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Built-in splitfanout in muxpack
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Alain Dargelas committed Jan 10, 2025
1 parent 99afdfa commit 2ae521b
Showing 1 changed file with 121 additions and 22 deletions.
143 changes: 121 additions & 22 deletions passes/opt/muxpack.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -123,6 +123,10 @@ struct MuxpackWorker
int mux_count, pmux_count;

pool<Cell*> remove_cells;
// Driver data
dict<SigBit, tuple<IdString, IdString, int>> bit_drivers_db;
// Load data
dict<SigBit, pool<tuple<IdString, IdString, int>>> bit_users_db;

dict<SigSpec, Cell*> sig_chain_next;
dict<SigSpec, Cell*> sig_chain_prev;
Expand All @@ -131,6 +135,26 @@ struct MuxpackWorker
pool<Cell*> candidate_cells;

ExclusiveDatabase excl_db;
// Splitfanout limit
int limit = -1;

bool fanout_in_range(SigSpec outsig)
{
// Check if output signal is "bit-split", skip if so
// This is a lookahead for the splitfanout pass that has this limitation
auto bit_users = bit_users_db[outsig[0]];
for (int i = 0; i < GetSize(outsig); i++) {
if (bit_users_db[outsig[i]] != bit_users) {
return false;
}
}

// Skip if fanout is above limit
if (limit != -1 && GetSize(bit_users) > limit) {
return false;
}
return true;
}

void make_sig_chain_next_prev()
{
Expand All @@ -156,20 +180,29 @@ struct MuxpackWorker
for (auto a_bit : a_sig)
sigbit_with_non_chain_users.insert(a_bit);
else {
sig_chain_next[a_sig] = cell;
if (fanout_in_range(y_sig)) {
sig_chain_next[a_sig] = cell;
candidate_cells.insert(cell);
}
}

if (!b_sig.empty()) {
if (sig_chain_next.count(b_sig))
for (auto b_bit : b_sig)
sigbit_with_non_chain_users.insert(b_bit);
else {
sig_chain_next[b_sig] = cell;
if (fanout_in_range(y_sig)) {
sig_chain_next[b_sig] = cell;
candidate_cells.insert(cell);
}
}
}
candidate_cells.insert(cell);

sig_chain_prev[y_sig] = cell;
if (fanout_in_range(y_sig)) {

// Mark cell as the previous in the chain relative to y_sig
sig_chain_prev[y_sig] = cell;
}
continue;
}

Expand Down Expand Up @@ -356,10 +389,11 @@ struct MuxpackWorker
}
}

void cleanup()
void cleanup(bool remove_cell)
{
for (auto cell : remove_cells)
module->remove(cell);
if (remove_cell)
for (auto cell : remove_cells)
module->remove(cell);

remove_cells.clear();
sig_chain_next.clear();
Expand All @@ -368,18 +402,87 @@ struct MuxpackWorker
candidate_cells.clear();
}

MuxpackWorker(Module *module, bool assume_excl, bool make_excl) :
module(module), sigmap(module), mux_count(0), pmux_count(0), excl_db(module, sigmap, assume_excl, make_excl)
MuxpackWorker(Design *design, Module *module, bool assume_excl, bool make_excl, int limit)
: module(module), sigmap(module), mux_count(0), pmux_count(0), excl_db(module, sigmap, assume_excl, make_excl), limit(limit)
{

// Build bit_drivers_db
log("Building bit_drivers_db...\n");
for (auto cell : module->cells()) {
for (auto conn : cell->connections()) {
if (!cell->output(conn.first))
continue;
for (int i = 0; i < GetSize(conn.second); i++) {
SigBit bit(sigmap(conn.second[i]));
bit_drivers_db[bit] = tuple<IdString, IdString, int>(cell->name, conn.first, i);
}
}
}

// Build bit_users_db
log("Building bit_users_db...\n");
for (auto cell : module->cells()) {
for (auto conn : cell->connections()) {
if (!cell->input(conn.first))
continue;
for (int i = 0; i < GetSize(conn.second); i++) {
SigBit bit(sigmap(conn.second[i]));
if (!bit_drivers_db.count(bit))
continue;
bit_users_db[bit].insert(
tuple<IdString, IdString, int>(cell->name, conn.first, i - std::get<2>(bit_drivers_db[bit])));
}
}
}

// Build bit_users_db for output ports
log("Building bit_users_db for output ports...\n");
for (auto wire : module->wires()) {
if (!wire->port_output)
continue;
SigSpec sig(sigmap(wire));
for (int i = 0; i < GetSize(sig); i++) {
SigBit bit(sig[i]);
if (!bit_drivers_db.count(bit))
continue;
bit_users_db[bit].insert(
tuple<IdString, IdString, int>(wire->name, IdString(), i - std::get<2>(bit_drivers_db[bit])));
}
}

make_sig_chain_next_prev();
find_chain_start_cells(assume_excl);

// Deselect all cells
Pass::call(design, "select -none");
bool has_cell_to_split = false;
for (auto c : chain_start_cells) {
vector<Cell *> chain = create_chain(c);
for (auto cell : chain) {
has_cell_to_split = true;
// Select the cells that are candidate
design->select(module, cell);
}
}
// Clean up
cleanup(false);

// Make sure we dup the cells with fanout, else the resulting
// transform is not logically equivalent
if (has_cell_to_split)
Pass::call(design, "splitfanout");
// Reset selection for other passes
Pass::call(design, "select -clear");
// Recreate sigmap
sigmap.set(module);

// Make the actual transform
for (auto c : chain_start_cells) {
vector<Cell*> chain = create_chain(c);
vector<Cell *> chain = create_chain(c);
process_chain(chain, make_excl);
}

cleanup();
// Clean up
cleanup(true);
}
};

Expand All @@ -399,8 +502,8 @@ struct MuxpackPass : public Pass {
log("whose select lines are driven by '$eq' cells with other such cells if it can be\n");
log("certain that their select inputs are mutually exclusive.\n");
log("\n");
log(" -splitfanout\n");
log(" run splitfanout pass first\n");
log(" -fanout_limit n\n");
log(" max fanout to split.\n");
log("\n");
log(" -assume_excl\n");
log(" assume mutually exclusive constraint when packing (may result in inequivalence)\n");
Expand All @@ -410,18 +513,17 @@ struct MuxpackPass : public Pass {
}
void execute(std::vector<std::string> args, RTLIL::Design *design) override
{
bool splitfanout = false;
bool assume_excl = false;
bool make_excl = false;

int limit = -1;

log_header(design, "Executing MUXPACK pass ($mux cell cascades to $pmux).\n");

size_t argidx;
for (argidx = 1; argidx < args.size(); argidx++)
{
if (args[argidx] == "-splitfanout") {
splitfanout = true;
if (args[argidx] == "-fanout_limit" && argidx + 1 < args.size()) {
limit = std::stoi(args[++argidx]);
continue;
}
if (args[argidx] == "-assume_excl") {
Expand All @@ -437,14 +539,11 @@ struct MuxpackPass : public Pass {
}
extra_args(args, argidx, design);

if (splitfanout)
Pass::call(design, "splitfanout -limit 256 t:$mux t:$pmux");

int mux_count = 0;
int pmux_count = 0;

for (auto module : design->selected_modules()) {
MuxpackWorker worker(module, assume_excl, make_excl);
MuxpackWorker worker(design, module, assume_excl, make_excl, limit);
mux_count += worker.mux_count;
pmux_count += worker.pmux_count;
}
Expand Down

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