ice40: Promote one clock to a global buffer

Signed-off-by: David Shah <davey1576@gmail.com>

ice40/cells.cc

@@ -79,7 +79,9 @@ CellInfo *create_ice_cell(Design *design, IdString type, IdString name)
 
         add_port(new_cell, "D_IN_0", PORT_OUT);
         add_port(new_cell, "D_IN_1", PORT_OUT);
-
+    } else if (type == "SB_GB") {
+        add_port(new_cell, "USER_SIGNAL_TO_GLOBAL_BUFFER", PORT_IN);
+        add_port(new_cell, "GLOBAL_BUFFER_OUTPUT", PORT_OUT);
     } else {
         log_error("unable to create iCE40 cell of type %s", type.c_str());
     }
@@ -164,4 +166,8 @@ void nxio_to_sb(CellInfo *nxio, CellInfo *sbio)
     }
 }
 
+bool is_global_net(NetInfo *net) {
+    return bool(net_driven_by(net, is_gbuf, "GLOBAL_BUFFER_OUTPUT"));
+}
+
 NEXTPNR_NAMESPACE_END

ice40/cells.h

@@ -50,6 +50,9 @@ inline bool is_ff(const CellInfo *cell)
 // Return true if a cell is a SB_IO
 inline bool is_sb_io(const CellInfo *cell) { return cell->type == "SB_IO"; }
 
+// Return true if a cell is a global buffer
+inline bool is_gbuf(const CellInfo *cell) {return cell->type == "SB_GB"; }
+
 // Convert a SB_LUT primitive to (part of) an ICESTORM_LC, swapping ports
 // as needed. Set no_dff if a DFF is not being used, so that the output
 // can be reconnected
@@ -64,6 +67,9 @@ void dff_to_lc(CellInfo *dff, CellInfo *lc, bool pass_thru_lut = false);
 // Convert a nextpnr IO buffer to a SB_IO
 void nxio_to_sb(CellInfo *nxio, CellInfo *sbio);
 
+// Return true if a net is a global net
+bool is_global_net(NetInfo *net);
+
 NEXTPNR_NAMESPACE_END
 
 #endif

ice40/pack.cc

@@ -222,10 +222,58 @@ static void pack_io(Design *design)
     }
 }
 
+// Simple global promoter (clock only)
+static void promote_globals(Design *design)
+{
+    std::unordered_map<IdString, int> clock_count;
+    for (auto net : design->nets) {
+        NetInfo *ni = net.second;
+        if (ni->driver.cell != nullptr && !is_global_net(ni)) {
+            clock_count[net.first] = 0;
+            for (auto user : ni->users) {
+                if (user.cell != nullptr && is_ff(user.cell) && user.port == "C")
+                    clock_count[net.first]++;
+            }
+        }
+    }
+    auto global_clock = std::max_element(clock_count.begin(), clock_count.end(), [](
+            const std::pair<IdString, int> &a, const std::pair<IdString, int> &b) {
+       return a.second < b.second;
+    });
+    if (global_clock->second > 0) {
+        NetInfo *clknet = design->nets[global_clock->first];
+        CellInfo *gb = create_ice_cell(design, "SB_GB");
+        gb->ports["USER_SIGNAL_TO_GLOBAL_BUFFER"].net = clknet;
+        PortRef pr;
+        pr.cell = gb;
+        pr.port = "USER_SIGNAL_TO_GLOBAL_BUFFER";
+        clknet->users.push_back(pr);
+
+        pr.cell = gb;
+        pr.port = "GLOBAL_BUFFER_OUTPUT";
+        NetInfo *glbnet = new NetInfo();
+        glbnet->name = clknet->name.str() + "_glb";
+        glbnet->driver = pr;
+        design->nets[glbnet->name] = glbnet;
+        std::vector<PortRef> keep_users;
+        for (auto user : clknet->users) {
+            if (user.cell != nullptr && is_ff(user.cell) && user.port == "C") {
+                user.cell->ports[user.port].net = glbnet;
+                glbnet->users.push_back(user);
+            } else {
+                keep_users.push_back(user);
+            }
+        }
+        clknet->users = keep_users;
+        design->cells[gb->name] = gb;
+    }
+}
+
 // Main pack function
 void pack_design(Design *design)
 {
     pack_constants(design);
+    promote_globals(design);
     pack_io(design);
     pack_lut_lutffs(design);
     pack_nonlut_ffs(design);