proc_arst: Add special-casing of clock signal in conditionals.

The already-existing special case for conditionals on clock has been
remade as follows:

- now triggered for the last remaining edge trigger after all others
  have been converted to async reset, not just when there is only one
  sync rule in the first place
- does not require all contained assignments to be constant, as opposed
  to a reset conditional — merely const-folds the condition

In addition, the code has been refactored a bit; as a bonus, the
priority order of async resets found is now preserved in resulting sync
rule ordering (though this is not yet respected by proc_dff).

Fixes #2656.

passes/proc/proc_arst.cc

@@ -179,22 +179,35 @@ RTLIL::SigSpec apply_reset(RTLIL::Module *mod, RTLIL::Process *proc, RTLIL::Sync
 
 void proc_arst(RTLIL::Module *mod, RTLIL::Process *proc, SigMap &assign_map)
 {
-restart_proc_arst:
+	std::vector<RTLIL::SyncRule *> arst_syncs;
-	if (proc->root_case.switches.size() != 1)
+	std::vector<RTLIL::SyncRule *> edge_syncs;
-		return;
+	std::vector<RTLIL::SyncRule *> other_syncs;
-
-	RTLIL::SigSpec root_sig = proc->root_case.switches[0]->signal;
 
 	for (auto &sync : proc->syncs) {
-		if (sync->type == RTLIL::SyncType::STp || sync->type == RTLIL::SyncType::STn) {
+		if (sync->type == RTLIL::SyncType::ST0 || sync->type == RTLIL::SyncType::ST1) {
+			arst_syncs.push_back(sync);
+		} else if (sync->type == RTLIL::SyncType::STp || sync->type == RTLIL::SyncType::STn) {
+			edge_syncs.push_back(sync);
+		} else {
+			other_syncs.push_back(sync);
+		}
+	}
+
+	bool did_something = false;
+
+	while (proc->root_case.switches.size() == 1) {
+		RTLIL::SigSpec root_sig = proc->root_case.switches[0]->signal;
+
+		bool found = false;
+		for (auto it = edge_syncs.begin(); it != edge_syncs.end(); ++it) {
+			auto sync = *it;
 			bool polarity = sync->type == RTLIL::SyncType::STp;
 			if (check_signal(mod, root_sig, sync->signal, polarity)) {
-				if (proc->syncs.size() == 1) {
+				if (edge_syncs.size() > 1) {
-					log("Found VHDL-style edge-trigger %s in `%s.%s'.\n", log_signal(sync->signal), mod->name.c_str(), proc->name.c_str());
-				} else {
 					log("Found async reset %s in `%s.%s'.\n", log_signal(sync->signal), mod->name.c_str(), proc->name.c_str());
 					sync->type = sync->type == RTLIL::SyncType::STp ? RTLIL::SyncType::ST1 : RTLIL::SyncType::ST0;
-				}
+					arst_syncs.push_back(sync);
+					edge_syncs.erase(it);
 					for (auto &action : sync->actions) {
 						action.second = apply_reset(mod, proc, sync, assign_map, root_sig, polarity, action.second, action.first);
 					}
@@ -209,9 +222,24 @@ restart_proc_arst:
 					}
 					sync->mem_write_actions.clear();
 					eliminate_const(mod, &proc->root_case, root_sig, polarity);
-				goto restart_proc_arst;
+				} else {
+					log("Found VHDL-style edge-trigger %s in `%s.%s'.\n", log_signal(sync->signal), mod->name.c_str(), proc->name.c_str());
+					eliminate_const(mod, &proc->root_case, root_sig, !polarity);
+				}
+				did_something = true;
+				found = true;
+				break;
 			}
 		}
+		if (!found)
+			break;
+	}
+
+	if (did_something) {
+		proc->syncs.clear();
+		proc->syncs.insert(proc->syncs.end(), arst_syncs.begin(), arst_syncs.end());
+		proc->syncs.insert(proc->syncs.end(), edge_syncs.begin(), edge_syncs.end());
+		proc->syncs.insert(proc->syncs.end(), other_syncs.begin(), other_syncs.end());
 	}
 }
 

tests/proc/bug2656.ys

@@ -0,0 +1,31 @@
+read_verilog <<EOT
+module top (...);
+
+input clk, rst, d1, d2;
+output q1, q2;
+
+always @(posedge clk)
+	if (clk)
+		q1 <= d1;
+
+always @(posedge clk, posedge rst)
+	if (rst)
+		q2 <= 0;
+	else if (clk)
+		q2 <= d2;
+
+endmodule
+EOT
+
+proc
+opt
+
+select -assert-count 1 t:$dff
+select -assert-count 1 w:clk %a %co t:$dff %i
+select -assert-count 1 w:d1 %a %co t:$dff %i
+select -assert-count 1 w:q1 %a %ci t:$dff %i
+select -assert-count 1 t:$adff
+select -assert-count 1 w:clk %a %co t:$adff %i
+select -assert-count 1 w:rst %a %co t:$adff %i
+select -assert-count 1 w:d2 %a %co t:$adff %i
+select -assert-count 1 w:q2 %a %ci t:$adff %i