peepopt: add formal only peepopt to rewrite latches to ffs in clock gates

* this is gated behind the -formalclk flag, which also disables the other
  synthesis focused optimizations

passes/pmgen/Makefile.inc

@@ -57,6 +57,7 @@ PEEPOPT_PATTERN  = passes/pmgen/peepopt_shiftmul_right.pmg
 PEEPOPT_PATTERN += passes/pmgen/peepopt_shiftmul_left.pmg
 PEEPOPT_PATTERN += passes/pmgen/peepopt_shiftadd.pmg
 PEEPOPT_PATTERN += passes/pmgen/peepopt_muldiv.pmg
+PEEPOPT_PATTERN += passes/pmgen/peepopt_formal_clockgateff.pmg
 
 passes/pmgen/peepopt_pm.h: passes/pmgen/pmgen.py $(PEEPOPT_PATTERN)
 	$(P) mkdir -p passes/pmgen && $(PYTHON_EXECUTABLE) $< -o $@ -p peepopt $(filter-out $<,$^)

passes/pmgen/peepopt.cc

@@ -40,7 +40,7 @@ struct PeepoptPass : public Pass {
 		log("\n");
 		log("This pass applies a collection of peephole optimizers to the current design.\n");
 		log("\n");
-		log("This pass employs the following rules:\n");
+		log("This pass employs the following rules by default:\n");
 		log("\n");
 		log("   * muldiv - Replace (A*B)/B with A\n");
 		log("\n");
@@ -57,14 +57,26 @@ struct PeepoptPass : public Pass {
 		log("                limits the amount of padding to a multiple of the data, \n");
 		log("                to avoid high resource usage from large temporary MUX trees.\n");
 		log("\n");
+		log("If -formalclk is specified it instead employs the following rules:\n");
+		log("\n");
+		log("   * clockgateff - Replace latch based clock gating patterns with a flip-flop\n");
+		log("                   based pattern to prevent combinational paths from the\n");
+		log("                   output to the enable input after running clk2fflogic.\n");
+		log("\n");
 	}
 	void execute(std::vector<std::string> args, RTLIL::Design *design) override
 	{
 		log_header(design, "Executing PEEPOPT pass (run peephole optimizers).\n");
 
+		bool formalclk = false;
+
 		size_t argidx;
 		for (argidx = 1; argidx < args.size(); argidx++)
 		{
+			if (args[argidx] == "-formalclk") {
+				formalclk = true;
+				continue;
+			}
 			break;
 		}
 		extra_args(args, argidx, design);
@@ -86,6 +98,9 @@ struct PeepoptPass : public Pass {
 
 				pm.setup(module->selected_cells());
 
+				if (formalclk) {
+					pm.run_formal_clockgateff();
+				} else {
 					pm.run_shiftadd();
 					pm.run_shiftmul_right();
 					pm.run_shiftmul_left();
@@ -93,6 +108,7 @@ struct PeepoptPass : public Pass {
 				}
 			}
 		}
+	}
 } PeepoptPass;
 
 PRIVATE_NAMESPACE_END

passes/pmgen/peepopt_formal_clockgateff.pmg

@@ -0,0 +1,59 @@
+pattern formal_clockgateff
+
+// Detects the most common clock gating pattern using a latch and replaces it
+// with a functionally equivalent pattern based on a flip-flop. The latch
+// based pattern has a combinational path from the enable input to output after
+// clk2fflogic, but this is a stable loop and the flip-flop based pattern does
+// not exhibit this.
+//
+// This optimization is suitable for formal to prevent false comb loops, but
+// should not be used for synthesis where the latch is an intentional choice
+//
+// Latch style:
+// always @* if (!clk_i) latched_en = en;
+// assign gated_clk_o = latched_en & clk_i;
+//
+// Flip-flop style:
+// always @(posedge clk) flopped_en <= en;
+// assign gated_clk_o = flopped_en & clk_i;
+
+state <SigSpec> clk en latched_en gated_clk
+state <IdString> latched_en_port_name
+
+match latch
+	select latch->type == $dlatch
+	select param(latch, \WIDTH) == 1
+	select param(latch, \EN_POLARITY).as_bool() == false
+	set clk port(latch, \EN)
+	set en port(latch, \D)
+	set latched_en port(latch, \Q)
+endmatch
+
+match and_gate
+	select and_gate->type.in($and, $logic_and)
+	select param(and_gate, \A_WIDTH) == 1
+	select param(and_gate, \B_WIDTH) == 1
+	select param(and_gate, \Y_WIDTH) == 1
+	choice <IdString> clk_port {\A, \B}
+	define <IdString> latch_port {clk_port == \A ? \B : \A}
+	index <SigSpec> port(and_gate, clk_port) === clk
+	index <SigSpec> port(and_gate, latch_port) === latched_en
+	set gated_clk port(and_gate, \Y)
+	set latched_en_port_name latch_port
+endmatch
+
+code
+	log("replacing clock gate pattern in %s with ff: latch=%s, and=%s\n",
+		log_id(module), log_id(latch), log_id(and_gate));
+
+	// Add a flip-flop and rewire the AND gate to use the output of this flop
+	// instead of the latch. We don't delete the latch in case its output is
+	// used to drive other nodes. If it isn't, it will be trivially removed by
+	// clean
+	SigSpec flopped_en = module->addWire(NEW_ID);
+	module->addDff(NEW_ID, clk, en, flopped_en, true, latch->get_src_attribute());
+	and_gate->setPort(latched_en_port_name, flopped_en);
+	did_something = true;
+
+	accept;
+endcode