kernel/ff: Refactor FfData to enable FFs with async load.

- *_en is split into *_ce (clock enable) and *_aload (async load aka
  latch gate enable), so both can be present at once
- has_d is removed
- has_gclk is added (to have a clear marker for $ff)
- d_is_const and val_d leftovers are removed
- async2sync, clk2fflogic, opt_dff are updated to operate correctly on
  FFs with async load

1 files changed, 139 insertions, 90 deletions

diff --git a/passes/opt/opt_dff.cc b/passes/opt/opt_dff.cc
index ddf08392b..0e25484b8 100644
--- a/passes/opt/opt_dff.cc
+++ b/passes/opt/opt_dff.cc
@@ -382,6 +382,69 @@ struct OptDffWorker
 				}
 			}
 
+			if (ff.has_aload) {
+				if (ff.sig_aload == (ff.pol_aload ? State::S0 : State::S1) || (!opt.keepdc && ff.sig_aload == State::Sx)) {
+					// Always-inactive enable — remove.
+					log("Removing never-active async load on %s (%s) from module %s.\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					ff.has_aload = false;
+					changed = true;
+				} else if (ff.sig_aload == (ff.pol_aload ? State::S1 : State::S0)) {
+					// Always-active enable.  Make a comb circuit, nuke the FF/latch.
+					log("Handling always-active async load on %s (%s) from module %s (changing to combinatorial circuit).\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					initvals.remove_init(ff.sig_q);
+					module->remove(cell);
+					if (ff.has_sr) {
+						SigSpec tmp;
+						if (ff.is_fine) {
+							if (ff.pol_set)
+								tmp = module->MuxGate(NEW_ID, ff.sig_ad, State::S1, ff.sig_set);
+							else
+								tmp = module->MuxGate(NEW_ID, State::S1, ff.sig_ad, ff.sig_set);
+							if (ff.pol_clr)
+								module->addMuxGate(NEW_ID, tmp, State::S0, ff.sig_clr, ff.sig_q);
+							else
+								module->addMuxGate(NEW_ID, State::S0, tmp, ff.sig_clr, ff.sig_q);
+						} else {
+							if (ff.pol_set)
+								tmp = module->Or(NEW_ID, ff.sig_ad, ff.sig_set);
+							else
+								tmp = module->Or(NEW_ID, ff.sig_ad, module->Not(NEW_ID, ff.sig_set));
+							if (ff.pol_clr)
+								module->addAnd(NEW_ID, tmp, module->Not(NEW_ID, ff.sig_clr), ff.sig_q);
+							else
+								module->addAnd(NEW_ID, tmp, ff.sig_clr, ff.sig_q);
+						}
+					} else if (ff.has_arst) {
+						if (ff.is_fine) {
+							if (ff.pol_arst)
+								module->addMuxGate(NEW_ID, ff.sig_ad, ff.val_arst[0], ff.sig_arst, ff.sig_q);
+							else
+								module->addMuxGate(NEW_ID, ff.val_arst[0], ff.sig_ad, ff.sig_arst, ff.sig_q);
+						} else {
+							if (ff.pol_arst)
+								module->addMux(NEW_ID, ff.sig_ad, ff.val_arst, ff.sig_arst, ff.sig_q);
+							else
+								module->addMux(NEW_ID, ff.val_arst, ff.sig_ad, ff.sig_arst, ff.sig_q);
+						}
+					} else {
+						module->connect(ff.sig_q, ff.sig_ad);
+					}
+					did_something = true;
+					continue;
+				} else if (ff.sig_ad.is_fully_const() && !ff.has_arst && !ff.has_sr) {
+					log("Changing const-value async load to async reset on %s (%s) from module %s.\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					ff.has_arst = true;
+					ff.has_aload = false;
+					ff.sig_arst = ff.sig_aload;
+					ff.pol_arst = ff.pol_aload;
+					ff.val_arst = ff.sig_ad.as_const();
+					changed = true;
+				}
+			}
+
 			if (ff.has_arst) {
 				if (ff.sig_arst == (ff.pol_arst ? State::S0 : State::S1)) {
 					// Always-inactive reset — remove.
@@ -414,111 +477,63 @@ struct OptDffWorker
 							log_id(cell), log_id(cell->type), log_id(module));
 					ff.has_srst = false;
 					if (!ff.ce_over_srst)
-						ff.has_en = false;
-					ff.sig_d = ff.val_d = ff.val_srst;
-					ff.d_is_const = true;
+						ff.has_ce = false;
+					ff.sig_d = ff.val_srst;
 					changed = true;
 				}
 			}
 
-			if (ff.has_en) {
-				if (ff.sig_en == (ff.pol_en ? State::S0 : State::S1) || (!opt.keepdc && ff.sig_en == State::Sx)) {
+			if (ff.has_ce) {
+				if (ff.sig_ce == (ff.pol_ce ? State::S0 : State::S1) || (!opt.keepdc && ff.sig_ce == State::Sx)) {
 					// Always-inactive enable — remove.
-					if (ff.has_clk && ff.has_srst && !ff.ce_over_srst) {
+					if (ff.has_srst && !ff.ce_over_srst) {
 						log("Handling never-active EN on %s (%s) from module %s (connecting SRST instead).\n",
 								log_id(cell), log_id(cell->type), log_id(module));
 						// FF with sync reset — connect the sync reset to D instead.
-						ff.pol_en = ff.pol_srst;
-						ff.sig_en = ff.sig_srst;
+						ff.pol_ce = ff.pol_srst;
+						ff.sig_ce = ff.sig_srst;
 						ff.has_srst = false;
-						ff.sig_d = ff.val_d = ff.val_srst;
-						ff.d_is_const = true;
+						ff.sig_d = ff.val_srst;
 						changed = true;
 					} else {
 						log("Handling never-active EN on %s (%s) from module %s (removing D path).\n",
 								log_id(cell), log_id(cell->type), log_id(module));
-						// The D input path is effectively useless, so remove it (this will be a const-input D latch, SR latch, or a const driver).
-						ff.has_d = ff.has_en = ff.has_clk = false;
+						// The D input path is effectively useless, so remove it (this will be a D latch, SR latch, or a const driver).
+						ff.has_ce = ff.has_clk = ff.has_srst = false;
 						changed = true;
 					}
-				} else if (ff.sig_en == (ff.pol_en ? State::S1 : State::S0)) {
-					// Always-active enable.
-					if (ff.has_clk) {
-						// For FF, just remove the useless enable.
-						log("Removing always-active EN on %s (%s) from module %s.\n",
-								log_id(cell), log_id(cell->type), log_id(module));
-						ff.has_en = false;
-						changed = true;
-					} else {
-						// For latches, make a comb circuit, nuke the latch.
-						log("Handling always-active EN on %s (%s) from module %s (changing to combinatorial circuit).\n",
-								log_id(cell), log_id(cell->type), log_id(module));
-						initvals.remove_init(ff.sig_q);
-						module->remove(cell);
-						if (ff.has_sr) {
-							SigSpec tmp;
-							if (ff.is_fine) {
-								if (ff.pol_set)
-									tmp = module->MuxGate(NEW_ID, ff.sig_d, State::S1, ff.sig_set);
-								else
-									tmp = module->MuxGate(NEW_ID, State::S1, ff.sig_d, ff.sig_set);
-								if (ff.pol_clr)
-									module->addMuxGate(NEW_ID, tmp, State::S0, ff.sig_clr, ff.sig_q);
-								else
-									module->addMuxGate(NEW_ID, State::S0, tmp, ff.sig_clr, ff.sig_q);
-							} else {
-								if (ff.pol_set)
-									tmp = module->Or(NEW_ID, ff.sig_d, ff.sig_set);
-								else
-									tmp = module->Or(NEW_ID, ff.sig_d, module->Not(NEW_ID, ff.sig_set));
-								if (ff.pol_clr)
-									module->addAnd(NEW_ID, tmp, module->Not(NEW_ID, ff.sig_clr), ff.sig_q);
-								else
-									module->addAnd(NEW_ID, tmp, ff.sig_clr, ff.sig_q);
-							}
-						} else if (ff.has_arst) {
-							if (ff.is_fine) {
-								if (ff.pol_arst)
-									module->addMuxGate(NEW_ID, ff.sig_d, ff.val_arst[0], ff.sig_arst, ff.sig_q);
-								else
-									module->addMuxGate(NEW_ID, ff.val_arst[0], ff.sig_d, ff.sig_arst, ff.sig_q);
-							} else {
-								if (ff.pol_arst)
-									module->addMux(NEW_ID, ff.sig_d, ff.val_arst, ff.sig_arst, ff.sig_q);
-								else
-									module->addMux(NEW_ID, ff.val_arst, ff.sig_d, ff.sig_arst, ff.sig_q);
-							}
-						} else {
-							module->connect(ff.sig_q, ff.sig_d);
-						}
-						did_something = true;
-						continue;
-					}
+				} else if (ff.sig_ce == (ff.pol_ce ? State::S1 : State::S0)) {
+					// Always-active enable.  Just remove it.
+					// For FF, just remove the useless enable.
+					log("Removing always-active EN on %s (%s) from module %s.\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					ff.has_ce = false;
+					changed = true;
 				}
 			}
 
 			if (ff.has_clk) {
 				if (ff.sig_clk.is_fully_const()) {
-					// Const clock — the D input path is effectively useless, so remove it (this will be a const-input D latch, SR latch, or a const driver).
+					// Const clock — the D input path is effectively useless, so remove it (this will be a D latch, SR latch, or a const driver).
 					log("Handling const CLK on %s (%s) from module %s (removing D path).\n",
 							log_id(cell), log_id(cell->type), log_id(module));
-					ff.has_d = ff.has_en = ff.has_clk = ff.has_srst = false;
+					ff.has_ce = ff.has_clk = ff.has_srst = false;
 					changed = true;
 				}
 			}
 
-			if (ff.has_d && ff.sig_d == ff.sig_q) {
+			if ((ff.has_clk || ff.has_gclk) && ff.sig_d == ff.sig_q) {
 				// Q wrapped back to D, can be removed.
 				if (ff.has_clk && ff.has_srst) {
 					// FF with sync reset — connect the sync reset to D instead.
 					log("Handling D = Q on %s (%s) from module %s (conecting SRST instead).\n",
 							log_id(cell), log_id(cell->type), log_id(module));
-					if (ff.has_en && ff.ce_over_srst) {
-						if (!ff.pol_en) {
+					if (ff.has_ce && ff.ce_over_srst) {
+						if (!ff.pol_ce) {
 							if (ff.is_fine)
-								ff.sig_en = module->NotGate(NEW_ID, ff.sig_en);
+								ff.sig_ce = module->NotGate(NEW_ID, ff.sig_ce);
 							else
-								ff.sig_en = module->Not(NEW_ID, ff.sig_en);
+								ff.sig_ce = module->Not(NEW_ID, ff.sig_ce);
 						}
 						if (!ff.pol_srst) {
 							if (ff.is_fine)
@@ -527,28 +542,34 @@ struct OptDffWorker
 								ff.sig_srst = module->Not(NEW_ID, ff.sig_srst);
 						}
 						if (ff.is_fine)
-							ff.sig_en = module->AndGate(NEW_ID, ff.sig_en, ff.sig_srst);
+							ff.sig_ce = module->AndGate(NEW_ID, ff.sig_ce, ff.sig_srst);
 						else
-							ff.sig_en = module->And(NEW_ID, ff.sig_en, ff.sig_srst);
-						ff.pol_en = true;
+							ff.sig_ce = module->And(NEW_ID, ff.sig_ce, ff.sig_srst);
+						ff.pol_ce = true;
 					} else {
-						ff.pol_en = ff.pol_srst;
-						ff.sig_en = ff.sig_srst;
+						ff.pol_ce = ff.pol_srst;
+						ff.sig_ce = ff.sig_srst;
 					}
-					ff.has_en = true;
+					ff.has_ce = true;
 					ff.has_srst = false;
-					ff.sig_d = ff.val_d = ff.val_srst;
-					ff.d_is_const = true;
+					ff.sig_d = ff.val_srst;
 					changed = true;
 				} else {
 					// The D input path is effectively useless, so remove it (this will be a const-input D latch, SR latch, or a const driver).
 					log("Handling D = Q on %s (%s) from module %s (removing D path).\n",
 							log_id(cell), log_id(cell->type), log_id(module));
-					ff.has_d = ff.has_en = ff.has_clk = false;
+					ff.has_clk = ff.has_ce = ff.has_clk = false;
 					changed = true;
 				}
 			}
 
+			if (ff.has_aload && !ff.has_clk && ff.sig_ad == ff.sig_q) {
+				log("Handling AD = Q on %s (%s) from module %s (removing async load path).\n",
+						log_id(cell), log_id(cell->type), log_id(module));
+				ff.has_aload = false;
+				changed = true;
+			}
+
 			// Now check if any bit can be replaced by a constant.
 			pool<int> removed_sigbits;
 			for (int i = 0; i < ff.width; i++) {
@@ -565,7 +586,7 @@ struct OptDffWorker
 				}
 				if (val == State::Sm)
 					continue;
-				if (ff.has_d) {
+				if (ff.has_clk || ff.has_gclk) {
 					if (!ff.sig_d[i].wire) {
 						val = combine_const(val, ff.sig_d[i].data);
 						if (val == State::Sm)
@@ -593,6 +614,34 @@ struct OptDffWorker
 							continue;
 					}
 				}
+				if (ff.has_aload) {
+					if (!ff.sig_ad[i].wire) {
+						val = combine_const(val, ff.sig_ad[i].data);
+						if (val == State::Sm)
+							continue;
+					} else {
+						if (!opt.sat)
+							continue;
+						// For each register bit, try to prove that it cannot change from the initial value. If so, remove it
+						if (!modwalker.has_drivers(ff.sig_ad.extract(i)))
+							continue;
+						if (val != State::S0 && val != State::S1)
+							continue;
+
+						int init_sat_pi = qcsat.importSigBit(val);
+						int q_sat_pi = qcsat.importSigBit(ff.sig_q[i]);
+						int d_sat_pi = qcsat.importSigBit(ff.sig_ad[i]);
+
+						qcsat.prepare();
+
+						// Try to find out whether the register bit can change under some circumstances
+						bool counter_example_found = qcsat.ez->solve(qcsat.ez->IFF(q_sat_pi, init_sat_pi), qcsat.ez->NOT(qcsat.ez->IFF(d_sat_pi, init_sat_pi)));
+
+						// If the register bit cannot change, we can replace it with a constant
+						if (counter_example_found)
+							continue;
+					}
+				}
 				log("Setting constant %d-bit at position %d on %s (%s) from module %s.\n", val ? 1 : 0,
 						i, log_id(cell), log_id(cell->type), log_id(module));
 
@@ -616,7 +665,7 @@ struct OptDffWorker
 
 			// The cell has been simplified as much as possible already.  Now try to spice it up with enables / sync resets.
 			if (ff.has_clk) {
-				if (!ff.has_arst && !ff.has_sr && (!ff.has_srst || !ff.has_en || ff.ce_over_srst) && !opt.nosdff) {
+				if (!ff.has_arst && !ff.has_sr && (!ff.has_srst || !ff.has_ce || ff.ce_over_srst) && !opt.nosdff) {
 					// Try to merge sync resets.
 					std::map<ctrls_t, std::vector<int>> groups;
 					std::vector<int> remaining_indices;
@@ -677,7 +726,7 @@ struct OptDffWorker
 						new_ff.has_srst = true;
 						new_ff.sig_srst = srst.first;
 						new_ff.pol_srst = srst.second;
-						if (new_ff.has_en)
+						if (new_ff.has_ce)
 							new_ff.ce_over_srst = true;
 						Cell *new_cell = new_ff.emit(module, NEW_ID);
 						if (new_cell)
@@ -695,7 +744,7 @@ struct OptDffWorker
 						changed = true;
 					}
 				}
-				if ((!ff.has_srst || !ff.has_en || !ff.ce_over_srst) && !opt.nodffe) {
+				if ((!ff.has_srst || !ff.has_ce || !ff.ce_over_srst) && !opt.nodffe) {
 					// Try to merge enables.
 					std::map<std::pair<patterns_t, ctrls_t>, std::vector<int>> groups;
 					std::vector<int> remaining_indices;
@@ -725,8 +774,8 @@ struct OptDffWorker
 						if (!opt.simple_dffe)
 							patterns = find_muxtree_feedback_patterns(ff.sig_d[i], ff.sig_q[i], pattern_t());
 						if (!patterns.empty() || !enables.empty()) {
-							if (ff.has_en)
-								enables.insert(ctrl_t(ff.sig_en, ff.pol_en));
+							if (ff.has_ce)
+								enables.insert(ctrl_t(ff.sig_ce, ff.pol_ce));
 							simplify_patterns(patterns);
 							groups[std::make_pair(patterns, enables)].push_back(i);
 						} else
@@ -737,9 +786,9 @@ struct OptDffWorker
 						FfData new_ff = ff.slice(it.second);
 						ctrl_t en = make_patterns_logic(it.first.first, it.first.second, ff.is_fine);
 
-						new_ff.has_en = true;
-						new_ff.sig_en = en.first;
-						new_ff.pol_en = en.second;
+						new_ff.has_ce = true;
+						new_ff.sig_ce = en.first;
+						new_ff.pol_ce = en.second;
 						new_ff.ce_over_srst = false;
 						Cell *new_cell = new_ff.emit(module, NEW_ID);
 						if (new_cell)