clkbufmap: Add support for inverters in clock path.

4 files changed, 69 insertions, 6 deletions

diff --git a/README.md b/README.md
index e46971526..1ce5d47ea 100644
--- a/README.md
+++ b/README.md
@@ -343,6 +343,13 @@ Verilog Attributes and non-standard features
 - The ``clkbuf_sink`` attribute can be set on an input port of a module to
   request clock buffer insertion by the ``clkbufmap`` pass.
 
+- The ``clkbuf_inv`` attribute can be set on an output port of a module
+  with the value set to the name of an input port of that module.  When
+  the ``clkbufmap`` would otherwise insert a clock buffer on this output,
+  it will instead try inserting the clock buffer on the input port (this
+  is used to implement clock inverter cells that clock buffer insertion
+  will "see through").
+
 - The ``clkbuf_inhibit`` is the default attribute to set on a wire to prevent
   automatic clock buffer insertion by ``clkbufmap``. This behaviour can be
   overridden by providing a custom selection to ``clkbufmap``.
diff --git a/passes/techmap/clkbufmap.cc b/passes/techmap/clkbufmap.cc
index 246932d81..b9cd68883 100644
--- a/passes/techmap/clkbufmap.cc
+++ b/passes/techmap/clkbufmap.cc
@@ -115,6 +115,8 @@ struct ClkbufmapPass : public Pass {
 		// Cell type, port name, bit index.
 		pool<pair<IdString, pair<IdString, int>>> sink_ports;
 		pool<pair<IdString, pair<IdString, int>>> buf_ports;
+		dict<pair<IdString, pair<IdString, int>>, pair<IdString, int>> inv_ports_out;
+		dict<pair<IdString, pair<IdString, int>>, pair<IdString, int>> inv_ports_in;
 
 		// Process submodules before module using them.
 		std::vector<Module *> modules_sorted;
@@ -133,6 +135,14 @@ struct ClkbufmapPass : public Pass {
 					if (wire->get_bool_attribute("\\clkbuf_sink"))
 						for (int i = 0; i < GetSize(wire); i++)
 							sink_ports.insert(make_pair(module->name, make_pair(wire->name, i)));
+					auto it = wire->attributes.find("\\clkbuf_inv");
+					if (it != wire->attributes.end()) {
+						IdString in_name = RTLIL::escape_id(it->second.decode_string());
+						for (int i = 0; i < GetSize(wire); i++) {
+							inv_ports_out[make_pair(module->name, make_pair(wire->name, i))] = make_pair(in_name, i);
+							inv_ports_in[make_pair(module->name, make_pair(in_name, i))] = make_pair(wire->name, i);
+						}
+					}
 				}
 				continue;
 			}
@@ -157,6 +167,37 @@ struct ClkbufmapPass : public Pass {
 				if (buf_ports.count(make_pair(cell->type, make_pair(port.first, i))))
 					buf_wire_bits.insert(sigmap(port.second[i]));
 
+			// Third, propagate tags through inverters.
+			bool retry = true;
+			while (retry) {
+				retry = false;
+				for (auto cell : module->cells())
+				for (auto port : cell->connections())
+				for (int i = 0; i < port.second.size(); i++) {
+					auto it = inv_ports_out.find(make_pair(cell->type, make_pair(port.first, i)));
+					auto bit = sigmap(port.second[i]);
+					// If output of an inverter is connected to a sink, mark it as buffered,
+					// and request a buffer on the inverter's input instead.
+					if (it != inv_ports_out.end() && !buf_wire_bits.count(bit) && sink_wire_bits.count(bit)) {
+						buf_wire_bits.insert(bit);
+						auto other_bit = sigmap(cell->getPort(it->second.first)[it->second.second]);
+						sink_wire_bits.insert(other_bit);
+						retry = true;
+					}
+					// If input of an inverter is marked as already-buffered,
+					// mark its output already-buffered as well.
+					auto it2 = inv_ports_in.find(make_pair(cell->type, make_pair(port.first, i)));
+					if (it2 != inv_ports_in.end() && buf_wire_bits.count(bit)) {
+						auto other_bit = sigmap(cell->getPort(it2->second.first)[it2->second.second]);
+						if (!buf_wire_bits.count(other_bit)) {
+							buf_wire_bits.insert(other_bit);
+							retry = true;
+						}
+					}
+
+				}
+			};
+
 			// Collect all driven bits.
 			for (auto cell : module->cells())
 			for (auto port : cell->connections())
diff --git a/techlibs/xilinx/cells_sim.v b/techlibs/xilinx/cells_sim.v
index 5faddcd52..fa33f4596 100644
--- a/techlibs/xilinx/cells_sim.v
+++ b/techlibs/xilinx/cells_sim.v
@@ -126,7 +126,11 @@ endmodule
 //   assign O = IO, IO = T ? 1'bz : I;
 // endmodule
 
-module INV(output O, input I);
+module INV(
+    (* clkbuf_inv = "I" *)
+    output O,
+    input I
+);
   assign O = !I;
 endmodule
 
diff --git a/tests/techmap/clkbufmap.ys b/tests/techmap/clkbufmap.ys
index f1277864e..b81a35e74 100644
--- a/tests/techmap/clkbufmap.ys
+++ b/tests/techmap/clkbufmap.ys
@@ -4,6 +4,7 @@ module dff ((* clkbuf_sink *) input clk, input d, output q); endmodule
 module dffe ((* clkbuf_sink *) input c, input d, e, output q); endmodule
 module latch (input e, d, output q); endmodule
 module clkgen (output o); endmodule
+module inv ((* clkbuf_inv = "i" *) output o, input i); endmodule
 
 module top(input clk1, clk2, clk3, d, e, output [4:0] q);
 wire clk4, clk5, clk6;
@@ -17,12 +18,18 @@ dff s6 (.clk(clk6), .d(d), .q(q[4]));
 endmodule
 
 module sub(output sclk4, output sclk5, output sclk6, input sd, output sq);
+wire sclk7, sclk8, sclk9;
+wire siq;
 wire tmp;
 clkgen s7(.o(sclk4));
 clkgen s8(.o(sclk5));
 clkgen s9(.o(tmp));
-clkbuf s10(.i(tmp), .o(sclk6));
-dff s11(.clk(sclk4), .d(sd), .q(sq));
+clkbuf s10(.i(tmp), .o(sclk7));
+dff s11(.clk(sclk4), .d(sd), .q(siq));
+inv s15(.i(sclk7), .o(sclk6));
+clkgen s12(.o(sclk8));
+inv s13(.o(sclk9), .i(sclk8));
+dff s14(.clk(sclk9), .d(siq), .q(sq));
 endmodule
 EOT
 
@@ -34,7 +41,7 @@ design -save ref
 design -load ref
 clkbufmap -buf clkbuf o:i
 select -assert-count 3 top/t:clkbuf
-select -assert-count 2 sub/t:clkbuf
+select -assert-count 3 sub/t:clkbuf
 select -set clk1 w:clk1 %a %co t:clkbuf %i          # Find 'clk1' fanouts that are 'clkbuf'
 select -assert-count 1 @clk1                        # Check there is one such fanout
 select -assert-count 1 @clk1 %x:+[o] %co c:s* %i    # Check that the 'o' of that clkbuf drives one fanout
@@ -51,6 +58,10 @@ select -set sclk4 w:sclk4 %a %ci t:clkbuf %i
 select -assert-count 1 @sclk4
 select -assert-count 1 @sclk4 %x:+[o] %co c:s11 %i
 select -assert-count 1 @sclk4 %x:+[i] %ci c:s7 %i
+select -set sclk8 w:sclk8 %a %ci t:clkbuf %i
+select -assert-count 1 @sclk8
+select -assert-count 1 @sclk8 %x:+[o] %co c:s13 %i
+select -assert-count 1 @sclk8 %x:+[i] %ci c:s12 %i
 
 # ----------------------
 
@@ -72,7 +83,7 @@ setattr -set clkbuf_inhibit 1 w:clk1
 setattr -set buffer_type "bufg" w:clk2
 clkbufmap -buf clkbuf o:i w:* a:buffer_type=none a:buffer_type=bufr %u %d
 select -assert-count 3 top/t:clkbuf
-select -assert-count 2 sub/t:clkbuf
+select -assert-count 3 sub/t:clkbuf
 select -set clk1 w:clk1 %a %co t:clkbuf %i          # Find 'clk1' fanouts that are 'clkbuf'
 select -assert-count 1 @clk1                        # Check there is one such fanout
 select -assert-count 1 @clk1 %x:+[o] %co c:s* %i    # Check that the 'o' of that clkbuf drives one fanout
@@ -93,4 +104,4 @@ clkbufmap -buf clkbuf o:i w:* a:buffer_type=none a:buffer_type=bufr %u %d
 select -assert-count 0 w:clk1 %a %co t:clkbuf %i
 select -assert-count 0 w:clk2 %a %co t:clkbuf %i
 select -assert-count 0 top/t:clkbuf
-select -assert-count 1 sub/t:clkbuf
+select -assert-count 2 sub/t:clkbuf