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.

1 files changed, 10 insertions, 4 deletions

diff --git a/backends/cxxrtl/cxxrtl_vcd.h b/backends/cxxrtl/cxxrtl_vcd.h
index 8ba94ea77..5f5f612b5 100644
--- 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);