/*
* nextpnr -- Next Generation Place and Route
*
* Copyright (C) 2018 gatecat <gatecat@ds0.me>
*
* Permission to use, copy, modify, and/or distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*
*/
/*
* Timing-optimised detailed placement algorithm using BFS of the neighbour graph created from cells
* on a critical path
*
* Based on "An Effective Timing-Driven Detailed Placement Algorithm for FPGAs"
* https://www.cerc.utexas.edu/utda/publications/C205.pdf
*
* Modifications made to deal with the smaller Bels that nextpnr uses instead of swapping whole tiles,
* and deal with the fact that not every cell on the crit path may be swappable.
*/
#include "timing_opt.h"
#include <boost/range/adaptor/reversed.hpp>
#include <queue>
#include "nextpnr.h"
#include "timing.h"
#include "util.h"
NEXTPNR_NAMESPACE_BEGIN
class TimingOptimiser
{
public:
TimingOptimiser(Context *ctx, TimingOptCfg cfg) : ctx(ctx), cfg(cfg), tmg(ctx){};
bool optimise()
{
log_info("Running timing-driven placement optimisation...\n");
ctx->lock();
if (ctx->verbose)
timing_analysis(ctx, false, true, false, false);
tmg.setup();
for (int i = 0; i < 30; i++) {
log_info(" Iteration %d...\n", i);
tmg.run();
setup_delay_limits();
auto crit_paths = find_crit_paths(0.98, 50000);
for (auto &path : crit_paths)
optimise_path(path);
if (ctx->verbose)
timing_analysis(ctx, false, true, false, false);
}
ctx->unlock();
return true;
}
private:
void setup_delay_limits()
{
max_net_delay.clear();
for (auto &net : ctx->nets) {
NetInfo *ni = net.second.get();
if (ni->driver.cell == nullptr)
continue;
for (auto usr : ni->users) {
max_net_delay[std::make_pair(usr.cell->name, usr.port)] = std::numeric_limits<delay_t>::max();
}
for (size_t i = 0; i < ni->users.size(); i++) {
auto &usr = ni->users.at(i);
delay_t net_delay = ctx->getNetinfoRouteDelay(ni, usr);
delay_t slack = tmg.get_setup_slack(CellPortKey(usr));
delay_t domain_slack = tmg.get_domain_setup_slack(CellPortKey(usr));
if (slack == std::numeric_limits<delay_t>::max())
continue;
max_net_delay[std::make_pair(usr.cell->name, usr.port)] = net_delay + ((slack - domain_slack) / 10);
}
}
}
bool check_cell_delay_limits(CellInfo *cell)
{
for (const auto &port : cell->ports) {
int nc;
if (ctx->getPortTimingClass(cell, port.first, nc) == TMG_IGNORE)
continue;
NetInfo *net = port.second.net;
if (net == nullptr)
continue;
if (port.second.type == PORT_IN) {
if (net->driver.cell == nullptr || net->driver.cell->bel == BelId())
continue;
for (auto user : net->users) {
if (user.cell == cell && user.port == port.first) {
if (ctx->predictDelay(net, user) >
1.1 * max_net_delay.at(std::make_pair(cell->name, port.first)))
return false;
}
}
} else if (port.second.type == PORT_OUT) {
for (auto user : net->users) {
// This could get expensive for high-fanout nets??
BelId dstBel = user.cell->bel;
if (dstBel == BelId())
continue;
if (ctx->predictDelay(net, user) >
1.1 * max_net_delay.at(std::make_pair(user.cell->name, user.port))) {
return false;
}
}
}
}
return true;
}
BelId cell_swap_bel(CellInfo *cell, BelId newBel)
{
BelId oldBel = cell->bel;
if (oldBel == newBel)
return oldBel;
CellInfo *other_cell = ctx->getBoundBelCell(newBel);
NPNR_ASSERT(other_cell == nullptr || other_cell->belStrength <= STRENGTH_WEAK);
ctx->unbindBel(oldBel);
if (other_cell != nullptr) {
ctx->unbindBel(newBel);
ctx->bindBel(oldBel, other_cell, STRENGTH_WEAK);
}
ctx->bindBel(newBel, cell, STRENGTH_WEAK);
return oldBel;
}
// Check that a series of moves are both legal and remain within maximum delay bounds
// Moves are specified as a vector of pairs <cell, oldBel>
bool acceptable_move(std::vector<std::pair<CellInfo *, BelId>> &move, bool check_delays = true)
{
for (auto &entry : move) {
if (!ctx->isBelLocationValid(entry.first->bel))
return false;
if (!ctx->isBelLocationValid(entry.second))
return false;
if (!check_delays)
continue;
if (!check_cell_delay_limits(entry.first))
return false;
// We might have swapped another cell onto the original bel. Check this for max delay violations
// too
CellInfo *swapped = ctx->getBoundBelCell(entry.second);
if (swapped != nullptr && !check_cell_delay_limits(swapped))
return false;
}
return true;
}
int find_neighbours(CellInfo *cell, IdString prev_cell, int d, bool allow_swap)
{
BelId curr = cell->bel;
Loc curr_loc = ctx->getBelLocation(curr);
int found_count = 0;
cell_neighbour_bels[cell->name] = pool<BelId>{};
for (int dy = -d; dy <= d; dy++) {
for (int dx = -d; dx <= d; dx++) {
// Go through all the Bels at this location
// First, find all bels of the correct type that are either unbound or bound normally
// Strongly bound bels are ignored
// FIXME: This means that we cannot touch carry chains or similar relatively constrained macros
std::vector<BelId> free_bels_at_loc;
std::vector<BelId> bound_bels_at_loc;
for (auto bel : ctx->getBelsByTile(curr_loc.x + dx, curr_loc.y + dy)) {
if (!ctx->isValidBelForCellType(cell->type, bel))
continue;
CellInfo *bound = ctx->getBoundBelCell(bel);
if (bound == nullptr) {
free_bels_at_loc.push_back(bel);
} else if (bound->belStrength <= STRENGTH_WEAK && bound->cluster == ClusterId()) {
bound_bels_at_loc.push_back(bel);
}
}
BelId candidate;
while (!free_bels_at_loc.empty() || !bound_bels_at_loc.empty()) {
BelId try_bel;
if (!free_bels_at_loc.empty()) {
int try_idx = ctx->rng(int(free_bels_at_loc.size()));
try_bel = free_bels_at_loc.at(try_idx);
free_bels_at_loc.erase(free_bels_at_loc.begin() + try_idx);
} else {
int try_idx = ctx->rng(int(bound_bels_at_loc.size()));
try_bel = bound_bels_at_loc.at(try_idx);
bound_bels_at_loc.erase(bound_bels_at_loc.begin() + try_idx);
}
if (bel_candidate_cells.count(try_bel) && !allow_swap) {
// Overlap is only allowed if it is with the previous cell (this is handled by removing those
// edges in the graph), or if allow_swap is true to deal with cases where overlap means few
// neighbours are identified
if (bel_candidate_cells.at(try_bel).size() > 1 ||
(bel_candidate_cells.at(try_bel).size() == 1 &&
*(bel_candidate_cells.at(try_bel).begin()) != prev_cell))
continue;
}
// TODO: what else to check here?
candidate = try_bel;
break;
}
if (candidate != BelId()) {
cell_neighbour_bels[cell->name].insert(candidate);
bel_candidate_cells[candidate].insert(cell->name);
// Work out if we need to delete any overlap
std::vector<IdString> overlap;
for (auto other : bel_candidate_cells[candidate])
if (other != cell->name && other != prev_cell)
overlap.push_back(other);
if (overlap.size() > 0)
NPNR_ASSERT(allow_swap);
for (auto ov : overlap) {
bel_candidate_cells[candidate].erase(ov);
cell_neighbour_bels[ov].erase(candidate);
}
}
}
}
return found_count;
}
std::vector<std::vector<PortRef *>> find_crit_paths(float crit_thresh, size_t max_count)
{
std::vector<std::vector<PortRef *>> crit_paths;
std::vector<std::pair<NetInfo *, int>> crit_nets;
std::vector<IdString> netnames;
std::transform(ctx->nets.begin(), ctx->nets.end(), std::back_inserter(netnames),
[](const std::pair<IdString, std::unique_ptr<NetInfo>> &kv) { return kv.first; });
ctx->sorted_shuffle(netnames);
for (auto net : netnames) {
if (crit_nets.size() >= max_count)
break;
float highest_crit = 0;
size_t crit_user_idx = 0;
NetInfo *ni = ctx->nets.at(net).get();
for (size_t i = 0; i < ni->users.size(); i++) {
float crit = tmg.get_criticality(CellPortKey(ni->users.at(i)));
if (crit > highest_crit) {
highest_crit = crit;
crit_user_idx = i;
}
}
if (highest_crit > crit_thresh)
crit_nets.push_back(std::make_pair(ni, crit_user_idx));
}
auto port_user_index = [](CellInfo *cell, PortInfo &port) -> size_t {
NPNR_ASSERT(port.net != nullptr);
for (size_t i = 0; i < port.net->users.size(); i++) {
auto &usr = port.net->users.at(i);
if (usr.cell == cell && usr.port == port.name)
return i;
}
NPNR_ASSERT_FALSE("port user not found on net");
};
pool<PortRef *, hash_ptr_ops> used_ports;
for (auto crit_net : crit_nets) {
if (used_ports.count(&(crit_net.first->users.at(crit_net.second))))
continue;
std::deque<PortRef *> crit_path;
// FIXME: This will fail badly on combinational loops
// Iterate backwards following greatest criticality
NetInfo *back_cursor = crit_net.first;
while (back_cursor != nullptr) {
float max_crit = 0;
std::pair<NetInfo *, size_t> crit_sink{nullptr, 0};
CellInfo *cell = back_cursor->driver.cell;
if (cell == nullptr)
break;
for (auto port : cell->ports) {
if (port.second.type != PORT_IN)
continue;
NetInfo *pn = port.second.net;
if (pn == nullptr)
continue;
int ccount;
DelayQuad combDelay;
TimingPortClass tpclass = ctx->getPortTimingClass(cell, port.first, ccount);
if (tpclass != TMG_COMB_INPUT)
continue;
bool is_path = ctx->getCellDelay(cell, port.first, back_cursor->driver.port, combDelay);
if (!is_path)
continue;
size_t user_idx = port_user_index(cell, port.second);
float usr_crit = tmg.get_criticality(CellPortKey(cell->name, port.first));
if (used_ports.count(&(pn->users.at(user_idx))))
continue;
if (usr_crit >= max_crit) {
max_crit = usr_crit;
crit_sink = std::make_pair(pn, user_idx);
}
}
if (crit_sink.first != nullptr) {
crit_path.push_front(&(crit_sink.first->users.at(crit_sink.second)));
used_ports.insert(&(crit_sink.first->users.at(crit_sink.second)));
}
back_cursor = crit_sink.first;
}
// Iterate forwards following greatest criticiality
PortRef *fwd_cursor = &(crit_net.first->users.at(crit_net.second));
while (fwd_cursor != nullptr) {
crit_path.push_back(fwd_cursor);
float max_crit = 0;
std::pair<NetInfo *, size_t> crit_sink{nullptr, 0};
CellInfo *cell = fwd_cursor->cell;
for (auto port : cell->ports) {
if (port.second.type != PORT_OUT)
continue;
NetInfo *pn = port.second.net;
if (pn == nullptr)
continue;
int ccount;
DelayQuad combDelay;
TimingPortClass tpclass = ctx->getPortTimingClass(cell, port.first, ccount);
if (tpclass != TMG_COMB_OUTPUT && tpclass != TMG_REGISTER_OUTPUT)
continue;
bool is_path = ctx->getCellDelay(cell, fwd_cursor->port, port.first, combDelay);
if (!is_path)
continue;
for (size_t i = 0; i < pn->users.size(); i++) {
if (used_ports.count(&(pn->users.at(i))))
continue;
float crit = tmg.get_criticality(CellPortKey(pn->users.at(i)));
if (crit >= max_crit) {
max_crit = crit;
crit_sink = std::make_pair(pn, i);
}
}
}
if (crit_sink.first != nullptr) {
fwd_cursor = &(crit_sink.first->users.at(crit_sink.second));
used_ports.insert(&(crit_sink.first->users.at(crit_sink.second)));
} else {
fwd_cursor = nullptr;
}
}
std::vector<PortRef *> crit_path_vec;
std::copy(crit_path.begin(), crit_path.end(), std::back_inserter(crit_path_vec));
crit_paths.push_back(crit_path_vec);
}
return crit_paths;
}
void optimise_path(std::vector<PortRef *> &path)
{
path_cells.clear();
cell_neighbour_bels.clear();
bel_candidate_cells.clear();
if (ctx->debug)
log_info("Optimising the following path: \n");
auto front_port = path.front();
NetInfo *front_net = front_port->cell->ports.at(front_port->port).net;
if (front_net != nullptr && front_net->driver.cell != nullptr) {
auto front_cell = front_net->driver.cell;
if (front_cell->belStrength <= STRENGTH_WEAK && cfg.cellTypes.count(front_cell->type) &&
front_cell->cluster == ClusterId()) {
path_cells.push_back(front_cell->name);
}
}
for (auto port : path) {
if (ctx->debug) {
float crit = tmg.get_criticality(CellPortKey(*port));
log_info(" %s.%s at %s crit %0.02f\n", port->cell->name.c_str(ctx), port->port.c_str(ctx),
ctx->nameOfBel(port->cell->bel), crit);
}
if (std::find(path_cells.begin(), path_cells.end(), port->cell->name) != path_cells.end())
continue;
if (port->cell->belStrength > STRENGTH_WEAK || !cfg.cellTypes.count(port->cell->type) ||
port->cell->cluster != ClusterId())
continue;
if (ctx->debug)
log_info(" can move\n");
path_cells.push_back(port->cell->name);
}
if (path_cells.size() < 2) {
if (ctx->debug) {
log_info("Too few moveable cells; skipping path\n");
log_break();
}
return;
}
// Calculate original delay before touching anything
delay_t original_delay = 0;
for (size_t i = 0; i < path.size(); i++) {
NetInfo *pn = path.at(i)->cell->ports.at(path.at(i)->port).net;
for (size_t j = 0; j < pn->users.size(); j++) {
auto &usr = pn->users.at(j);
if (usr.cell == path.at(i)->cell && usr.port == path.at(i)->port) {
original_delay += ctx->predictDelay(pn, usr);
break;
}
}
}
IdString last_cell;
const int d = 2; // FIXME: how to best determine d
for (auto cell : path_cells) {
// FIXME: when should we allow swapping due to a lack of candidates
find_neighbours(ctx->cells[cell].get(), last_cell, d, false);
last_cell = cell;
}
if (ctx->debug) {
for (auto cell : path_cells) {
log_info("Candidate neighbours for %s (%s):\n", cell.c_str(ctx), ctx->nameOfBel(ctx->cells[cell]->bel));
for (auto neigh : cell_neighbour_bels.at(cell)) {
log_info(" %s\n", ctx->nameOfBel(neigh));
}
}
}
// Actual BFS path optimisation algorithm
dict<IdString, dict<BelId, delay_t>> cumul_costs;
dict<std::pair<IdString, BelId>, std::pair<IdString, BelId>> backtrace;
std::queue<std::pair<int, BelId>> visit;
pool<std::pair<int, BelId>> to_visit;
for (auto startbel : cell_neighbour_bels[path_cells.front()]) {
// Swap for legality check
CellInfo *cell = ctx->cells.at(path_cells.front()).get();
BelId origBel = cell_swap_bel(cell, startbel);
std::vector<std::pair<CellInfo *, BelId>> move{std::make_pair(cell, origBel)};
if (acceptable_move(move)) {
auto entry = std::make_pair(0, startbel);
visit.push(entry);
cumul_costs[path_cells.front()][startbel] = 0;
}
// Swap back
cell_swap_bel(cell, origBel);
}
while (!visit.empty()) {
auto entry = visit.front();
visit.pop();
auto cellname = path_cells.at(entry.first);
if (entry.first == int(path_cells.size()) - 1)
continue;
std::vector<std::pair<CellInfo *, BelId>> move;
// Apply the entire backtrace for accurate legality and delay checks
// This is probably pretty expensive (but also probably pales in comparison to the number of swaps
// SA will make...)
std::vector<std::pair<IdString, BelId>> route_to_entry;
auto cursor = std::make_pair(cellname, entry.second);
route_to_entry.push_back(cursor);
while (backtrace.count(cursor)) {
cursor = backtrace.at(cursor);
route_to_entry.push_back(cursor);
}
for (auto rt_entry : boost::adaptors::reverse(route_to_entry)) {
CellInfo *cell = ctx->cells.at(rt_entry.first).get();
BelId origBel = cell_swap_bel(cell, rt_entry.second);
move.push_back(std::make_pair(cell, origBel));
}
// Have a look at where we can travel from here
for (auto neighbour : cell_neighbour_bels.at(path_cells.at(entry.first + 1))) {
// Edges between overlapping bels are deleted
if (neighbour == entry.second)
continue;
// Experimentally swap the next path cell onto the neighbour bel we are trying
IdString ncname = path_cells.at(entry.first + 1);
CellInfo *next_cell = ctx->cells.at(ncname).get();
BelId origBel = cell_swap_bel(next_cell, neighbour);
move.push_back(std::make_pair(next_cell, origBel));
delay_t total_delay = 0;
for (size_t i = 0; i < path.size(); i++) {
NetInfo *pn = path.at(i)->cell->ports.at(path.at(i)->port).net;
for (size_t j = 0; j < pn->users.size(); j++) {
auto &usr = pn->users.at(j);
if (usr.cell == path.at(i)->cell && usr.port == path.at(i)->port) {
total_delay += ctx->predictDelay(pn, usr);
break;
}
}
if (path.at(i)->cell == next_cell)
break;
}
// First, check if the move is actually worthwhile from a delay point of view before the expensive
// legality check
if (!cumul_costs.count(ncname) || !cumul_costs.at(ncname).count(neighbour) ||
cumul_costs.at(ncname).at(neighbour) > total_delay) {
// Now check that the swaps we have made to get here are legal and meet max delay requirements
if (acceptable_move(move)) {
cumul_costs[ncname][neighbour] = total_delay;
backtrace[std::make_pair(ncname, neighbour)] = std::make_pair(cellname, entry.second);
if (!to_visit.count(std::make_pair(entry.first + 1, neighbour)))
visit.push(std::make_pair(entry.first + 1, neighbour));
}
}
// Revert the experimental swap
cell_swap_bel(move.back().first, move.back().second);
move.pop_back();
}
// Revert move by swapping cells back to their original order
// Execute swaps in reverse order to how we made them originally
for (auto move_entry : boost::adaptors::reverse(move)) {
cell_swap_bel(move_entry.first, move_entry.second);
}
}
// Did we find a solution??
if (cumul_costs.count(path_cells.back())) {
// Find the end position with the lowest total delay
auto &end_options = cumul_costs.at(path_cells.back());
auto lowest = std::min_element(end_options.begin(), end_options.end(),
[](const std::pair<BelId, delay_t> &a, const std::pair<BelId, delay_t> &b) {
return a.second < b.second;
});
NPNR_ASSERT(lowest != end_options.end());
std::vector<std::pair<IdString, BelId>> route_to_solution;
auto cursor = std::make_pair(path_cells.back(), lowest->first);
route_to_solution.push_back(cursor);
while (backtrace.count(cursor)) {
cursor = backtrace.at(cursor);
route_to_solution.push_back(cursor);
}
if (ctx->debug)
log_info("Found a solution with cost %.02f ns (existing path %.02f ns)\n",
ctx->getDelayNS(lowest->second), ctx->getDelayNS(original_delay));
for (auto rt_entry : boost::adaptors::reverse(route_to_solution)) {
CellInfo *cell = ctx->cells.at(rt_entry.first).get();
cell_swap_bel(cell, rt_entry.second);
if (ctx->debug)
log_info(" %s at %s\n", rt_entry.first.c_str(ctx), ctx->nameOfBel(rt_entry.second));
}
} else {
if (ctx->debug)
log_info("Solution was not found\n");
}
if (ctx->debug)
log_break();
}
// Current candidate Bels for cells (linked in both direction>
std::vector<IdString> path_cells;
dict<IdString, pool<BelId>> cell_neighbour_bels;
dict<BelId, pool<IdString>> bel_candidate_cells;
// Map cell ports to net delay limit
dict<std::pair<IdString, IdString>, delay_t> max_net_delay;
Context *ctx;
TimingOptCfg cfg;
TimingAnalyser tmg;
};
bool timing_opt(Context *ctx, TimingOptCfg cfg) { return TimingOptimiser(ctx, cfg).optimise(); }
NEXTPNR_NAMESPACE_END