cxxrtl: track aliases in VCD writer.

This commit changes the VCD writer such that for all signals that
share `debug_item.curr`, it would only emit a single VCD identifier,
and sample the value once.

Commit 9b39c6f7 added redundancy to debug information by including
alias wires, and increased the size of VCD files proportionally; this
commit eliminates the redundancy from VCD files so that their size
is the same as before.

diff --git a/backends/cxxrtl/cxxrtl_vcd.h b/backends/cxxrtl/cxxrtl_vcd.h
index 5706917caa309491247580b1d791c6d8d32e34c5..8ba94ea7794f8afbad96dd0ba525183c13da0ceb 100644 (file)
--- a/backends/cxxrtl/cxxrtl_vcd.h
+++ b/backends/cxxrtl/cxxrtl_vcd.h
@@ -34,6 +34,7 @@ class vcd_writer {
        std::vector<std::string> current_scope;
        std::vector<variable> variables;
        std::vector<chunk_t> cache;
+       std::map<chunk_t*, size_t> aliases;
        bool streaming = false;
 
        void emit_timescale(unsigned number, const std::string &unit) {
@@ -103,10 +104,16 @@ 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]);
+       const variable &register_variable(size_t width, chunk_t *curr) {
+               if (aliases.count(curr)) {
+                       return variables[aliases[curr]];
+               } else {
+                       const size_t chunks = (width + (sizeof(chunk_t) * 8 - 1)) / (sizeof(chunk_t) * 8);
+                       aliases[curr] = variables.size();
+                       variables.emplace_back(variable { variables.size(), width, curr, cache.size() });
+                       cache.insert(cache.end(), &curr[0], &curr[chunks]);
+                       return variables.back();
+               }
        }
 
        bool test_variable(const variable &var) {
@@ -151,20 +158,17 @@ public:
                switch (item.type) {
                        // Not the best naming but oh well...
                        case debug_item::VALUE:
-                               append_variable(item.width, item.curr);
-                               emit_var(variables.back(), "wire", name);
+                               emit_var(register_variable(item.width, item.curr), "wire", name);
                                break;
                        case debug_item::WIRE:
-                               append_variable(item.width, item.curr);
-                               emit_var(variables.back(), "reg", name);
+                               emit_var(register_variable(item.width, item.curr), "reg", name);
                                break;
                        case debug_item::MEMORY: {
                                const size_t stride = (item.width + (sizeof(chunk_t) * 8 - 1)) / (sizeof(chunk_t) * 8);
                                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) + ']';
-                                       append_variable(item.width, nth_curr);
-                                       emit_var(variables.back(), "reg", nth_name);
+                                       emit_var(register_variable(item.width, nth_curr), "reg", nth_name);
                                }
                                break;
                        }