frontend: Improved error handling and fixes

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

2 files changed, 12 insertions, 1 deletions

diff --git a/frontend/frontend_base.h b/frontend/frontend_base.h
index 3484b5b4..40d03863 100644
--- a/frontend/frontend_base.h
+++ b/frontend/frontend_base.h
@@ -104,6 +104,7 @@
 #include "design_utils.h"
 #include "log.h"
 #include "nextpnr.h"
+#include "util.h"
 NEXTPNR_NAMESPACE_BEGIN
 
 namespace {
@@ -193,8 +194,14 @@ template <typename FrontendType> struct GenericFrontend
         for (auto &mod : mods)
             for (auto &c : mod.second.instantiated_celltypes)
                 candidate_top.erase(c);
-        if (candidate_top.size() != 1)
+        if (candidate_top.size() != 1) {
+            if (candidate_top.size() == 0)
+                log_info("No candidate top level modules.\n");
+            else
+                for (auto ctp : sorted(candidate_top))
+                    log_info("Candidate top module: '%s'\n", ctx->nameOf(ctp));
             log_error("Failed to autodetect top module, please specify using --top.\n");
+        }
         top = *(candidate_top.begin());
     }
 
diff --git a/frontend/json_frontend.cc b/frontend/json_frontend.cc
index 37c1dabd..2eb0a39b 100644
--- a/frontend/json_frontend.cc
+++ b/frontend/json_frontend.cc
@@ -181,6 +181,10 @@ bool parse_json(std::istream &in, const std::string &filename, Context *ctx)
         root = Json::parse(json_str, error, JsonParse::COMMENTS);
         if (root.is_null())
             log_error("Failed to parse JSON file '%s': %s.\n", filename.c_str(), error.c_str());
+        root = root["modules"];
+        if (root.is_null())
+            log_error("JSON file '%s' doesn't look like a netlist (doesn't contain \"modules\" key)\n",
+                      filename.c_str());
     }
     GenericFrontend<JsonFrontendImpl>(ctx, JsonFrontendImpl(root))();
     return true;