Add testcase

tests/arch/ice40/ice40_opt.ys

@@ -24,3 +24,63 @@ equiv_opt -assert -map +/ice40/cells_map.v -map +/ice40/cells_sim.v ice40_opt
 design -load postopt
 select -assert-count 1 t:*
 select -assert-count 1 t:$lut
+
+# https://github.com/YosysHQ/yosys/issues/1543
+design -reset
+read_verilog <<EOT
+module delay_element (input wire clk, input wire reset, input wire enable,
+                      input wire chainin,  output wire chainout, output reg latch);
+
+
+   reg                           const_zero = 0;
+   reg                           const_one = 1;
+
+    wire                         delay_tap;
+
+
+   //carry logic
+   (* keep *) SB_CARRY carry ( .CO(chainout), .I0(const_zero),
+                    .I1(const_one), .CI(chainin));
+
+
+   //flip flop latch
+   (* keep *) SB_DFFER flipflop( .Q(latch), .C(clk), .E(enable),
+                      .D(delay_tap), .R(reset));
+
+
+   //LUT table
+   // the LUT should just echo the carry in (I3)
+   // carry I0 = LUT I1
+   // carry I1 = LUT I2
+   // carry in = LUT I3
+   // LUT_INIT[0] = 0
+   // LUT_INIT[1] = 0
+   // LUT_INIT[2] = 0
+   // LUT_INIT[3] = 0
+   // LUT_INIT[4] = 0
+   // LUT_INIT[5] = 0
+   // LUT_INIT[6] = 0
+   // LUT_INIT[7] = 0
+   // LUT_INIT[8] = 1
+   // LUT_INIT[9] = 1
+   // LUT_INIT[10] = 1
+   // LUT_INIT[11] = 1
+   // LUT_INIT[12] = 1
+   // LUT_INIT[13] = 1
+   // LUT_INIT[14] = 1
+   // LUT_INIT[15] = 1
+
+   (* keep *) SB_LUT4 lut( .O(delay_tap), .I0(const_zero), .I1(const_zero),
+                .I2(const_one), .I3(chainin));
+
+   //TODO: is this the right way round??
+   defparam lut.LUT_INIT=16'hFF00;
+
+
+endmodule // delay_element
+EOT
+
+synth_ice40
+select -assert-count 1 t:SB_LUT4
+select -assert-count 1 t:SB_CARRY
+select -assert-count 1 t:SB_CARRY a:keep %i