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add some comments to timeest

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Emily Schmidt 2025-04-22 12:21:32 +01:00 • committed by Emily (aiju)
parent 28c7f202ca
commit 9c9a0e3e45
1 changed files with 25 additions and 4 deletions
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29
passes/cmds/timeest.cc
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@ -61,6 +61,8 @@ struct EstimateSta {
samplers.push_back(std::make_pair(cell, bit));
}
// we include a discount factor for cells that can be implemented using carry chain logic
// and to account for the AIG model not being balanced
int cell_type_factor(IdString type)
{
if (type.in(ID($gt), ID($ge), ID($lt), ID($le), ID($add), ID($sub),
@ -81,11 +83,13 @@ struct EstimateSta {
{
log("Domain %s\n", log_signal(clk));
// first, we collect launch and sample points and convert the combinational logic to AIG
std::vector<Cell *> combinational;
for (auto cell : m->cells()) {
SigSpec launch, sample;
if (RTLIL::builtin_ff_cell_types().count(cell->type)) {
// collect launch and sample points for FF cell
FfData ff(nullptr, cell);
if (!ff.has_clk) {
log_warning("Ignoring unsupported storage element '%s' (%s)\n",
@ -107,6 +111,7 @@ struct EstimateSta {
} else if (cell->type == ID($scopeinfo)) {
continue;
} else {
// find or build AIG model of combinational cell
auto fingerprint = std::make_pair(cell->type, cell->parameters);
if (!aigs.count(fingerprint)) {
aigs.emplace(fingerprint, Aig(cell));
@ -121,11 +126,14 @@ struct EstimateSta {
}
}
// since we're now taking reference into `aigs`, we can no longer modify it
// and thus have to fill `cell_aigs` in a separate loop
for (auto cell : combinational) {
auto fingerprint = std::make_pair(cell->type, cell->parameters);
cell_aigs.emplace(cell, &aigs.at(fingerprint));
}
// collect launch and sample points for memory cells
for (auto &mem : Mem::get_all_memories(m)) {
for (auto &rd : mem.rd_ports) {
if (!rd.clk_enable) {
@ -143,6 +151,9 @@ struct EstimateSta {
}
}
// now we toposort the combinational logic
// each toposort node is either a SigBit or a pair of Cell * / AigNode *
TopoSort<std::tuple<SigBit, Cell *, AigNode *>> topo;
auto desc_aig = [&](Cell *cell, AigNode &node) {
@ -152,6 +163,7 @@ struct EstimateSta {
return std::make_tuple(sigmap(bit), (Cell *) NULL, (AigNode *) NULL);
};
// collect edges of the AIG graph
for (auto cell : combinational) {
assert(cell_aigs.count(cell));
Aig &aig = *cell_aigs.at(cell);
@ -185,12 +197,16 @@ struct EstimateSta {
if (!topo.sort())
log_error("Module '%s' contains combinational loops", log_id(m));
// now we determine how long it takes for signals to stabilize
// `levels` records the time after a clock edge after which a signal is stable
dict<std::tuple<SigBit, Cell *, AigNode *>, arrivalint> levels;
for (auto node : topo.sorted)
levels[node] = INF_PAST;
// launch points are at 0 by definition
for (auto pair : launchers)
levels[desc_sig(pair.second)] = 0;
@ -200,22 +216,26 @@ struct EstimateSta {
Cell *cell = std::get<1>(node);
Aig &aig = *cell_aigs.at(cell);
if (!aig_node->portname.empty()) {
// for a cell port, copy `levels` value from port bit
SigBit bit = cell->getPort(aig_node->portname)[aig_node->portbit];
levels[node] = levels[desc_sig(bit)];
} else if (aig_node->left_parent < 0 && aig_node->right_parent < 0) {
// constant, nothing to do
} else {
// for each AIG node, find maximum of parents and add a cell-specific delay
int left = levels[desc_aig(cell, aig.nodes[aig_node->left_parent])];
int right = levels[desc_aig(cell, aig.nodes[aig_node->right_parent])];
levels[node] = (std::max(left, right) + cell_type_factor(cell->type));
}
// copy `levels` value to any output ports
for (auto &oport : aig_node->outports) {
levels[desc_sig(cell->getPort(oport.first)[oport.second])] = levels[node];
}
}
}
// now find the length of the critical path (slowest path in the design)
arrivalint crit = INF_PAST;
for (auto pair : samplers)
if (levels[desc_sig(pair.second)] > crit)
@ -232,6 +252,7 @@ struct EstimateSta {
// some compile-time errors related to hashing
dict<std::tuple<SigBit, Cell *, AigNode *>, bool> critical;
// actually find one critical path, or all such paths if requested
for (auto pair : samplers) {
if (levels[desc_sig(pair.second)] == crit) {
critical[desc_sig(pair.second)] = true;
@ -240,6 +261,7 @@ struct EstimateSta {
}
}
// walk backwards through toposorted nodes and set critical flag on nodes in critical path
for (auto it = topo.sorted.rbegin(); it != topo.sorted.rend(); it++) {
auto node = *it;
AigNode *aig_node = std::get<2>(node);
@ -248,22 +270,20 @@ struct EstimateSta {
Aig &aig = *cell_aigs.at(cell);
for (auto &oport : aig_node->outports) {
//levels[desc_sig(cell->getPort(oport.first)[oport.second])] = levels[node];
if (critical.count(desc_sig(cell->getPort(oport.first)[oport.second])))
critical[node] = true;
}
if (!aig_node->portname.empty()) {
SigBit bit = cell->getPort(aig_node->portname)[aig_node->portbit];
//levels[node] = levels[desc_sig(bit)];
if (critical.count(node))
critical[desc_sig(bit)] = true;
} else if (aig_node->left_parent < 0 && aig_node->right_parent < 0) {
// constant, nothing to do
} else {
// figure out which parent is on the critical path
auto left = desc_aig(cell, aig.nodes[aig_node->left_parent]);
auto right = desc_aig(cell, aig.nodes[aig_node->right_parent]);
//levels[node] = (std::max(left, right) + 1);
int crit_input_lvl = levels[node] - cell_type_factor(cell->type);
if (critical.count(node)) {
bool left_critical = (levels[left] == crit_input_lvl);
@ -284,6 +304,7 @@ struct EstimateSta {
}
}
// finally print the path we found
SigPool bits_to_select;
pool<IdString> to_select;