444 lines
20 KiB
C++
444 lines
20 KiB
C++
/*
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* nextpnr -- Next Generation Place and Route
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*
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* Copyright (C) 2018 David Shah <david@symbioticeda.com>
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*
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* Permission to use, copy, modify, and/or distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*
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*/
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/*
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* Timing-optimised detailed placement algorithm using BFS of the neighbour graph created from cells
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* on a critical path
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*
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* Based on "An Effective Timing-Driven Detailed Placement Algorithm for FPGAs"
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* https://www.cerc.utexas.edu/utda/publications/C205.pdf
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*
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* Modifications made to deal with the smaller Bels that nextpnr uses instead of swapping whole tiles,
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* and deal with the fact that not every cell on the crit path may be swappable.
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*/
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#include "timing.h"
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#include "timing_opt.h"
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#include "nextpnr.h"
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#include "util.h"
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#include <boost/range/adaptor/reversed.hpp>
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#include <queue>
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NEXTPNR_NAMESPACE_BEGIN
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class TimingOptimiser
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{
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public:
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TimingOptimiser(Context *ctx) : ctx(ctx){};
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bool optimise() {}
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private:
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// Ratio of available to already-candidates to begin borrowing
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const float borrow_thresh = 0.2;
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void setup_delay_limits() {
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for (auto net : sorted(ctx->nets)) {
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NetInfo *ni = net.second;
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max_net_delay[ni].clear();
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max_net_delay[ni].resize(ni->users.size(), std::numeric_limits<delay_t>::max());
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if (!net_crit.count(net.first))
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continue;
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auto &nc = net_crit.at(net.first);
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if (nc.slack.empty())
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continue;
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for (size_t i = 0; i < ni->users.size(); i++) {
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delay_t net_delay = ctx->getNetinfoRouteDelay(ni, ni->users.at(i));
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max_net_delay[ni].at(i) = net_delay + ((nc.slack.at(i) - nc.cd_worst_slack) / nc.max_path_length);
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}
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}
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}
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bool check_cell_delay_limits(CellInfo *cell) {
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for (const auto &port : cell->ports) {
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int nc;
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if (ctx->getPortTimingClass(cell, port.first, nc) == TMG_IGNORE)
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continue;
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NetInfo *net = port.second.net;
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if (net == nullptr)
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continue;
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if (port.second.type == PORT_IN) {
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if (net->driver.cell == nullptr || net->driver.cell->bel == BelId())
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continue;
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BelId srcBel = net->driver.cell->bel;
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if (ctx->estimateDelay(ctx->getBelPinWire(srcBel, net->driver.port),
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ctx->getBelPinWire(cell->bel, port.first)) > max_net_delay.at(std::make_pair(cell->name, port.first)))
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return false;
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} else if (port.second.type == PORT_OUT) {
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for (auto user : net->users) {
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// This could get expensive for high-fanout nets??
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BelId dstBel = user.cell->bel;
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if (dstBel == BelId())
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continue;
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if (ctx->estimateDelay(ctx->getBelPinWire(cell->bel, port.first),
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ctx->getBelPinWire(dstBel, user.port)) > max_net_delay.at(std::make_pair(user.cell->name, user.port)))
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return false;
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}
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}
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}
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return true;
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}
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BelId cell_swap_bel(CellInfo *cell, BelId newBel) {
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BelId oldBel = cell->bel;
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CellInfo *other_cell = ctx->getBoundBelCell(newBel);
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NPNR_ASSERT(other_cell == nullptr || other_cell->belStrength <= STRENGTH_WEAK);
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ctx->unbindBel(oldBel);
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if (other_cell != nullptr) {
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ctx->unbindBel(newBel);
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ctx->bindBel(oldBel, other_cell, STRENGTH_WEAK);
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}
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ctx->bindBel(newBel, cell, STRENGTH_WEAK);
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return oldBel;
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}
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// Check that a series of moves are both legal and remain within maximum delay bounds
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// Moves are specified as a vector of pairs <cell, oldBel>
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bool acceptable_move(std::vector<std::pair<CellInfo *, BelId>> &move, bool check_delays = true) {
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for (auto &entry : move) {
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if (!ctx->isBelLocationValid(entry.first->bel))
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return false;
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if (!ctx->isBelLocationValid(entry.second))
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return false;
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if (!check_delays)
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continue;
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if (!check_cell_delay_limits(entry.first))
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return false;
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// We might have swapped another cell onto the original bel. Check this for max delay violations
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// too
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CellInfo *swapped = ctx->getBoundBelCell(entry.second);
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if (swapped != nullptr && !check_cell_delay_limits(swapped))
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return false;
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}
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return true;
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}
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int find_neighbours(CellInfo *cell, IdString prev_cell, int d, bool allow_swap) {
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BelId curr = cell->bel;
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Loc curr_loc = ctx->getBelLocation(curr);
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int found_count = 0;
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for (int dy = -d; dy <= d; dy++) {
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for (int dx = -d; dx <= d; dx++) {
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// Go through all the Bels at this location
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// First, find all bels of the correct type that are either unbound or bound normally
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// Strongly bound bels are ignored
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// FIXME: This means that we cannot touch carry chains or similar relatively constrained macros
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std::vector<BelId> free_bels_at_loc;
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std::vector<BelId> bound_bels_at_loc;
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for (auto bel : ctx->getBelsByTile(curr_loc.x + dx, curr_loc.y + dy)) {
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if (ctx->getBelType(bel) != cell->type)
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continue;
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CellInfo *bound = ctx->getBoundBelCell(bel);
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if (bound == nullptr) {
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free_bels_at_loc.push_back(bel);
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} else if (bound->belStrength <= STRENGTH_WEAK) {
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bound_bels_at_loc.push_back(bel);
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}
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}
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BelId candidate;
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while (!free_bels_at_loc.empty() && !bound_bels_at_loc.empty()) {
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BelId try_bel;
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if (!free_bels_at_loc.empty()) {
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int try_idx = ctx->rng(int(free_bels_at_loc.size()));
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try_bel = free_bels_at_loc.at(try_idx);
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free_bels_at_loc.erase(free_bels_at_loc.begin() + try_idx);
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} else {
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int try_idx = ctx->rng(int(bound_bels_at_loc.size()));
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try_bel = bound_bels_at_loc.at(try_idx);
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bound_bels_at_loc.erase(bound_bels_at_loc.begin() + try_idx);
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}
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if (bel_candidate_cells.count(try_bel) && !allow_swap) {
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// Overlap is only allowed if it is with the previous cell (this is handled by removing those
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// edges in the graph), or if allow_swap is true to deal with cases where overlap means few neighbours
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// are identified
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if (bel_candidate_cells.at(try_bel).size() > 1 || (bel_candidate_cells.at(try_bel).size() == 0 ||
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*(bel_candidate_cells.at(try_bel).begin()) != prev_cell))
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continue;
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}
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// TODO: what else to check here?
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candidate = try_bel;
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break;
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}
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if (candidate != BelId()) {
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cell_neighbour_bels[cell->name].insert(candidate);
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bel_candidate_cells[candidate].insert(cell->name);
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// Work out if we need to delete any overlap
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std::vector<IdString> overlap;
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for (auto other : bel_candidate_cells[candidate])
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if (other != cell->name && other != prev_cell)
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overlap.push_back(other);
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if (overlap.size() > 0)
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NPNR_ASSERT(allow_swap);
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for (auto ov : overlap) {
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bel_candidate_cells[candidate].erase(ov);
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cell_neighbour_bels[ov].erase(candidate);
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}
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}
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}
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}
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return found_count;
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}
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std::vector<std::vector<PortRef*>> find_crit_paths(float crit_thresh, int max_count) {
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std::vector<std::vector<PortRef*>> crit_paths;
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std::vector<std::pair<NetInfo *, int>> crit_nets;
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std::vector<IdString> netnames;
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std::transform(ctx->nets.begin(), ctx->nets.end(), std::back_inserter(netnames),
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[](const std::pair<IdString, std::unique_ptr<NetInfo>> &kv){
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return kv.first;
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});
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ctx->sorted_shuffle(netnames);
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for (auto net : netnames) {
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if (crit_nets.size() >= max_count)
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break;
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if (!net_crit.count(net))
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continue;
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auto crit_user = std::max_element(net_crit[net].criticality.begin(),
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net_crit[net].criticality.end());
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if (*crit_user > crit_thresh)
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crit_nets.push_back(std::make_pair(ctx->nets[net].get(), crit_user - net_crit[net].criticality.begin()));
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}
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auto port_user_index = [](CellInfo *cell, PortInfo &port) -> size_t {
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NPNR_ASSERT(port.net != nullptr);
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for (size_t i = 0; i < port.net->users.size(); i++) {
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auto &usr = port.net->users.at(i);
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if (usr.cell == cell && usr.port == port.name)
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return i;
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}
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NPNR_ASSERT_FALSE("port user not found on net");
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};
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for (auto crit_net : crit_nets) {
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std::deque<PortRef*> crit_path;
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// FIXME: This will fail badly on combinational loops
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// Iterate backwards following greatest criticality
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NetInfo* back_cursor = crit_net.first;
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while (back_cursor != nullptr) {
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float max_crit = 0;
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std::pair<NetInfo *, size_t> crit_sink{nullptr, 0};
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CellInfo *cell = back_cursor->driver.cell;
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if (cell == nullptr)
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break;
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for (auto port : cell->ports) {
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if (port.second.type != PORT_IN)
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continue;
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NetInfo *pn = port.second.net;
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if (pn == nullptr)
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continue;
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if (!net_crit.count(pn->name) || net_crit.at(pn->name).criticality.empty())
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continue;
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int ccount;
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DelayInfo combDelay;
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TimingPortClass tpclass = ctx->getPortTimingClass(cell, port.first, ccount);
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if (tpclass != TMG_COMB_INPUT && tpclass != TMG_REGISTER_INPUT)
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continue;
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bool is_path = ctx->getCellDelay(cell, port.first, back_cursor->driver.port, combDelay);
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if (!is_path)
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continue;
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size_t user_idx = port_user_index(cell, port.second);
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float usr_crit = net_crit.at(pn->name).criticality.at(user_idx);
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if (usr_crit >= max_crit) {
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max_crit = usr_crit;
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crit_sink = std::make_pair(pn, user_idx);
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}
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}
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if (crit_sink.first != nullptr) {
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crit_path.push_front(&(crit_sink.first->users.at(crit_sink.second)));
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}
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back_cursor = crit_sink.first;
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}
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// Iterate forwards following greatest criticiality
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PortRef *fwd_cursor = &(crit_net.first->users.at(crit_net.second));
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while (fwd_cursor != nullptr) {
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crit_path.push_back(fwd_cursor);
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float max_crit = 0;
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std::pair<NetInfo *, size_t> crit_sink{nullptr, 0};
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CellInfo *cell = fwd_cursor->cell;
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for (auto port : cell->ports) {
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if (port.second.type != PORT_OUT)
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continue;
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NetInfo *pn = port.second.net;
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if (pn == nullptr)
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continue;
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if (!net_crit.count(pn->name) || net_crit.at(pn->name).criticality.empty())
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continue;
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int ccount;
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DelayInfo combDelay;
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TimingPortClass tpclass = ctx->getPortTimingClass(cell, port.first, ccount);
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if (tpclass != TMG_COMB_OUTPUT && tpclass != TMG_REGISTER_OUTPUT)
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continue;
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auto &crits = net_crit.at(pn->name).criticality;
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auto most_crit_usr = std::max_element(crits.begin(), crits.end());
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if (*most_crit_usr >= max_crit) {
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max_crit = *most_crit_usr;
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crit_sink = std::make_pair(pn, std::distance(crits.begin(), most_crit_usr));
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}
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}
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if (crit_sink.first != nullptr) {
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fwd_cursor = &(crit_sink.first->users.at(crit_sink.second));
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} else {
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fwd_cursor = nullptr;
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}
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}
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std::vector<PortRef*> crit_path_vec;
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std::copy(crit_path.begin(), crit_path.end(), std::back_inserter(crit_path_vec));
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crit_paths.push_back(crit_path_vec);
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}
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return crit_paths;
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}
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void optimise_path(std::vector<PortRef*> &path) {
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path_cells.clear();
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cell_neighbour_bels.clear();
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bel_candidate_cells.clear();
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for (auto port : path) {
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if (std::find(path_cells.begin(), path_cells.end(), port->cell->name) != path_cells.end())
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continue;
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if (port->cell->belStrength > STRENGTH_WEAK || !cfg.cellTypes.count(port->cell->type))
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continue;
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path_cells.push_back(port->cell->name);
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}
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if (path_cells.empty())
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return;
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IdString last_cell;
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const int d = 3; // FIXME: how to best determine d
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for (auto cell : path_cells) {
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// FIXME: when should we allow swapping due to a lack of candidates
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find_neighbours(ctx->cells[cell].get(), last_cell, d, false);
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last_cell = cell;
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}
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// Map cells that we will actually modify to the arc we will use for cost
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// calculation
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// for delay calc purposes
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std::unordered_map<IdString, std::pair<PortRef *, PortRef *>> cost_ports;
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PortRef *last_port = nullptr;
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auto pcell = path_cells.begin();
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for (auto port : path) {
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if (port->cell->name == *pcell) {
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cost_ports[*pcell] = std::make_pair(last_port, port);
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pcell++;
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}
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last_port = port;
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}
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// Actual BFS path optimisation algorithm
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std::unordered_map<IdString, std::unordered_map<BelId, delay_t>> cumul_costs;
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std::unordered_map<std::pair<IdString, BelId>, std::pair<IdString, BelId>> backtrace;
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std::queue<std::pair<int, BelId>> visit;
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std::unordered_set<std::pair<int, BelId>> to_visit;
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for (auto startbel : cell_neighbour_bels[path_cells.front()]) {
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auto entry = std::make_pair(0, startbel);
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visit.push(entry);
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cumul_costs[path_cells.front()][startbel] = 0;
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}
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while(!visit.empty()) {
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auto entry = visit.front();
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visit.pop();
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auto cellname = path_cells.at(entry.first);
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if (entry.first == path_cells.size() - 1)
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continue;
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std::vector<std::pair<CellInfo *, BelId>> move;
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// Apply the entire backtrace for accurate legality and delay checks
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// This is probably pretty expensive (but also probably pales in comparison to the number of swaps
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// SA will make...)
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std::vector<std::pair<IdString, BelId>> route_to_entry;
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auto cursor = std::make_pair(cellname, entry.second);
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route_to_entry.push_back(cursor);
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while (backtrace.count(cursor)) {
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cursor = backtrace.at(cursor);
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route_to_entry.push_back(cursor);
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}
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for (auto rt_entry : boost::adaptors::reverse(route_to_entry)) {
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CellInfo *cell = ctx->cells.at(rt_entry.first).get();
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BelId origBel = cell_swap_bel(cell, rt_entry.second);
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move.push_back(std::make_pair(cell, origBel));
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}
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delay_t cdelay = cumul_costs[cellname][entry.second];
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// Have a look at where we can travel from here
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for (auto neighbour : cell_neighbour_bels.at(path_cells.at(entry.first + 1))) {
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// Edges between overlapping bels are deleted
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if (neighbour == entry.second)
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continue;
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// Experimentally swap the next path cell onto the neighbour bel we are trying
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IdString ncname = path_cells.at(entry.first + 1);
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CellInfo *next_cell = ctx->cells.at(ncname).get();
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BelId origBel = cell_swap_bel(next_cell, neighbour);
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move.push_back(std::make_pair(next_cell, origBel));
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// Check the new cumulative delay
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auto port_pair = cost_ports.at(ncname);
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delay_t edge_delay = ctx->estimateDelay(ctx->getBelPinWire(port_pair.first->cell->bel, port_pair.first->port),
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ctx->getBelPinWire(port_pair.second->cell->bel, port_pair.second->port));
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delay_t total_delay = cdelay + edge_delay;
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// First, check if the move is actually worthwhile from a delay point of view before the expensive
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// legality check
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if (!cumul_costs.count(ncname) || !cumul_costs.at(ncname).count(neighbour)
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|| cumul_costs.at(ncname).at(neighbour) > total_delay) {
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// Now check that the swaps we have made to get here are legal and meet max delay requirements
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if (acceptable_move(move)) {
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cumul_costs[ncname][neighbour] = total_delay;
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backtrace[std::make_pair(ncname, neighbour)] = std::make_pair(cellname, entry.second);
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if (!to_visit.count(std::make_pair(entry.first + 1, neighbour)))
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visit.push(std::make_pair(entry.first + 1, neighbour));
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}
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}
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// Revert the experimental swap
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cell_swap_bel(move.back().first, move.back().second);
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move.pop_back();
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}
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// Revert move by swapping cells back to their original order
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// Execute swaps in reverse order to how we made them originally
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for (auto move_entry : boost::adaptors::reverse(move)) {
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cell_swap_bel(move_entry.first, move_entry.second);
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}
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}
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}
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// Current candidate Bels for cells (linked in both direction>
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std::vector<IdString> path_cells;
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std::unordered_map<IdString, std::unordered_set<BelId>> cell_neighbour_bels;
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std::unordered_map<BelId, std::unordered_set<IdString>> bel_candidate_cells;
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// Map cell ports to net delay limit
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std::unordered_map<std::pair<IdString, IdString>, delay_t> max_net_delay;
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// Criticality data from timing analysis
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NetCriticalityMap net_crit;
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TimingOptCfg cfg;
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Context *ctx;
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};
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bool timing_opt(Context *ctx, TimingOptCfg cfg) { return TimingOptimiser(ctx).optimise(); }
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NEXTPNR_NAMESPACE_END
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