cxxrtl: don't check immutable values for changes in VCD writer.

This commit changes the VCD writer such that for all signals that
have `debug_item.type == VALUE && debug_item.next == nullptr`, it
would only sample the value once.

Commit f2d7a187 added more debug information by including constant
wires, and decreased the performance of VCD writer proportionally
because the constant wires were still repeatedly sampled; this commit
eliminates the performance hit.

diff --git a/backends/cxxrtl/cxxrtl_vcd.h b/backends/cxxrtl/cxxrtl_vcd.h
index 8ba94ea7794f8afbad96dd0ba525183c13da0ceb..5f5f612b5acde0b00655d450e07587ea52fc5b6c 100644 (file)
--- a/backends/cxxrtl/cxxrtl_vcd.h
+++ b/backends/cxxrtl/cxxrtl_vcd.h
@@ -104,19 +104,25 @@ class vcd_writer {
                buffer += '\n';
        }
 
-       const variable &register_variable(size_t width, chunk_t *curr) {
+       const variable &register_variable(size_t width, chunk_t *curr, bool immutable = false) {
                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]);
+                       if (immutable) {
+                               variables.emplace_back(variable { variables.size(), width, curr, (size_t)-1 });
+                       } else {
+                               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) {
+               if (var.prev_off == (size_t)-1)
+                       return false; // immutable
                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;
@@ -158,7 +164,7 @@ public:
                switch (item.type) {
                        // Not the best naming but oh well...
                        case debug_item::VALUE:
-                               emit_var(register_variable(item.width, item.curr), "wire", name);
+                               emit_var(register_variable(item.width, item.curr, /*immutable=*/item.next == nullptr), "wire", name);
                                break;
                        case debug_item::WIRE:
                                emit_var(register_variable(item.width, item.curr), "reg", name);