When choosing between labels for one net, always prefer ports.

1. Ports are already used in PCF and LPF files, so it is natural
that the developer would use them for constraints as well.

2. Unpredictable (the set of netlabels nextpnr looks at depends on
the optimizations during synthesis) net naming makes it impossible
for code generators like nMigen to reliably apply clock constraints.

1 files changed, 13 insertions, 4 deletions

diff --git a/json/jsonparse.cc b/json/jsonparse.cc
index a0479c2e..eec7041c 100644
--- a/json/jsonparse.cc
+++ b/json/jsonparse.cc
@@ -692,11 +692,22 @@ void json_import(Context *ctx, string modname, JsonNode *node)
 
     log_info("Importing module %s\n", modname.c_str());
 
+    JsonNode *ports_parent = nullptr;
+    if (node->data_dict.count("ports") > 0)
+        ports_parent = node->data_dict.at("ports");
+
     // Multiple labels might refer to the same net. For now we resolve conflicts thus:
+    //  - (toplevel) ports are always preferred
     //  - names with fewer $ are always prefered
     //  - between equal $ counts, fewer .s are prefered
     //  - ties are resolved alphabetically
-    auto prefer_netlabel = [](const std::string &a, const std::string &b) {
+    auto prefer_netlabel = [ports_parent](const std::string &a, const std::string &b) {
+        if (ports_parent != nullptr) {
+            if (ports_parent->data_dict.count(a))
+                return true;
+            if (ports_parent->data_dict.count(b))
+                return false;
+        }
         if (b.empty())
             return true;
         long a_dollars = std::count(a.begin(), a.end(), '$'), b_dollars = std::count(b.begin(), b.end(), '$');
@@ -753,9 +764,7 @@ void json_import(Context *ctx, string modname, JsonNode *node)
         }
     }
 
-    if (node->data_dict.count("ports")) {
-        JsonNode *ports_parent = node->data_dict.at("ports");
-
+    if (ports_parent != nullptr) {
         // N.B. ports must be imported after cells for tristate behaviour
         // to be correct
         // Loop through all ports, first non-tristate then tristate to handle