Merge pull request #356 from YosysHQ/ecp5-ff-density
ecp5: Improve flipflop packing density
diff --git a/ecp5/pack.cc b/ecp5/pack.cc
index 7ec08d2..d2a21c8 100644
--- a/ecp5/pack.cc
+++ b/ecp5/pack.cc
@@ -57,12 +57,14 @@
}
// Print logic usgage
+ int available_slices = 0;
void print_logic_usage()
{
int total_luts = 0, total_ffs = 0;
int total_ramluts = 0, total_ramwluts = 0;
for (auto bel : ctx->getBels()) {
if (ctx->getBelType(bel) == id_TRELLIS_SLICE) {
+ available_slices += 1;
total_luts += 2;
total_ffs += 2;
Loc l = ctx->getBelLocation(bel);
@@ -119,6 +121,58 @@
}
}
+ // Check if a flipflop is available in a slice
+ bool is_ff_available(CellInfo *slice, int ff)
+ {
+ if (get_net_or_empty(slice, (ff == 1) ? id_Q1 : id_Q0) != nullptr)
+ return false;
+ if (get_net_or_empty(slice, (ff == 1) ? id_M1 : id_M0) != nullptr)
+ return false;
+ return true;
+ }
+
+ // Check if a flipflop can be added to a slice
+ bool can_add_ff_to_slice(CellInfo *slice, CellInfo *ff)
+ {
+ std::string clkmux = str_or_default(ff->params, ctx->id("CLKMUX"), "CLK");
+ std::string lsrmux = str_or_default(ff->params, ctx->id("LSRMUX"), "LSR");
+
+ bool has_dpram = str_or_default(slice->params, ctx->id("MODE"), "LOGIC") == "DPRAM";
+ if (has_dpram) {
+ std::string wckmux = str_or_default(slice->params, ctx->id("WCKMUX"), "WCK");
+ std::string wremux = str_or_default(slice->params, ctx->id("WREMUX"), "WRE");
+ if (wckmux != clkmux && !(wckmux == "WCK" && clkmux == "CLK"))
+ return false;
+ if (wremux != lsrmux && !(wremux == "WRE" && lsrmux == "LSR"))
+ return false;
+ }
+ bool has_ff0 = get_net_or_empty(slice, id_Q0) != nullptr;
+ bool has_ff1 = get_net_or_empty(slice, id_Q1) != nullptr;
+ if (!has_ff0 && !has_ff1)
+ return true;
+ if (str_or_default(ff->params, ctx->id("GSR"), "DISABLED") !=
+ str_or_default(slice->params, ctx->id("GSR"), "DISABLED"))
+ return false;
+ if (str_or_default(ff->params, ctx->id("SRMODE"), "LSR_OVER_CE") !=
+ str_or_default(slice->params, ctx->id("SRMODE"), "LSR_OVER_CE"))
+ return false;
+ if (str_or_default(ff->params, ctx->id("CEMUX"), "1") != str_or_default(slice->params, ctx->id("CEMUX"), "1"))
+ return false;
+ if (str_or_default(ff->params, ctx->id("LSRMUX"), "LSR") !=
+ str_or_default(slice->params, ctx->id("LSRMUX"), "LSR"))
+ return false;
+ if (str_or_default(ff->params, ctx->id("CLKMUX"), "CLK") !=
+ str_or_default(slice->params, ctx->id("CLKMUX"), "CLK"))
+ return false;
+ if (net_or_nullptr(ff, ctx->id("CLK")) != net_or_nullptr(slice, ctx->id("CLK")))
+ return false;
+ if (net_or_nullptr(ff, ctx->id("CE")) != net_or_nullptr(slice, ctx->id("CE")))
+ return false;
+ if (net_or_nullptr(ff, ctx->id("LSR")) != net_or_nullptr(slice, ctx->id("LSR")))
+ return false;
+ return true;
+ }
+
const NetInfo *net_or_nullptr(CellInfo *cell, IdString port)
{
auto fnd = cell->ports.find(port);
@@ -1069,17 +1123,116 @@
flush_cells();
}
+ // Find a cell that meets some criterea near an origin cell
+ // Used for packing an FF into a nearby SLICE
+ template <typename TFunc> CellInfo *find_nearby_cell(CellInfo *origin, TFunc Func)
+ {
+ std::unordered_set<CellInfo *> visited_cells;
+ std::queue<CellInfo *> to_visit;
+ visited_cells.insert(origin);
+ to_visit.push(origin);
+ int iter = 0;
+ while (!to_visit.empty() && iter < 10000) {
+ CellInfo *cursor = to_visit.front();
+ to_visit.pop();
+ if (Func(cursor))
+ return cursor;
+ for (const auto &port : cursor->ports) {
+ NetInfo *pn = port.second.net;
+ if (pn == nullptr)
+ continue;
+ // Skip high-fanout nets that are unlikely to be relevant
+ if (pn->users.size() > 25)
+ continue;
+ // Add other ports on this net if not already visited
+ auto visit_port = [&](const PortRef &port) {
+ if (port.cell == nullptr)
+ return;
+ if (visited_cells.count(port.cell))
+ return;
+ // If not already visited; add the cell of this port to the queue
+ to_visit.push(port.cell);
+ visited_cells.insert(port.cell);
+ };
+ visit_port(pn->driver);
+ for (const auto &usr : pn->users)
+ visit_port(usr);
+ }
+ ++iter;
+ }
+ return nullptr;
+ }
+
// Pack flipflops that weren't paired with a LUT
+ float dense_pack_mode_thresh = 0.95f;
void pack_remaining_ffs()
{
+ // Enter dense flipflop packing mode once utilisation exceeds a threshold (default: 95%)
+ int used_slices = 0;
+ for (auto &cell : ctx->cells)
+ if (cell.second->type == id_TRELLIS_SLICE)
+ ++used_slices;
+
log_info("Packing unpaired FFs into a SLICE...\n");
for (auto cell : sorted(ctx->cells)) {
CellInfo *ci = cell.second;
if (is_ff(ctx, ci)) {
+ bool pack_dense = used_slices > (dense_pack_mode_thresh * available_slices);
+ if (pack_dense) {
+ // If dense packing threshold exceeded; always try and pack the FF into an existing slice
+ // Find a SLICE with space "near" the flipflop in the netlist
+ std::vector<CellInfo *> ltile;
+ CellInfo *target = find_nearby_cell(ci, [&](CellInfo *cursor) {
+ if (cursor->type != id_TRELLIS_SLICE)
+ return false;
+ if (!cursor->constr_children.empty() || cursor->constr_parent != nullptr) {
+ auto &constr_children = (cursor->constr_parent != nullptr)
+ ? cursor->constr_parent->constr_children
+ : cursor->constr_children;
+ // Skip big chains for performance
+ if (constr_children.size() > 8)
+ return false;
+ // Have to check the whole of the tile for legality when dealing with chains, not just slice
+ ltile.clear();
+ if (cursor->constr_parent != nullptr)
+ ltile.push_back(cursor->constr_parent);
+ else
+ ltile.push_back(cursor);
+ for (auto c : constr_children)
+ ltile.push_back(c);
+ if (!can_add_ff_to_tile(ltile, cursor))
+ return false;
+ }
+ if (!can_add_ff_to_slice(cursor, ci))
+ return false;
+ for (int i = 0; i < 2; i++)
+ if (is_ff_available(cursor, i))
+ return true;
+ return false;
+ });
+
+ // If found, add the FF to this slice instead of creating a new one
+ if (target != nullptr) {
+ for (int i = 0; i < 2; i++) {
+ if (is_ff_available(target, i)) {
+ ff_to_slice(ctx, ci, target, i, false);
+ goto ff_packed;
+ }
+ }
+ }
+
+ if (false) {
+ ff_packed:
+ packed_cells.insert(ci->name);
+ continue;
+ }
+ }
+
std::unique_ptr<CellInfo> slice =
create_ecp5_cell(ctx, ctx->id("TRELLIS_SLICE"), ci->name.str(ctx) + "_SLICE");
ff_to_slice(ctx, ci, slice.get(), 0, false);
new_cells.push_back(std::move(slice));
+ ++used_slices;
packed_cells.insert(ci->name);
}
}
