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cxxrtl: implement debug information outlining.

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Aggressive wire localization and inlining is necessary for CXXRTL to
achieve high performance. However, that comes with a cost: reduced
debug information coverage. Previously, as a workaround, the `-Og`
option could have been used to guarantee complete coverage, at a cost
of a significant performance penalty.

This commit introduces debug information outlining. The main eval()
function is compiled with the user-specified optimization settings.
In tandem, an auxiliary debug_eval() function, compiled from the same
netlist, can be used to reconstruct the values of localized/inlined
signals on demand. To the extent that it is possible, debug_eval()
reuses the results of computations performed by eval(), only filling
in the missing values.

Benchmarking a representative design (Minerva SoC SRAM) shows that:
  * Switching from `-O4`/`-Og` to `-O6` reduces runtime by ~40%.
  * Switching from `-g1` to `-g2`, both used with `-O6`, increases
    compile time by ~25%.
  * Although `-g2` increases the resident size of generated modules,
    this has no effect on runtime.

Because the impact of `-g2` is minimal and the benefits of having
unconditional 100% debug information coverage (and the performance
improvement as well) are major, this commit removes `-Og` and changes
the defaults to `-O6 -g2`.

We'll have our cake and eat it too!
This commit is contained in:
whitequark 2020-12-13 07:03:16 +00:00
parent 3b5a1314cd
commit ece25a45d4
5 changed files with 278 additions and 71 deletions
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232
backends/cxxrtl/cxxrtl_backend.cc
View file

@ -539,6 +539,7 @@ struct CxxrtlWorker {
bool inline_public = false;
bool debug_info = false;
bool debug_eval = false;
std::ostringstream f;
std::string indent;
@ -553,8 +554,9 @@ struct CxxrtlWorker {
pool<const RTLIL::Wire*> unbuffered_wires;
pool<const RTLIL::Wire*> localized_wires;
dict<const RTLIL::Wire*, FlowGraph::Node> inlined_wires;
dict<const RTLIL::Wire*, const RTLIL::Wire*> debug_alias_wires;
dict<const RTLIL::Wire*, RTLIL::Const> debug_const_wires;
dict<const RTLIL::Wire*, const RTLIL::Wire*> debug_alias_wires;
pool<const RTLIL::Wire*> debug_outlined_wires;
dict<RTLIL::SigBit, bool> bit_has_state;
dict<const RTLIL::Module*, pool<std::string>> blackbox_specializations;
dict<const RTLIL::Module*, bool> eval_converges;
@ -786,22 +788,22 @@ struct CxxrtlWorker {
dump_const(data, data.size());
}
bool dump_sigchunk(const RTLIL::SigChunk &chunk, bool is_lhs)
bool dump_sigchunk(const RTLIL::SigChunk &chunk, bool is_lhs, bool for_debug = false)
{
if (chunk.wire == NULL) {
dump_const(chunk.data, chunk.width, chunk.offset);
return false;
} else {
if (inlined_wires.count(chunk.wire)) {
if (inlined_wires.count(chunk.wire) && (!for_debug || !debug_outlined_wires[chunk.wire])) {
log_assert(!is_lhs);
const FlowGraph::Node &node = inlined_wires[chunk.wire];
switch (node.type) {
case FlowGraph::Node::Type::CONNECT:
dump_connect_expr(node.connect);
dump_connect_expr(node.connect, for_debug);
break;
case FlowGraph::Node::Type::CELL_EVAL:
log_assert(is_inlinable_cell(node.cell->type));
dump_cell_expr(node.cell);
dump_cell_expr(node.cell, for_debug);
break;
default:
log_assert(false);
@ -821,36 +823,36 @@ struct CxxrtlWorker {
}
}
bool dump_sigspec(const RTLIL::SigSpec &sig, bool is_lhs)
bool dump_sigspec(const RTLIL::SigSpec &sig, bool is_lhs, bool for_debug = false)
{
if (sig.empty()) {
f << "value<0>()";
return false;
} else if (sig.is_chunk()) {
return dump_sigchunk(sig.as_chunk(), is_lhs);
return dump_sigchunk(sig.as_chunk(), is_lhs, for_debug);
} else {
dump_sigchunk(*sig.chunks().rbegin(), is_lhs);
dump_sigchunk(*sig.chunks().rbegin(), is_lhs, for_debug);
for (auto it = sig.chunks().rbegin() + 1; it != sig.chunks().rend(); ++it) {
f << ".concat(";
dump_sigchunk(*it, is_lhs);
dump_sigchunk(*it, is_lhs, for_debug);
f << ")";
}
return true;
}
}
void dump_sigspec_lhs(const RTLIL::SigSpec &sig)
void dump_sigspec_lhs(const RTLIL::SigSpec &sig, bool for_debug = false)
{
dump_sigspec(sig, /*is_lhs=*/true);
dump_sigspec(sig, /*is_lhs=*/true, for_debug);
}
void dump_sigspec_rhs(const RTLIL::SigSpec &sig)
void dump_sigspec_rhs(const RTLIL::SigSpec &sig, bool for_debug = false)
{
// In the contexts where we want template argument deduction to occur for `template<size_t Bits> ... value<Bits>`,
// it is necessary to have the argument to already be a `value<N>`, since template argument deduction and implicit
// type conversion are mutually exclusive. In these contexts, we use dump_sigspec_rhs() to emit an explicit
// type conversion, but only if the expression needs it.
bool is_complex = dump_sigspec(sig, /*is_lhs=*/false);
bool is_complex = dump_sigspec(sig, /*is_lhs=*/false, for_debug);
if (is_complex)
f << ".val()";
}
@ -875,9 +877,9 @@ struct CxxrtlWorker {
}
}
void dump_connect_expr(const RTLIL::SigSig &conn)
void dump_connect_expr(const RTLIL::SigSig &conn, bool for_debug = false)
{
dump_sigspec_rhs(conn.second);
dump_sigspec_rhs(conn.second, for_debug);
}
bool is_connect_inlined(const RTLIL::SigSig &conn)
@ -885,6 +887,14 @@ struct CxxrtlWorker {
return conn.first.is_wire() && inlined_wires.count(conn.first.as_wire());
}
bool is_connect_outlined(const RTLIL::SigSig &conn)
{
for (auto chunk : conn.first.chunks())
if (debug_outlined_wires.count(chunk.wire))
return true;
return false;
}
void collect_connect(const RTLIL::SigSig &conn, std::vector<RTLIL::IdString> &cells)
{
if (!is_connect_inlined(conn))
@ -893,16 +903,18 @@ struct CxxrtlWorker {
collect_sigspec_rhs(conn.second, cells);
}
void dump_connect(const RTLIL::SigSig &conn)
void dump_connect(const RTLIL::SigSig &conn, bool for_debug = false)
{
if (is_connect_inlined(conn))
if (!for_debug && is_connect_inlined(conn))
return;
if (for_debug && !is_connect_outlined(conn))
return;
f << indent << "// connection\n";
f << indent;
dump_sigspec_lhs(conn.first);
dump_sigspec_lhs(conn.first, for_debug);
f << " = ";
dump_connect_expr(conn);
dump_connect_expr(conn, for_debug);
f << ";\n";
}
@ -919,7 +931,7 @@ struct CxxrtlWorker {
}
}
void dump_cell_expr(const RTLIL::Cell *cell)
void dump_cell_expr(const RTLIL::Cell *cell, bool for_debug = false)
{
// Unary cells
if (is_unary_cell(cell->type)) {
@ -927,7 +939,7 @@ struct CxxrtlWorker {
if (is_extending_cell(cell->type))
f << '_' << (cell->getParam(ID::A_SIGNED).as_bool() ? 's' : 'u');
f << "<" << cell->getParam(ID::Y_WIDTH).as_int() << ">(";
dump_sigspec_rhs(cell->getPort(ID::A));
dump_sigspec_rhs(cell->getPort(ID::A), for_debug);
f << ")";
// Binary cells
} else if (is_binary_cell(cell->type)) {
@ -936,18 +948,18 @@ struct CxxrtlWorker {
f << '_' << (cell->getParam(ID::A_SIGNED).as_bool() ? 's' : 'u') <<
(cell->getParam(ID::B_SIGNED).as_bool() ? 's' : 'u');
f << "<" << cell->getParam(ID::Y_WIDTH).as_int() << ">(";
dump_sigspec_rhs(cell->getPort(ID::A));
dump_sigspec_rhs(cell->getPort(ID::A), for_debug);
f << ", ";
dump_sigspec_rhs(cell->getPort(ID::B));
dump_sigspec_rhs(cell->getPort(ID::B), for_debug);
f << ")";
// Muxes
} else if (cell->type == ID($mux)) {
f << "(";
dump_sigspec_rhs(cell->getPort(ID::S));
dump_sigspec_rhs(cell->getPort(ID::S), for_debug);
f << " ? ";
dump_sigspec_rhs(cell->getPort(ID::B));
dump_sigspec_rhs(cell->getPort(ID::B), for_debug);
f << " : ";
dump_sigspec_rhs(cell->getPort(ID::A));
dump_sigspec_rhs(cell->getPort(ID::A), for_debug);
f << ")";
// Parallel (one-hot) muxes
} else if (cell->type == ID($pmux)) {
@ -955,24 +967,24 @@ struct CxxrtlWorker {
int s_width = cell->getParam(ID::S_WIDTH).as_int();
for (int part = 0; part < s_width; part++) {
f << "(";
dump_sigspec_rhs(cell->getPort(ID::S).extract(part));
dump_sigspec_rhs(cell->getPort(ID::S).extract(part), for_debug);
f << " ? ";
dump_sigspec_rhs(cell->getPort(ID::B).extract(part * width, width));
dump_sigspec_rhs(cell->getPort(ID::B).extract(part * width, width), for_debug);
f << " : ";
}
dump_sigspec_rhs(cell->getPort(ID::A));
dump_sigspec_rhs(cell->getPort(ID::A), for_debug);
for (int part = 0; part < s_width; part++) {
f << ")";
}
// Concats
} else if (cell->type == ID($concat)) {
dump_sigspec_rhs(cell->getPort(ID::B));
dump_sigspec_rhs(cell->getPort(ID::B), for_debug);
f << ".concat(";
dump_sigspec_rhs(cell->getPort(ID::A));
dump_sigspec_rhs(cell->getPort(ID::A), for_debug);
f << ").val()";
// Slices
} else if (cell->type == ID($slice)) {
dump_sigspec_rhs(cell->getPort(ID::A));
dump_sigspec_rhs(cell->getPort(ID::A), for_debug);
f << ".slice<";
f << cell->getParam(ID::OFFSET).as_int() + cell->getParam(ID::Y_WIDTH).as_int() - 1;
f << ",";
@ -989,6 +1001,17 @@ struct CxxrtlWorker {
inlined_wires.count(cell->getPort(ID::Y).as_wire());
}
bool is_cell_outlined(const RTLIL::Cell *cell)
{
if (is_internal_cell(cell->type))
for (auto conn : cell->connections())
if (cell->output(conn.first))
for (auto chunk : conn.second.chunks())
if (debug_outlined_wires.count(chunk.wire))
return true;
return false;
}
void collect_cell_eval(const RTLIL::Cell *cell, std::vector<RTLIL::IdString> &cells)
{
if (!is_cell_inlined(cell))
@ -1000,9 +1023,11 @@ struct CxxrtlWorker {
collect_sigspec_rhs(port.second, cells);
}
void dump_cell_eval(const RTLIL::Cell *cell)
void dump_cell_eval(const RTLIL::Cell *cell, bool for_debug = false)
{
if (is_cell_inlined(cell))
if (!for_debug && is_cell_inlined(cell))
return;
if (for_debug && !is_cell_outlined(cell))
return;
if (cell->type == ID($meminit))
return; // Handled elsewhere.
@ -1026,9 +1051,9 @@ struct CxxrtlWorker {
// Elidable cells
if (is_inlinable_cell(cell->type)) {
f << indent;
dump_sigspec_lhs(cell->getPort(ID::Y));
dump_sigspec_lhs(cell->getPort(ID::Y), for_debug);
f << " = ";
dump_cell_expr(cell);
dump_cell_expr(cell, for_debug);
f << ";\n";
// Flip-flops
} else if (is_ff_cell(cell->type)) {
@ -1460,14 +1485,11 @@ struct CxxrtlWorker {
void dump_wire(const RTLIL::Wire *wire, bool is_local)
{
if (inlined_wires.count(wire))
return;
if (localized_wires[wire] && is_local) {
if (is_local && localized_wires[wire] && !inlined_wires.count(wire)) {
dump_attrs(wire);
f << indent << "value<" << wire->width << "> " << mangle(wire) << ";\n";
}
if (!localized_wires[wire] && !is_local) {
if (!is_local && !localized_wires[wire]) {
std::string width;
if (wire->module->has_attribute(ID(cxxrtl_blackbox)) && wire->has_attribute(ID(cxxrtl_width))) {
width = wire->get_string_attribute(ID(cxxrtl_width));
@ -1530,6 +1552,23 @@ struct CxxrtlWorker {
}
}
void dump_debug_wire(const RTLIL::Wire *wire, bool is_local)
{
if (!debug_outlined_wires[wire])
return;
bool is_outlined_member = wire->name.isPublic() &&
!(debug_const_wires.count(wire) || debug_alias_wires.count(wire));
if (is_local && !is_outlined_member) {
dump_attrs(wire);
f << indent << "value<" << wire->width << "> " << mangle(wire) << ";\n";
}
if (!is_local && is_outlined_member) {
dump_attrs(wire);
f << indent << "/*outline*/ value<" << wire->width << "> " << mangle(wire) << ";\n";
}
}
void dump_memory(RTLIL::Module *module, const RTLIL::Memory *memory)
{
vector<const RTLIL::Cell*> init_cells;
@ -1619,6 +1658,27 @@ struct CxxrtlWorker {
dec_indent();
}
void dump_debug_eval_method(RTLIL::Module *module)
{
inc_indent();
for (auto wire : module->wires())
dump_debug_wire(wire, /*is_local=*/true);
for (auto node : schedule[module]) {
switch (node.type) {
case FlowGraph::Node::Type::CONNECT:
dump_connect(node.connect, /*for_debug=*/true);
break;
case FlowGraph::Node::Type::CELL_EVAL:
dump_cell_eval(node.cell, /*for_debug=*/true);
break;
case FlowGraph::Node::Type::CELL_SYNC:
case FlowGraph::Node::Type::PROCESS:
break;
}
}
dec_indent();
}
void dump_commit_method(RTLIL::Module *module)
{
inc_indent();
@ -1656,6 +1716,7 @@ struct CxxrtlWorker {
size_t count_public_wires = 0;
size_t count_const_wires = 0;
size_t count_alias_wires = 0;
size_t count_inline_wires = 0;
size_t count_member_wires = 0;
size_t count_skipped_wires = 0;
size_t count_driven_sync = 0;
@ -1685,6 +1746,12 @@ struct CxxrtlWorker {
f << ", debug_item(debug_alias(), " << mangle(debug_alias_wires[wire]) << ", ";
f << wire->start_offset << "));\n";
count_alias_wires++;
} else if (debug_outlined_wires.count(wire)) {
// Inlined but rematerializable wire
f << indent << "items.add(path + " << escape_cxx_string(get_hdl_name(wire));
f << ", debug_item(debug_eval_outline, " << mangle(wire) << ", ";
f << wire->start_offset << "));\n";
count_inline_wires++;
} else if (!localized_wires.count(wire)) {
// Member wire
std::vector<std::string> flags;
@ -1738,6 +1805,7 @@ struct CxxrtlWorker {
f << "));\n";
count_member_wires++;
} else {
// Localized or inlined wire with no debug information
count_skipped_wires++;
}
}
@ -1761,14 +1829,16 @@ struct CxxrtlWorker {
log_debug("Debug information statistics for module `%s':\n", log_id(module));
log_debug(" Public wires: %zu, of which:\n", count_public_wires);
log_debug(" Const wires: %zu\n", count_const_wires);
log_debug(" Alias wires: %zu\n", count_alias_wires);
log_debug(" Member wires: %zu, of which:\n", count_member_wires);
log_debug(" Driven sync: %zu\n", count_driven_sync);
log_debug(" Driven comb: %zu\n", count_driven_comb);
log_debug(" Undriven: %zu\n", count_undriven);
log_debug(" Mixed driver: %zu\n", count_mixed_driver);
log_debug(" Other wires: %zu (no debug information)\n", count_skipped_wires);
log_debug(" Undriven: %zu\n", count_undriven);
log_debug(" Inline wires: %zu\n", count_inline_wires);
log_debug(" Alias wires: %zu\n", count_alias_wires);
log_debug(" Const wires: %zu\n", count_const_wires);
log_debug(" Other wires: %zu%s\n", count_skipped_wires,
count_skipped_wires > 0 ? " (debug information unavailable)" : "");
}
void dump_metadata_map(const dict<RTLIL::IdString, RTLIL::Const> &metadata_map)
@ -1855,7 +1925,8 @@ struct CxxrtlWorker {
inc_indent();
for (auto wire : module->wires())
dump_wire(wire, /*is_local=*/false);
f << "\n";
for (auto wire : module->wires())
dump_debug_wire(wire, /*is_local=*/false);
bool has_memories = false;
for (auto memory : module->memories) {
dump_memory(module, memory.second);
@ -1927,8 +1998,20 @@ struct CxxrtlWorker {
f << "\n";
f << indent << "bool eval() override;\n";
f << indent << "bool commit() override;\n";
if (debug_info)
if (debug_info) {
if (debug_eval) {
f << "\n";
f << indent << "void debug_eval();\n";
for (auto wire : module->wires())
if (debug_outlined_wires.count(wire)) {
f << indent << "debug_outline debug_eval_outline { std::bind(&"
<< mangle(module) << "::debug_eval, this) };\n";
break;
}
}
f << "\n";
f << indent << "void debug_info(debug_items &items, std::string path = \"\") override;\n";
}
dec_indent();
f << indent << "}; // struct " << mangle(module) << "\n";
f << "\n";
@ -1948,6 +2031,12 @@ struct CxxrtlWorker {
f << indent << "}\n";
f << "\n";
if (debug_info) {
if (debug_eval) {
f << indent << "void " << mangle(module) << "::debug_eval() {\n";
dump_debug_eval_method(module);
f << indent << "}\n";
f << "\n";
}
f << indent << "void " << mangle(module) << "::debug_info(debug_items &items, std::string path) {\n";
dump_debug_info_method(module);
f << indent << "}\n";
@ -2251,6 +2340,11 @@ struct CxxrtlWorker {
for (auto node : wire_comb_def.second)
node_defs[node].insert(wire_comb_def.first);
dict<FlowGraph::Node*, pool<const RTLIL::Wire*>, hash_ptr_ops> node_uses;
for (auto wire_use : flow.wire_uses)
for (auto node : wire_use.second)
node_uses[node].insert(wire_use.first);
Scheduler<FlowGraph::Node> scheduler;
dict<FlowGraph::Node*, Scheduler<FlowGraph::Node>::Vertex*, hash_ptr_ops> node_map;
for (auto node : flow.nodes)
@ -2368,6 +2462,30 @@ struct CxxrtlWorker {
}
}
}
if (debug_info && debug_eval) {
// Find wires that can be be outlined, i.e. whose values can be always recovered from
// the values of other wires. (This is the inverse of inlining--any wire that can be
// inlined can also be outlined.) Although this may seem strictly less efficient, since
// such values are computed at least twice, second-order effects make outlining useful.
pool<const RTLIL::Wire*> worklist, visited;
for (auto wire : module->wires()) {
if (!wire->name.isPublic())
continue; // only outline public wires
worklist.insert(wire);
}
while (!worklist.empty()) {
const RTLIL::Wire *wire = worklist.pop();
visited.insert(wire);
if (!localized_wires.count(wire) && !inlined_wires.count(wire))
continue; // member wire, doesn't need outlining
if (wire->name.isPublic() || !inlined_wires.count(wire))
debug_outlined_wires.insert(wire); // allow outlining of internal wires only
for (auto node : flow.wire_comb_defs[wire])
for (auto node_use : node_uses[node])
if (!visited.count(node_use))
worklist.insert(node_use);
}
}
}
if (has_feedback_arcs || has_buffered_comb_wires) {
// Although both non-feedback buffered combinatorial wires and apparent feedback wires may be eliminated
@ -2457,8 +2575,7 @@ struct CxxrtlWorker {
struct CxxrtlBackend : public Backend {
static const int DEFAULT_OPT_LEVEL = 6;
static const int OPT_LEVEL_DEBUG = 4;
static const int DEFAULT_DEBUG_LEVEL = 1;
static const int DEFAULT_DEBUG_LEVEL = 2;
CxxrtlBackend() : Backend("cxxrtl", "convert design to C++ RTL simulation") { }
void help() override
@ -2671,10 +2788,6 @@ struct CxxrtlBackend : public Backend {
log(" -O6\n");
log(" like -O5, and inline public wires not marked (*keep*) if possible.\n");
log("\n");
log(" -Og\n");
log(" highest optimization level that provides debug information for all\n");
log(" public wires. currently, alias for -O%d.\n", OPT_LEVEL_DEBUG);
log("\n");
log(" -g <level>\n");
log(" set the debug level. the default is -g%d. higher debug levels provide\n", DEFAULT_DEBUG_LEVEL);
log(" more visibility and generate more code, but do not pessimize evaluation.\n");
@ -2686,6 +2799,10 @@ struct CxxrtlBackend : public Backend {
log(" debug information for non-optimized public wires. this also makes it\n");
log(" possible to use the C API.\n");
log("\n");
log(" -g2\n");
log(" like -g1, and compute debug information on demand for all public wires\n");
log(" that were optimized out.\n");
log("\n");
}
void execute(std::ostream *&f, std::string filename, std::vector<std::string> args, RTLIL::Design *design) override
@ -2715,12 +2832,14 @@ struct CxxrtlBackend : public Backend {
continue;
}
if (args[argidx] == "-Og") {
opt_level = OPT_LEVEL_DEBUG;
log_warning("The `-Og` option has been removed. Use `-g2` instead for complete "
"design coverage regardless of optimization level.\n");
continue;
}
if (args[argidx] == "-O" && argidx+1 < args.size() && args[argidx+1] == "g") {
argidx++;
opt_level = OPT_LEVEL_DEBUG;
log_warning("The `-Og` option has been removed. Use `-g2` instead for complete "
"design coverage regardless of optimization level.\n");
continue;
}
if (args[argidx] == "-O" && argidx+1 < args.size()) {
@ -2781,6 +2900,9 @@ struct CxxrtlBackend : public Backend {
}
switch (debug_level) {
// the highest level here must match DEFAULT_DEBUG_LEVEL
case 2:
worker.debug_eval = true;
YS_FALLTHROUGH
case 1:
worker.debug_info = true;
YS_FALLTHROUGH