cxxrtl: provide a way to perform unobtrusive power-on reset.

Although it is always possible to destroy and recreate the design to
simulate a power-on reset, this has two drawbacks:
  * Black boxes are also destroyed and recreated, which causes them
    to reacquire their resources, which might be costly and/or erase
    important state.
  * Pointers into the design are invalidated and have to be acquired
    again, which is costly and might be very inconvenient if they are
    captured elsewhere (especially through the C API).

4 files changed, 78 insertions, 3 deletions

diff --git a/backends/cxxrtl/cxxrtl.h b/backends/cxxrtl/cxxrtl.h
index 41089a153..4528a7d8f 100644
--- a/backends/cxxrtl/cxxrtl.h
+++ b/backends/cxxrtl/cxxrtl.h
@@ -96,9 +96,11 @@ struct value : public expr_base<value<Bits>> {
 	explicit constexpr value(Init ...init) : data{init...} {}
 
 	value(const value<Bits> &) = default;
-	value(value<Bits> &&) = default;
 	value<Bits> &operator=(const value<Bits> &) = default;
 
+	value(value<Bits> &&) = default;
+	value<Bits> &operator=(value<Bits> &&) = default;
+
 	// A (no-op) helper that forces the cast to value<>.
 	CXXRTL_ALWAYS_INLINE
 	const value<Bits> &val() const {
@@ -647,10 +649,16 @@ struct wire {
 	template<typename... Init>
 	explicit constexpr wire(Init ...init) : curr{init...}, next{init...} {}
 
+	// Copying and copy-assigning values is natural. If, however, a value is replaced with a wire,
+	// e.g. because a module is built with a different optimization level, then existing code could
+	// unintentionally copy a wire instead, which would create a subtle but serious bug. To make sure
+	// this doesn't happen, prohibit copying and copy-assigning wires.
 	wire(const wire<Bits> &) = delete;
-	wire(wire<Bits> &&) = default;
 	wire<Bits> &operator=(const wire<Bits> &) = delete;
 
+	wire(wire<Bits> &&) = default;
+	wire<Bits> &operator=(wire<Bits> &&) = default;
+
 	template<class IntegerT>
 	CXXRTL_ALWAYS_INLINE
 	IntegerT get() const {
@@ -692,6 +700,9 @@ struct memory {
 	memory(const memory<Width> &) = delete;
 	memory<Width> &operator=(const memory<Width> &) = delete;
 
+	memory(memory<Width> &&) = default;
+	memory<Width> &operator=(memory<Width> &&) = default;
+
 	// The only way to get the compiler to put the initializer in .rodata and do not copy it on stack is to stuff it
 	// into a plain array. You'd think an std::initializer_list would work here, but it doesn't, because you can't
 	// construct an initializer_list in a constexpr (or something) and so if you try to do that the whole thing is
@@ -815,7 +826,7 @@ struct metadata {
 
 typedef std::map<std::string, metadata> metadata_map;
 
-// Helper class to disambiguate values/wires and their aliases.
+// Tag class to disambiguate values/wires and their aliases.
 struct debug_alias {};
 
 // This structure is intended for consumption via foreign function interfaces, like Python's ctypes.
@@ -965,13 +976,25 @@ struct debug_items {
 	}
 };
 
+// Tag class to disambiguate module move constructor and module constructor that takes black boxes
+// out of another instance of the module.
+struct adopt {};
+
 struct module {
 	module() {}
 	virtual ~module() {}
 
+	// Modules with black boxes cannot be copied. Although not all designs include black boxes,
+	// delete the copy constructor and copy assignment operator to make sure that any downstream
+	// code that manipulates modules doesn't accidentally depend on their availability.
 	module(const module &) = delete;
 	module &operator=(const module &) = delete;
 
+	module(module &&) = default;
+	module &operator=(module &&) = default;
+
+	virtual void reset() = 0;
+
 	virtual bool eval() = 0;
 	virtual bool commit() = 0;
 
diff --git a/backends/cxxrtl/cxxrtl_backend.cc b/backends/cxxrtl/cxxrtl_backend.cc
index a48ea5b23..aab415720 100644
--- a/backends/cxxrtl/cxxrtl_backend.cc
+++ b/backends/cxxrtl/cxxrtl_backend.cc
@@ -1869,6 +1869,46 @@ struct CxxrtlWorker {
 				}
 				if (has_cells)
 					f << "\n";
+				f << indent << mangle(module) << "() {}\n";
+				if (has_cells) {
+					f << indent << mangle(module) << "(adopt, " << mangle(module) << " other) :\n";
+					bool first = true;
+					for (auto cell : module->cells()) {
+						if (is_internal_cell(cell->type))
+							continue;
+						if (first) {
+							first = false;
+						} else {
+							f << ",\n";
+						}
+						RTLIL::Module *cell_module = module->design->module(cell->type);
+						if (cell_module->get_bool_attribute(ID(cxxrtl_blackbox))) {
+							f << indent << "  " << mangle(cell) << "(std::move(other." << mangle(cell) << "))";
+						} else {
+							f << indent << "  " << mangle(cell) << "(adopt {}, std::move(other." << mangle(cell) << "))";
+						}
+					}
+					f << " {\n";
+					inc_indent();
+						for (auto cell : module->cells()) {
+							if (is_internal_cell(cell->type))
+								continue;
+							RTLIL::Module *cell_module = module->design->module(cell->type);
+							if (cell_module->get_bool_attribute(ID(cxxrtl_blackbox)))
+								f << indent << mangle(cell) << "->reset();\n";
+						}
+					dec_indent();
+					f << indent << "}\n";
+				} else {
+					f << indent << mangle(module) << "(adopt, " << mangle(module) << " other) {}\n";
+				}
+				f << "\n";
+				f << indent << "void reset() override {\n";
+				inc_indent();
+					f << indent << "*this = " << mangle(module) << "(adopt {}, std::move(*this));\n";
+				dec_indent();
+				f << indent << "}\n";
+				f << "\n";
 				f << indent << "bool eval() override;\n";
 				f << indent << "bool commit() override;\n";
 				if (debug_info)
diff --git a/backends/cxxrtl/cxxrtl_capi.cc b/backends/cxxrtl/cxxrtl_capi.cc
index b77e4c491..1c3c63e3e 100644
--- a/backends/cxxrtl/cxxrtl_capi.cc
+++ b/backends/cxxrtl/cxxrtl_capi.cc
@@ -43,6 +43,10 @@ void cxxrtl_destroy(cxxrtl_handle handle) {
 	delete handle;
 }
 
+void cxxrtl_reset(cxxrtl_handle handle) {
+	handle->module->reset();
+}
+
 int cxxrtl_eval(cxxrtl_handle handle) {
 	return handle->module->eval();
 }
diff --git a/backends/cxxrtl/cxxrtl_capi.h b/backends/cxxrtl/cxxrtl_capi.h
index 385d6dcf3..662bf2c20 100644
--- a/backends/cxxrtl/cxxrtl_capi.h
+++ b/backends/cxxrtl/cxxrtl_capi.h
@@ -55,6 +55,14 @@ cxxrtl_handle cxxrtl_create(cxxrtl_toplevel design);
 // Release all resources used by a design and its handle.
 void cxxrtl_destroy(cxxrtl_handle handle);
 
+// Reinitialize the design, replacing the internal state with the reset values while preserving
+// black boxes.
+//
+// This operation is essentially equivalent to a power-on reset. Values, wires, and memories are
+// returned to their reset state while preserving the state of black boxes and keeping all of
+// the interior pointers obtained with e.g. `cxxrtl_get` valid.
+void cxxrtl_reset(cxxrtl_handle handle);
+
 // Evaluate the design, propagating changes on inputs to the `next` value of internal state and
 // output wires.
 //