cxxrtl: only write VCD values that were actually updated.

On a representative design (Minerva SoC) this reduces VCD file size
by ~20× and runtime by ~3×.

diff --git a/backends/cxxrtl/cxxrtl_vcd.h b/backends/cxxrtl/cxxrtl_vcd.h
index 537bd5661a3f14404b833ee814c7e8e0297f04cb..5706917caa309491247580b1d791c6d8d32e34c5 100644 (file)
--- a/backends/cxxrtl/cxxrtl_vcd.h
+++ b/backends/cxxrtl/cxxrtl_vcd.h
@@ -28,10 +28,12 @@ class vcd_writer {
                size_t ident;
                size_t width;
                chunk_t *curr;
+               size_t prev_off;
        };
 
        std::vector<std::string> current_scope;
        std::vector<variable> variables;
+       std::vector<chunk_t> cache;
        bool streaming = false;
 
        void emit_timescale(unsigned number, const std::string &unit) {
@@ -101,6 +103,22 @@ class vcd_writer {
                buffer += '\n';
        }
 
+       void append_variable(size_t width, chunk_t *curr) {
+               const size_t chunks = (width + (sizeof(chunk_t) * 8 - 1)) / (sizeof(chunk_t) * 8);
+               variables.emplace_back(variable { variables.size(), width, curr, cache.size() });
+               cache.insert(cache.end(), &curr[0], &curr[chunks]);
+       }
+
+       bool test_variable(const variable &var) {
+               const size_t chunks = (var.width + (sizeof(chunk_t) * 8 - 1)) / (sizeof(chunk_t) * 8);
+               if (std::equal(&var.curr[0], &var.curr[chunks], &cache[var.prev_off])) {
+                       return false;
+               } else {
+                       std::copy(&var.curr[0], &var.curr[chunks], &cache[var.prev_off]);
+                       return true;
+               }
+       }
+
        static std::vector<std::string> split_hierarchy(const std::string &hier_name) {
                std::vector<std::string> hierarchy;
                size_t prev = 0;
@@ -133,11 +151,11 @@ public:
                switch (item.type) {
                        // Not the best naming but oh well...
                        case debug_item::VALUE:
-                               variables.emplace_back(variable { variables.size(), item.width, item.curr });
+                               append_variable(item.width, item.curr);
                                emit_var(variables.back(), "wire", name);
                                break;
                        case debug_item::WIRE:
-                               variables.emplace_back(variable { variables.size(), item.width, item.curr });
+                               append_variable(item.width, item.curr);
                                emit_var(variables.back(), "reg", name);
                                break;
                        case debug_item::MEMORY: {
@@ -145,7 +163,7 @@ public:
                                for (size_t index = 0; index < item.depth; index++) {
                                        chunk_t *nth_curr = &item.curr[stride * index];
                                        std::string nth_name = name + '[' + std::to_string(index) + ']';
-                                       variables.emplace_back(variable { variables.size(), item.width, nth_curr });
+                                       append_variable(item.width, nth_curr);
                                        emit_var(variables.back(), "reg", nth_name);
                                }
                                break;
@@ -175,17 +193,19 @@ public:
        }
 
        void sample(uint64_t timestamp) {
-               if (!streaming) {
+               bool first_sample = !streaming;
+               if (first_sample) {
                        emit_scope({});
                        emit_enddefinitions();
                }
                emit_time(timestamp);
-               for (auto var : variables) {
-                       if (var.width == 1)
-                               emit_scalar(var);
-                       else
-                               emit_vector(var);
-               }
+               for (auto var : variables)
+                       if (test_variable(var) || first_sample) {
+                               if (var.width == 1)
+                                       emit_scalar(var);
+                               else
+                                       emit_vector(var);
+                       }
        }
 };