ice40: Don't split carry chain in simple feed-out cases

Signed-off-by: David Shah <dave@ds0.me>

1 files changed, 50 insertions, 7 deletions

diff --git a/ice40/chains.cc b/ice40/chains.cc
index 07eb100b..b8fbee0f 100644
--- a/ice40/chains.cc
+++ b/ice40/chains.cc
@@ -74,10 +74,10 @@ class ChainConstrainer
                         (net_only_drives(ctx, carry_net, is_lc, ctx->id("I3"), false) !=
                          net_only_drives(ctx, carry_net, is_lc, ctx->id("CIN"), false)) ||
                         (at_end && !net_only_drives(ctx, carry_net, is_lc, ctx->id("I3"), true))) {
-                        CellInfo *passout = make_carry_pass_out(cell->ports.at(ctx->id("COUT")));
+                        CellInfo *passout = make_carry_pass_out(cell->ports.at(ctx->id("COUT")),
+                                                                at_end ? nullptr : *(curr_cell + 1));
                         chains.back().cells.push_back(passout);
                         tile.push_back(passout);
-                        start_of_chain = true;
                     }
                 }
                 ++curr_cell;
@@ -87,30 +87,73 @@ class ChainConstrainer
     }
 
     // Insert a logic cell to legalise a COUT->fabric connection
-    CellInfo *make_carry_pass_out(PortInfo &cout_port)
+    CellInfo *make_carry_pass_out(PortInfo &cout_port, CellInfo *cin_cell = nullptr)
     {
         NPNR_ASSERT(cout_port.net != nullptr);
         std::unique_ptr<CellInfo> lc = create_ice_cell(ctx, ctx->id("ICESTORM_LC"));
         lc->params[ctx->id("LUT_INIT")] = "65280"; // 0xff00: O = I3
         lc->params[ctx->id("CARRY_ENABLE")] = "1";
-        lc->ports.at(ctx->id("O")).net = cout_port.net;
+        lc->ports.at(id_O).net = cout_port.net;
         std::unique_ptr<NetInfo> co_i3_net(new NetInfo());
         co_i3_net->name = ctx->id(lc->name.str(ctx) + "$I3");
         co_i3_net->driver = cout_port.net->driver;
         PortRef i3_r;
-        i3_r.port = ctx->id("I3");
+        i3_r.port = id_I3;
         i3_r.cell = lc.get();
         co_i3_net->users.push_back(i3_r);
         PortRef o_r;
-        o_r.port = ctx->id("O");
+        o_r.port = id_O;
         o_r.cell = lc.get();
         cout_port.net->driver = o_r;
-        lc->ports.at(ctx->id("I3")).net = co_i3_net.get();
+        lc->ports.at(id_I3).net = co_i3_net.get();
         cout_port.net = co_i3_net.get();
 
         IdString co_i3_name = co_i3_net->name;
         NPNR_ASSERT(ctx->nets.find(co_i3_name) == ctx->nets.end());
         ctx->nets[co_i3_name] = std::move(co_i3_net);
+
+        // If COUT also connects to a CIN; preserve the carry chain
+        if (cin_cell) {
+            std::unique_ptr<NetInfo> co_cin_net(new NetInfo());
+            co_cin_net->name = ctx->id(lc->name.str(ctx) + "$COUT");
+
+            // Connect I1 to 1 to preserve carry chain
+            NetInfo *vcc = ctx->nets.at(ctx->id("$PACKER_VCC_NET")).get();
+            lc->ports.at(id_I1).net = vcc;
+            PortRef i1_r;
+            i1_r.port = id_I1;
+            i1_r.cell = lc.get();
+            vcc->users.push_back(i1_r);
+
+            // Connect co_cin_net to the COUT of the LC
+            PortRef co_r;
+            co_r.port = id_COUT;
+            co_r.cell = lc.get();
+            co_cin_net->driver = co_r;
+            lc->ports.at(id_COUT).net = co_cin_net.get();
+
+            // Find the user corresponding to the next CIN
+            int replaced_ports = 0;
+            log_info("cell: %s\n", cin_cell->name.c_str(ctx));
+            for (auto port : {id_CIN, id_I3}) {
+                auto &usr = lc->ports.at(id_O).net->users;
+                for (auto user : usr)
+                    log_info("%s.%s\n", user.cell->name.c_str(ctx), user.port.c_str(ctx));
+                auto fnd_user = std::find_if(usr.begin(), usr.end(),
+                                             [&](const PortRef &pr) { return pr.cell == cin_cell && pr.port == port; });
+                if (fnd_user != usr.end()) {
+                    co_cin_net->users.push_back(*fnd_user);
+                    usr.erase(fnd_user);
+                    cin_cell->ports.at(port).net = co_cin_net.get();
+                    ++replaced_ports;
+                }
+            }
+            NPNR_ASSERT(replaced_ports > 0);
+            IdString co_cin_name = co_cin_net->name;
+            NPNR_ASSERT(ctx->nets.find(co_cin_name) == ctx->nets.end());
+            ctx->nets[co_cin_name] = std::move(co_cin_net);
+        }
+
         IdString name = lc->name;
         ctx->assignCellInfo(lc.get());
         ctx->cells[lc->name] = std::move(lc);