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cxxrtl: disambiguate values/wires and their aliases in debug info.

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With this change, it is easier to see which signals carry state (only
wire<>s appear as `reg` in VCD files) and to construct a minimal
checkpoint (CXXRTL_WIRE debug items represent the canonical smallest
set of state required to fully reconstruct the simulation).
This commit is contained in:
whitequark 2020-06-10 14:39:45 +00:00
parent 8f1a320646
commit 0955a603c8
4 changed files with 50 additions and 9 deletions
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17
backends/cxxrtl/cxxrtl_vcd.h
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@ -104,13 +104,13 @@ class vcd_writer {
buffer += '\n';
}
const variable &register_variable(size_t width, chunk_t *curr, bool immutable = false) {
const variable &register_variable(size_t width, chunk_t *curr, bool constant = 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();
if (immutable) {
if (constant) {
variables.emplace_back(variable { variables.size(), width, curr, (size_t)-1 });
} else {
variables.emplace_back(variable { variables.size(), width, curr, cache.size() });
@ -122,7 +122,7 @@ class vcd_writer {
bool test_variable(const variable &var) {
if (var.prev_off == (size_t)-1)
return false; // immutable
return false; // constant
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;
@ -164,7 +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, /*immutable=*/item.next == nullptr), "wire", name);
emit_var(register_variable(item.width, item.curr, /*constant=*/item.next == nullptr), "wire", name);
break;
case debug_item::WIRE:
emit_var(register_variable(item.width, item.curr), "reg", name);
@ -178,6 +178,13 @@ public:
}
break;
}
case debug_item::ALIAS:
// Like VALUE, but, even though `item.next == nullptr` always holds, the underlying value
// can actually change, and must be tracked. In most cases the VCD identifier will be
// unified with the aliased reg, but we should handle the case where only the alias is
// added to the VCD writer, too.
emit_var(register_variable(item.width, item.curr), "wire", name);
break;
}
}
@ -198,7 +205,7 @@ public:
void add_without_memories(const debug_items &items) {
this->template add(items, [](const std::string &, const debug_item &item) {
return item.type == debug_item::VALUE || item.type == debug_item::WIRE;
return item.type != debug_item::MEMORY;
});
}