fabulous: Improve names for BRAM bels

Signed-off-by: gatecat <gatecat@ds0.me>

generic/viaduct/fabulous/fabric_parsing.h

@@ -68,6 +68,14 @@ struct parser_view
         return npos;
     }
 
+    size_t rfind(char tok) const
+    {
+        for (size_t i = m_length; i > 0; i--)
+            if (m_ptr[i - 1] == tok)
+                return i - 1;
+        return npos;
+    }
+
     IdString to_id(const BaseCtx *ctx)
     {
         // This isn't really ideal, let's hope one day we can move to C++20 and have proper string_views instead :3
@@ -133,6 +141,12 @@ struct parser_view
         NPNR_ASSERT(pos != npos);
         return std::make_pair(parser_view(m_ptr, pos), parser_view(m_ptr + pos + 1, m_length - (pos + 1)));
     }
+    std::pair<parser_view, parser_view> rsplit(char delim) const
+    {
+        size_t pos = rfind(delim);
+        NPNR_ASSERT(pos != npos);
+        return std::make_pair(parser_view(m_ptr, pos), parser_view(m_ptr + pos + 1, m_length - (pos + 1)));
+    }
 };
 
 struct CsvParser

generic/viaduct/fabulous/fabulous.cc

@@ -237,6 +237,16 @@ struct FabulousImpl : ViaductAPI
             NPNR_ASSERT(bel_idx.size() == 1);
             int bel_z = bel_idx[0] - 'A';
             NPNR_ASSERT(bel_z >= 0 && bel_z < 26);
+            std::vector<parser_view> ports;
+            parser_view port;
+            while (!(port = csv.next_field()).empty()) {
+                ports.push_back(port);
+            }
+            IdString bel_name = bel_idx.to_id(ctx);
+            if (bel_type.in(id_InPass4_frame_config, id_OutPass4_frame_config)) {
+                // Assign BRAM IO a nicer name than just a letter
+                bel_name = ports.front().rsplit('_').first.to_id(ctx);
+            }
             /*
             In the future we will need to handle optionally splitting SLICEs into separate LUT/COMB and FF bels
             This is the preferred approach in nextpnr for arches where the LUT and FF can be used separately of
@@ -245,12 +255,7 @@ struct FabulousImpl : ViaductAPI
             While this isn't yet the standard fabulous SLICE, it should be considered as a future option in fabulous.
             */
             Loc loc(bel_x, bel_y, bel_z);
-            BelId bel = ctx->addBel(IdStringList::concat(tile, bel_idx.to_id(ctx)), bel_type, loc, false, false);
+            BelId bel = ctx->addBel(IdStringList::concat(tile, bel_name), bel_type, loc, false, false);
-            std::vector<parser_view> ports;
-            parser_view port;
-            while (!(port = csv.next_field()).empty()) {
-                ports.push_back(port);
-            }
             handle_bel_ports(bel, tile, bel_type, ports);
         }
         postprocess_bels();