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cxxrtl: don't overwrite buffered inputs.

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Before this commit, a cell's input was always assigned like:

    p_cell.p_input = (value...);

If `p_input` is buffered (e.g. if the design is built at -O0), this
is not correct. (In practice, this breaks clocking.) Unfortunately,
the incorrect design was compiled without diagnostics because wire<>
was move-assignable and also implicitly constructible from value<>.

After this commit, cell inputs are no longer incorrectly assumed to
always be unbuffered, and wires are not assignable from values.
This commit is contained in:
whitequark 2020-12-11 23:30:32 +00:00
parent ec410c9b19
commit e4aa8bc96b
2 changed files with 33 additions and 25 deletions
Download patch file Download diff file Expand all files Collapse all files

2
backends/cxxrtl/cxxrtl.h
View file

@ -659,7 +659,7 @@ struct wire {
value<Bits> next;
wire() = default;
constexpr wire(const value<Bits> &init) : curr(init), next(init) {}
explicit constexpr wire(const value<Bits> &init) : curr(init), next(init) {}
template<typename... Init>
explicit constexpr wire(Init ...init) : curr{init...}, next{init...} {}

56
backends/cxxrtl/cxxrtl_backend.cc
View file

@ -1240,10 +1240,19 @@ struct CxxrtlWorker {
// User cells
} else {
log_assert(cell->known());
bool buffered_inputs = false;
const char *access = is_cxxrtl_blackbox_cell(cell) ? "->" : ".";
for (auto conn : cell->connections())
if (cell->input(conn.first) && !cell->output(conn.first)) {
f << indent << mangle(cell) << access << mangle_wire_name(conn.first) << " = ";
if (cell->input(conn.first)) {
RTLIL::Module *cell_module = cell->module->design->module(cell->type);
log_assert(cell_module != nullptr && cell_module->wire(conn.first) && conn.second.is_wire());
RTLIL::Wire *cell_module_wire = cell_module->wire(conn.first);
f << indent << mangle(cell) << access << mangle_wire_name(conn.first);
if (!is_cxxrtl_blackbox_cell(cell) && !unbuffered_wires[cell_module_wire]) {
buffered_inputs = true;
f << ".next";
}
f << " = ";
dump_sigspec_rhs(conn.second);
f << ";\n";
if (getenv("CXXRTL_VOID_MY_WARRANTY")) {
@ -1255,19 +1264,11 @@ struct CxxrtlWorker {
// with:
// top.prev_p_clk = value<1>{0u}; top.p_clk = value<1>{1u}; top.step();
// Don't rely on this; it will be removed without warning.
RTLIL::Module *cell_module = cell->module->design->module(cell->type);
if (cell_module != nullptr && cell_module->wire(conn.first) && conn.second.is_wire()) {
RTLIL::Wire *cell_module_wire = cell_module->wire(conn.first);
if (edge_wires[conn.second.as_wire()] && edge_wires[cell_module_wire]) {
f << indent << mangle(cell) << access << "prev_" << mangle(cell_module_wire) << " = ";
f << "prev_" << mangle(conn.second.as_wire()) << ";\n";
}
if (edge_wires[conn.second.as_wire()] && edge_wires[cell_module_wire]) {
f << indent << mangle(cell) << access << "prev_" << mangle(cell_module_wire) << " = ";
f << "prev_" << mangle(conn.second.as_wire()) << ";\n";
}
}
} else if (cell->input(conn.first)) {
f << indent << mangle(cell) << access << mangle_wire_name(conn.first) << ".next = ";
dump_sigspec_rhs(conn.second);
f << ";\n";
}
auto assign_from_outputs = [&](bool cell_converged) {
for (auto conn : cell->connections()) {
@ -1285,9 +1286,9 @@ struct CxxrtlWorker {
// have any buffered wires if they were not output ports. Imagine inlining the cell's eval() function,
// and consider the fate of the localized wires that used to be output ports.)
//
// Unlike cell inputs (which are never buffered), it is not possible to know apriori whether the cell
// (which may be late bound) will converge immediately. Because of this, the choice between using .curr
// (appropriate for buffered outputs) and .next (appropriate for unbuffered outputs) is made at runtime.
// It is not possible to know apriori whether the cell (which may be late bound) will converge immediately.
// Because of this, the choice between using .curr (appropriate for buffered outputs) and .next (appropriate
// for unbuffered outputs) is made at runtime.
if (cell_converged && is_cxxrtl_comb_port(cell, conn.first))
f << ".next;\n";
else
@ -1295,16 +1296,23 @@ struct CxxrtlWorker {
}
}
};
f << indent << "if (" << mangle(cell) << access << "eval()) {\n";
inc_indent();
assign_from_outputs(/*cell_converged=*/true);
dec_indent();
f << indent << "} else {\n";
inc_indent();
if (buffered_inputs) {
// If we have any buffered inputs, there's no chance of converging immediately.
f << indent << mangle(cell) << access << "eval();\n";
f << indent << "converged = false;\n";
assign_from_outputs(/*cell_converged=*/false);
dec_indent();
f << indent << "}\n";
} else {
f << indent << "if (" << mangle(cell) << access << "eval()) {\n";
inc_indent();
assign_from_outputs(/*cell_converged=*/true);
dec_indent();
f << indent << "} else {\n";
inc_indent();
f << indent << "converged = false;\n";
assign_from_outputs(/*cell_converged=*/false);
dec_indent();
f << indent << "}\n";
}
}
}