From 467152d79fd4c8e25b48481f742fe318b1395728 Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Mon, 8 Jun 2020 17:38:11 +0000
Subject: [PATCH] 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.
---
 backends/cxxrtl/cxxrtl_vcd.h | 14 ++++++++++----
 1 file 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);