std::printf(
"entry=%-6s geometry=%-14s kv=%-6s mapping=%-10s execution=%-5s cache=%-4s "
"B=%d W=%d contexts=%s valid=%s rows=%s "
"gate=%-10s B=%d W=%d contexts=%s valid=%s rows=%s "
"workspace=%9zu peak=%9zu nodes=%zu calls=%d median=%10.3f us min=%10.3f us p95=%10.3f us "
"logical_payload=%8.1f GB/s physical_payload=%8.1f GB/s math=%7.2f TFLOP/s\n",
entry_name(result.entry), result.geometry.name, storage_name(result.storage),
mapping_name(result.mapping), execution_name(result.execution), cache_name(result.cache),
result.batch, result.tokens, result.row_contexts.c_str(), result.valid_columns.c_str(),
result.table_rows.c_str(), result.workspace_bytes, result.workspace_peak,
result.graph_nodes, result.graph_calls, result.timing.median_us, result.timing.min_us,
result.timing.p95_us, logical_gbps, physical_gbps, tflops);
gate_name(result.gate), result.batch, result.tokens, result.row_contexts.c_str(),
result.valid_columns.c_str(), result.table_rows.c_str(), result.workspace_bytes,
result.workspace_peak, result.graph_nodes, result.graph_calls, result.timing.median_us,
result.timing.min_us, result.timing.p95_us, logical_gbps, physical_gbps, tflops);
std::printf(" vectors K=%.0f V=%.0f bytes cache logical=%.0f physical=%.0f bytes "
"qk_flops=%.0f pv_flops=%.0f qk_full_op=%7.2f TFLOP/s "
"pv_full_op=%7.2f TFLOP/s unique_kv=%.0f bytes "
Expand All @@ -731,7 +775,7 @@ void write_csv(const Options& options, const std::vector<Result>& results) {
std::ofstream output(path);
if (!output) { throw std::runtime_error("failed to open CSV output"); }
output
<< "entry,geometry,kv_dtype,mapping,execution,cache,B,W,row_contexts,valid_columns,"
<< "entry,geometry,kv_dtype,mapping,execution,cache,gate,B,W,row_contexts,valid_columns,"
"table_rows,workspace_bytes,logical_bytes,physical_bytes,logical_cache_bytes,"
"key_vector_bytes,value_vector_bytes,physical_cache_bytes,qk_flops,pv_flops,"
"qk_full_op_tflops,pv_full_op_tflops,"
Expand All @@ -741,12 +785,13 @@ void write_csv(const Options& options, const std::vector<Result>& results) {
output << entry_name(result.entry) << ',' << result.geometry.name << ','
<< storage_name(result.storage) << ',' << mapping_name(result.mapping) << ','
<< execution_name(result.execution) << ',' << cache_name(result.cache) << ','
<< result.batch << ',' << result.tokens << ',' << result.row_contexts << ','
<< result.valid_columns << ',' << result.table_rows << ',' << result.workspace_bytes
<< ',' << result.logical_bytes << ',' << result.physical_bytes << ','
<< result.logical_cache_bytes << ',' << result.key_vector_bytes << ','
<< result.value_vector_bytes << ',' << result.physical_cache_bytes << ','
<< result.qk_flops << ',' << result.pv_flops << ',';
<< gate_name(result.gate) << ',' << result.batch << ',' << result.tokens << ','
<< result.row_contexts << ',' << result.valid_columns << ',' << result.table_rows
<< ',' << result.workspace_bytes << ',' << result.logical_bytes << ','
<< result.physical_bytes << ',' << result.logical_cache_bytes << ','
<< result.key_vector_bytes << ',' << result.value_vector_bytes << ','
<< result.physical_cache_bytes << ',' << result.qk_flops << ',' << result.pv_flops
<< ',';
const double seconds = result.timing.median_us * 1.0e-6;
output << result.qk_flops / seconds / 1.0e12 << ',' << result.pv_flops / seconds / 1.0e12
<< ',' << result.unique_kv_bytes << ',' << result.unique_kv_bytes / seconds / 1.0e9;
Expand Down Expand Up @@ -781,9 +826,10 @@ void profile(Case& data, Entry entry, const Geometry& geometry, KvCacheStorage s
}
std::printf(
"PROFILE entry=%s geometry=%s kv=%s mapping=%s dispatch=public execution=%s cache=%s "
"B=%d W=%d contexts=%.*s valid=%.*s rows=%.*s graph_calls=%d\n",
"gate=%s B=%d W=%d contexts=%.*s valid=%.*s rows=%.*s graph_calls=%d\n",
entry_name(entry), geometry.name, storage_name(storage), mapping_name(options.mapping),
execution_name(execution), cache_name(cache), batch, width,
execution_name(execution), cache_name(cache),
gate_name(effective_gate(options.gate, entry)), batch, width,
static_cast<int>(contexts.size()), contexts.data(), static_cast<int>(valid_columns.size()),
valid_columns.data(), static_cast<int>(table_rows.size()), table_rows.data(),
options.graph_calls);
Expand Down Expand Up @@ -874,7 +920,7 @@ int main(int argc, char** argv) {
options.row_contexts.empty() ? options.contexts.front() : 0;
const RowProfile rows = make_row_profile(options, batch, width, context);
Case data(geometry, storage, width, rows.contexts, rows.valid_columns, rows.table_rows,
options.mapping);
options.mapping, options.gate);
const std::string context_name = profile_name(rows.contexts);
const std::string valid_name = profile_name(rows.valid_columns);
const std::string table_name = profile_name(rows.table_rows);
Expand All @@ -884,6 +930,7 @@ int main(int argc, char** argv) {
return 0;
}

std::printf("gate=%s\n", gate_name(options.gate));
std::vector<Result> results;
const std::vector<std::int32_t> context_profiles =
options.row_contexts.empty() ? options.contexts : std::vector<std::int32_t>{0};
Expand All @@ -895,7 +942,7 @@ int main(int argc, char** argv) {
const RowProfile rows =
make_row_profile(options, batch, tokens, context);
Case data(geometry, storage, tokens, rows.contexts, rows.valid_columns,
rows.table_rows, options.mapping);
rows.table_rows, options.mapping, options.gate);
for (const Entry entry : {Entry::Append, Entry::Cached}) {
if ((options.entry == Entry::Append && entry != Entry::Append) ||
(options.entry == Entry::Cached && entry != Entry::Cached) ||
Expand Down Expand Up @@ -965,6 +1012,7 @@ int main(int argc, char** argv) {
result.workspace_peak = data.workspace_peak();
result.graph_calls =
execution == Execution::Graph ? options.graph_calls : 1;
result.gate = effective_gate(options.gate, entry);
result.timing.median_us /= result.graph_calls;
result.timing.min_us /= result.graph_calls;
result.timing.p95_us /= result.graph_calls;
Expand Down
8 changes: 7 additions & 1 deletion include/ninfer/ops/softmax_attention.h
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Original file line number Diff line number Diff line change
Expand Up @@ -132,13 +132,19 @@ void packed_softmax_attention(const Tensor& q, const Tensor& k, const Tensor& v,
* Inputs, output, every cache plane/table, and live workspace suballocations are pairwise
* non-overlapping. The Op overwrites every addressed cache row but owns no cache allocation,
* frontier, request identity, or commit authority.
*
* An optional gate asks the Op to finish with out *= sigmoid(gate). When present it is a contiguous
* BF16 tensor shaped exactly like out and disjoint from every other operand. Where the route allows
* it the multiply is folded into the reduce epilogue; every other route applies the standalone
* elementwise kernel inside the Op. Either way the result is bit-identical to calling sigmoid_mul
* on the ungated output, so no caller has to know which route it landed on.
*/
void causal_softmax_attention(const Tensor& q, const Tensor& k, const Tensor& v,
const Tensor& positions, const Tensor& valid_columns,
const Tensor& kv_table_rows, AttentionHeadGeometry geometry,
float scale, PagedKVBatchLayerView cache,
CausalAttentionExecutionEnvelope envelope, WorkspaceArena& workspace,
Tensor& out, cudaStream_t stream);
Tensor& out, cudaStream_t stream, const Tensor* gate = nullptr);

/**
* Read-only single-sequence causal attention over an already populated cache.
Expand Down
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