Add (* gclk *) attribute support

Signed-off-by: Clifford Wolf <clifford@clifford.at>

README.md

@@ -418,7 +418,9 @@ Non-standard or SystemVerilog features for formal verification
   supported in any clocked block.
 
 - The syntax ``@($global_clock)`` can be used to create FFs that have no
-  explicit clock input ($ff cells).
+  explicit clock input ($ff cells). The same can be achieved by using
+  ``@(posedge <netname>)`` or ``@(negedge <netname>)`` when ``<netname>``
+  is marked with the ``(* gclk *)`` Verilog attribute.
 
 
 Supported features from SystemVerilog

frontends/ast/genrtlil.cc

@@ -223,12 +223,18 @@ struct AST_INTERNAL::ProcessGenerator
 		bool found_global_syncs = false;
 		bool found_anyedge_syncs = false;
 		for (auto child : always->children)
+		{
+			if ((child->type == AST_POSEDGE || child->type == AST_NEGEDGE) && GetSize(child->children) == 1 && child->children.at(0)->type == AST_IDENTIFIER &&
+					child->children.at(0)->id2ast && child->children.at(0)->id2ast->type == AST_WIRE && child->children.at(0)->id2ast->get_bool_attribute("\\gclk")) {
+				found_global_syncs = true;
+			}
 			if (child->type == AST_EDGE) {
 				if (GetSize(child->children) == 1 && child->children.at(0)->type == AST_IDENTIFIER && child->children.at(0)->str == "\\$global_clock")
 					found_global_syncs = true;
 				else
 					found_anyedge_syncs = true;
 			}
+		}
 
 		if (found_anyedge_syncs) {
 			if (found_global_syncs)
@@ -242,6 +248,9 @@ struct AST_INTERNAL::ProcessGenerator
 		bool found_clocked_sync = false;
 		for (auto child : always->children)
 			if (child->type == AST_POSEDGE || child->type == AST_NEGEDGE) {
+				if (GetSize(child->children) == 1 && child->children.at(0)->type == AST_IDENTIFIER && child->children.at(0)->id2ast &&
+						child->children.at(0)->id2ast->type == AST_WIRE && child->children.at(0)->id2ast->get_bool_attribute("\\gclk"))
+					continue;
 				found_clocked_sync = true;
 				if (found_global_syncs || found_anyedge_syncs)
 					log_error("Found non-synthesizable event list at %s:%d!\n", always->filename.c_str(), always->linenum);

frontends/verific/verific.cc

@@ -1470,6 +1470,10 @@ VerificClocking::VerificClocking(VerificImporter *importer, Net *net, bool sva_a
 
 	clock_net = net;
 	clock_sig = importer->net_map_at(clock_net);
+
+	const char *gclk_attr = clock_net->GetAttValue("gclk");
+	if (gclk_attr != nullptr && (!strcmp(gclk_attr, "1") || !strcmp(gclk_attr, "'1'")))
+		gclk = true;
 }
 
 Cell *VerificClocking::addDff(IdString name, SigSpec sig_d, SigSpec sig_q, Const init_value)
@@ -1492,15 +1496,20 @@ Cell *VerificClocking::addDff(IdString name, SigSpec sig_d, SigSpec sig_q, Const
 		sig_d = module->Mux(NEW_ID, sig_q, sig_d, enable_sig);
 
 	if (disable_sig != State::S0) {
+		log_assert(gclk == false);
 		log_assert(GetSize(sig_q) == GetSize(init_value));
 		return module->addAdff(name, clock_sig, disable_sig, sig_d, sig_q, init_value, posedge);
 	}
 
+	if (gclk)
+		return module->addFf(name, sig_d, sig_q);
+
 	return module->addDff(name, clock_sig, sig_d, sig_q, posedge);
 }
 
 Cell *VerificClocking::addAdff(IdString name, RTLIL::SigSpec sig_arst, SigSpec sig_d, SigSpec sig_q, Const arst_value)
 {
+	log_assert(gclk == false);
 	log_assert(disable_sig == State::S0);
 
 	if (enable_sig != State::S1)
@@ -1511,6 +1520,7 @@ Cell *VerificClocking::addAdff(IdString name, RTLIL::SigSpec sig_arst, SigSpec s
 
 Cell *VerificClocking::addDffsr(IdString name, RTLIL::SigSpec sig_set, RTLIL::SigSpec sig_clr, SigSpec sig_d, SigSpec sig_q)
 {
+	log_assert(gclk == false);
 	log_assert(disable_sig == State::S0);
 
 	if (enable_sig != State::S1)

frontends/verific/verific.h

@@ -40,6 +40,7 @@ struct VerificClocking {
 	SigBit enable_sig = State::S1;
 	SigBit disable_sig = State::S0;
 	bool posedge = true;
+	bool gclk = false;
 
 	VerificClocking() { }
 	VerificClocking(VerificImporter *importer, Verific::Net *net, bool sva_at_only = false);