Initial lookahead for FPGA interchange.

Currently the lookahead is disabled by default because of the time to
compute and RAM usage.  However it does appear to work reasonably well
in testing.  Further effort is required to lower RAM usage after initial
computation, and explore trade-off for cheaper time to compute.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

1 files changed, 400 insertions, 0 deletions

diff --git a/fpga_interchange/cost_map.cc b/fpga_interchange/cost_map.cc
new file mode 100644
index 00000000..b42f115d
--- /dev/null
+++ b/fpga_interchange/cost_map.cc
@@ -0,0 +1,400 @@
+/*
+ *  nextpnr -- Next Generation Place and Route
+ *
+ *  Copyright (C) 2021  Symbiflow Authors
+ *
+ *
+ *  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.
+ *
+ */
+
+#include "cost_map.h"
+
+#include "context.h"
+#include "log.h"
+
+NEXTPNR_NAMESPACE_BEGIN
+
+///@brief Factor to adjust the penalty calculation for deltas outside the segment bounding box:
+//      factor < 1.0: penalty has less impact on the final returned delay
+//      factor > 1.0: penalty has more impact on the final returned delay
+static constexpr float PENALTY_FACTOR = 1.f;
+
+///@brief Minimum penalty cost that is added when penalizing a delta outside the segment bounding box.
+static constexpr delay_t PENALTY_MIN = 1;
+
+// also known as the L1 norm
+static int manhattan_distance(const std::pair<int32_t, int32_t> &a, const std::pair<int32_t, int32_t> &b)
+{
+    return std::abs(b.first - a.first) + std::abs(b.second - a.second);
+}
+
+static delay_t penalize(const delay_t &entry, int distance, delay_t penalty)
+{
+    penalty = std::max(penalty, PENALTY_MIN);
+    return entry + distance * penalty * PENALTY_FACTOR;
+}
+
+delay_t CostMap::get_delay(const Context *ctx, WireId src_wire, WireId dst_wire) const
+{
+    TypeWirePair type_pair;
+    type_pair.src = TypeWireId(ctx, src_wire);
+    type_pair.dst = TypeWireId(ctx, dst_wire);
+
+    int src_tile;
+    if (src_wire.tile == -1) {
+        src_tile = ctx->chip_info->nodes[src_wire.index].tile_wires[0].tile;
+    } else {
+        src_tile = src_wire.tile;
+    }
+
+    int32_t src_x, src_y;
+    ctx->get_tile_x_y(src_tile, &src_x, &src_y);
+
+    int dst_tile;
+    if (dst_wire.tile == -1) {
+        dst_tile = ctx->chip_info->nodes[dst_wire.index].tile_wires[0].tile;
+    } else {
+        dst_tile = dst_wire.tile;
+    }
+
+    int32_t dst_x, dst_y;
+    ctx->get_tile_x_y(dst_tile, &dst_x, &dst_y);
+
+    auto iter = cost_map_.find(type_pair);
+    if (iter == cost_map_.end()) {
+        auto &src_type = ctx->chip_info->tile_types[type_pair.src.type];
+        IdString src_tile_type(src_type.name);
+        IdString src_wire_name(src_type.wire_data[type_pair.src.index].name);
+
+        auto &dst_type = ctx->chip_info->tile_types[type_pair.dst.type];
+        IdString dst_tile_type(dst_type.name);
+        IdString dst_wire_name(dst_type.wire_data[type_pair.dst.index].name);
+
+#if 0
+        log_warning("Delay matrix is missing %s/%s -> %s/%s\n",
+                src_tile_type.c_str(ctx),
+                src_wire_name.c_str(ctx),
+                dst_tile_type.c_str(ctx),
+                dst_wire_name.c_str(ctx));
+#endif
+        return std::numeric_limits<delay_t>::max();
+    }
+
+    const auto &delay_matrix = iter->second;
+
+    int32_t off_x = delay_matrix.offset.first + (dst_x - src_x);
+    int32_t off_y = delay_matrix.offset.second + (dst_y - src_y);
+
+    int32_t x_dim = delay_matrix.data.shape()[0];
+    int32_t y_dim = delay_matrix.data.shape()[1];
+    NPNR_ASSERT(x_dim > 0);
+    NPNR_ASSERT(y_dim > 0);
+
+    // Bound closest_x/y to [0, dim)
+    int32_t closest_x = std::min(std::max(off_x, 0), x_dim - 1);
+    int32_t closest_y = std::min(std::max(off_y, 0), y_dim - 1);
+
+    // Get the cost entry from the cost map at the deltas values
+    auto cost = delay_matrix.data[closest_x][closest_y];
+    NPNR_ASSERT(cost >= 0);
+
+    // Get the base penalty corresponding to the current segment.
+    auto penalty = delay_matrix.penalty;
+
+    // Get the distance between the closest point in the bounding box and the original coordinates.
+    // Note that if the original coordinates are within the bounding box, the distance will be equal to zero.
+    auto distance = manhattan_distance(std::make_pair(off_x, off_y), std::make_pair(closest_x, closest_y));
+
+    // Return the penalized cost (no penalty is added if the coordinates are within the bounding box).
+    return penalize(cost, distance, penalty);
+}
+
+void CostMap::set_cost_map(const Context *ctx, const TypeWirePair &wire_pair,
+                           const HashTables::HashMap<std::pair<int32_t, int32_t>, delay_t> &delays)
+{
+    CostMapEntry delay_matrix;
+
+    auto &offset = delay_matrix.offset;
+    offset.first = 0;
+    offset.second = 0;
+
+    int32_t max_x_offset = 0;
+    int32_t max_y_offset = 0;
+
+    for (const auto &delay_pair : delays) {
+        auto &dx_dy = delay_pair.first;
+        offset.first = std::max(-dx_dy.first, offset.first);
+        offset.second = std::max(-dx_dy.second, offset.second);
+        max_x_offset = std::max(dx_dy.first, max_x_offset);
+        max_y_offset = std::max(dx_dy.second, max_y_offset);
+    }
+
+    int32_t x_dim = offset.first + max_x_offset + 1;
+    int32_t y_dim = offset.second + max_y_offset + 1;
+
+    delay_matrix.data.resize(boost::extents[x_dim][y_dim]);
+
+    // Fill matrix with sentinel of -1 to know where the holes in the matrix
+    // are.
+    std::fill_n(delay_matrix.data.data(), delay_matrix.data.num_elements(), -1);
+
+    for (const auto &delay_pair : delays) {
+        auto &dx_dy = delay_pair.first;
+        int32_t off_x = dx_dy.first + offset.first;
+        int32_t off_y = dx_dy.second + offset.second;
+        NPNR_ASSERT(off_x >= 0);
+        NPNR_ASSERT(off_x < x_dim);
+        NPNR_ASSERT(off_y >= 0);
+        NPNR_ASSERT(off_y < y_dim);
+
+        delay_matrix.data[off_x][off_y] = delay_pair.second;
+    }
+
+    delay_matrix.penalty = get_penalty(delay_matrix.data);
+    fill_holes(ctx, wire_pair, delay_matrix.data, delay_matrix.penalty);
+
+    {
+        cost_map_mutex_.lock();
+        auto result = cost_map_.emplace(wire_pair, delay_matrix);
+        NPNR_ASSERT(result.second);
+        cost_map_mutex_.unlock();
+    }
+}
+
+static void assign_min_entry(delay_t *dst, const delay_t &src)
+{
+    if (src >= 0) {
+        if (*dst < 0) {
+            *dst = src;
+        } else if (src < *dst) {
+            *dst = src;
+        }
+    }
+}
+
+std::pair<delay_t, int> CostMap::get_nearby_cost_entry(const boost::multi_array<delay_t, 2> &matrix, int cx, int cy,
+                                                       const ArcBounds &bounds)
+{
+#ifdef DEBUG_FILL
+    log_info("Filling %d, %d within (%d, %d, %d, %d)\n", cx, cy, bounds.x0, bounds.y0, bounds.x1, bounds.y1);
+#endif
+
+    // spiral around (cx, cy) looking for a nearby entry
+    bool in_bounds = bounds.contains(cx, cy);
+    if (!in_bounds) {
+#ifdef DEBUG_FILL
+        log_info("Already out of bounds, return!\n");
+#endif
+        return std::make_pair(-1, 0);
+    }
+    int n = 0;
+    delay_t fill(matrix[cx][cy]);
+
+    while (in_bounds && (fill < 0)) {
+        n++;
+#ifdef DEBUG_FILL
+        log_info("At n = %d\n", n);
+#endif
+        in_bounds = false;
+        delay_t min_entry = -1;
+        for (int ox = -n; ox <= n; ox++) {
+            int x = cx + ox;
+            int oy = n - abs(ox);
+            int yp = cy + oy;
+            int yn = cy - oy;
+#ifdef DEBUG_FILL
+            log_info("Testing %d, %d\n", x, yp);
+#endif
+            if (bounds.contains(x, yp)) {
+                assign_min_entry(&min_entry, matrix[x][yp]);
+                in_bounds = true;
+#ifdef DEBUG_FILL
+                log_info("matrix[%d, %d] = %d, min_entry = %d\n", x, yp, matrix[x][yp], min_entry);
+#endif
+            }
+#ifdef DEBUG_FILL
+            log_info("Testing %d, %d\n", x, yn);
+#endif
+            if (bounds.contains(x, yn)) {
+                assign_min_entry(&min_entry, matrix[x][yn]);
+                in_bounds = true;
+#ifdef DEBUG_FILL
+                log_info("matrix[%d, %d] = %d, min_entry = %d\n", x, yn, matrix[x][yn], min_entry);
+#endif
+            }
+        }
+
+        if (fill < 0 && min_entry >= 0) {
+            fill = min_entry;
+        }
+    }
+
+    return std::make_pair(fill, n);
+}
+
+void CostMap::fill_holes(const Context *ctx, const TypeWirePair &type_pair, boost::multi_array<delay_t, 2> &matrix,
+                         delay_t delay_penalty)
+{
+    // find missing cost entries and fill them in by copying a nearby cost entry
+    std::vector<std::tuple<unsigned, unsigned, delay_t>> missing;
+    bool couldnt_fill = false;
+    auto shifted_bounds = ArcBounds(0, 0, matrix.shape()[0] - 1, matrix.shape()[1] - 1);
+    int max_fill = 0;
+    for (unsigned ix = 0; ix < matrix.shape()[0]; ix++) {
+        for (unsigned iy = 0; iy < matrix.shape()[1]; iy++) {
+            delay_t &cost_entry = matrix[ix][iy];
+            if (cost_entry < 0) {
+                // maximum search radius
+                delay_t filler;
+                int distance;
+                std::tie(filler, distance) = get_nearby_cost_entry(matrix, ix, iy, shifted_bounds);
+                if (filler >= 0) {
+                    missing.push_back(std::make_tuple(ix, iy, penalize(filler, distance, delay_penalty)));
+                    max_fill = std::max(max_fill, distance);
+                } else {
+                    couldnt_fill = true;
+                }
+            }
+        }
+        if (couldnt_fill) {
+            // give up trying to fill an empty matrix
+            break;
+        }
+    }
+
+    if (!couldnt_fill && max_fill > 0) {
+        if (ctx->verbose) {
+            auto &src_type_data = ctx->chip_info->tile_types[type_pair.src.type];
+            IdString src_type(src_type_data.name);
+            IdString src_wire(src_type_data.wire_data[type_pair.src.index].name);
+
+            auto &dst_type_data = ctx->chip_info->tile_types[type_pair.dst.type];
+            IdString dst_type(dst_type_data.name);
+            IdString dst_wire(dst_type_data.wire_data[type_pair.dst.index].name);
+
+#ifdef DEBUG_FILL
+            log_info("At %s/%s -> %s/%s: max_fill = %d, delay_penalty = %d\n", src_type.c_str(ctx), src_wire.c_str(ctx),
+                     dst_type.c_str(ctx), dst_wire.c_str(ctx), max_fill, delay_penalty);
+#endif
+        }
+    }
+
+    // write back the missing entries
+    for (auto &xy_entry : missing) {
+        matrix[std::get<0>(xy_entry)][std::get<1>(xy_entry)] = std::get<2>(xy_entry);
+    }
+
+    if (couldnt_fill) {
+        auto &src_type_data = ctx->chip_info->tile_types[type_pair.src.type];
+        IdString src_type(src_type_data.name);
+        IdString src_wire(src_type_data.wire_data[type_pair.src.index].name);
+
+        auto &dst_type_data = ctx->chip_info->tile_types[type_pair.dst.type];
+        IdString dst_type(dst_type_data.name);
+        IdString dst_wire(dst_type_data.wire_data[type_pair.dst.index].name);
+
+        log_warning("Couldn't fill holes in the cost matrix %s/%s -> %s/%s %d x %d bounding box\n", src_type.c_str(ctx),
+                    src_wire.c_str(ctx), dst_type.c_str(ctx), dst_wire.c_str(ctx), shifted_bounds.x1,
+                    shifted_bounds.y1);
+        for (unsigned y = 0; y < matrix.shape()[1]; y++) {
+            for (unsigned x = 0; x < matrix.shape()[0]; x++) {
+                NPNR_ASSERT(matrix[x][y] >= 0);
+            }
+        }
+    }
+}
+
+delay_t CostMap::get_penalty(const boost::multi_array<delay_t, 2> &matrix) const
+{
+    delay_t min_delay = std::numeric_limits<delay_t>::max();
+    delay_t max_delay = std::numeric_limits<delay_t>::min();
+
+    std::pair<int32_t, int32_t> min_location(0, 0), max_location(0, 0);
+    for (unsigned ix = 0; ix < matrix.shape()[0]; ix++) {
+        for (unsigned iy = 0; iy < matrix.shape()[1]; iy++) {
+            const delay_t &cost_entry = matrix[ix][iy];
+            if (cost_entry >= 0) {
+                if (cost_entry < min_delay) {
+                    min_delay = cost_entry;
+                    min_location = std::make_pair(ix, iy);
+                }
+                if (cost_entry > max_delay) {
+                    max_delay = cost_entry;
+                    max_location = std::make_pair(ix, iy);
+                }
+            }
+        }
+    }
+
+    delay_t delay_penalty =
+            (max_delay - min_delay) / static_cast<float>(std::max(1, manhattan_distance(max_location, min_location)));
+
+    return delay_penalty;
+}
+
+void CostMap::from_reader(lookahead_storage::CostMap::Reader reader)
+{
+    for (auto cost_entry : reader.getCostMap()) {
+        TypeWirePair key(cost_entry.getKey());
+
+        auto result = cost_map_.emplace(key, CostMapEntry());
+        NPNR_ASSERT(result.second);
+
+        CostMapEntry &entry = result.first->second;
+        auto data = cost_entry.getData();
+        auto in_iter = data.begin();
+
+        entry.data.resize(boost::extents[cost_entry.getXDim()][cost_entry.getYDim()]);
+        if (entry.data.num_elements() != data.size()) {
+            log_error("entry.data.num_elements() %zu != data.size() %u", entry.data.num_elements(), data.size());
+        }
+
+        delay_t *out = entry.data.origin();
+        for (; in_iter != data.end(); ++in_iter, ++out) {
+            *out = *in_iter;
+        }
+
+        entry.penalty = cost_entry.getPenalty();
+
+        entry.offset.first = cost_entry.getXOffset();
+        entry.offset.second = cost_entry.getYOffset();
+    }
+}
+
+void CostMap::to_builder(lookahead_storage::CostMap::Builder builder) const
+{
+    auto cost_map = builder.initCostMap(cost_map_.size());
+    auto entry_iter = cost_map.begin();
+    auto in = cost_map_.begin();
+    for (; entry_iter != cost_map.end(); ++entry_iter, ++in) {
+        NPNR_ASSERT(in != cost_map_.end());
+
+        in->first.to_builder(entry_iter->getKey());
+        const CostMapEntry &entry = in->second;
+
+        auto data = entry_iter->initData(entry.data.num_elements());
+        const delay_t *data_in = entry.data.origin();
+        for (size_t i = 0; i < entry.data.num_elements(); ++i) {
+            data.set(i, data_in[i]);
+        }
+
+        entry_iter->setXDim(entry.data.shape()[0]);
+        entry_iter->setYDim(entry.data.shape()[1]);
+        entry_iter->setXOffset(entry.offset.first);
+        entry_iter->setYOffset(entry.offset.second);
+        entry_iter->setPenalty(entry.penalty);
+    }
+}
+
+NEXTPNR_NAMESPACE_END