Add opt_dff pass.

10 files changed, 914 insertions, 3 deletions

diff --git a/tests/opt/opt_dff_arst.ys b/tests/opt/opt_dff_arst.ys
new file mode 100644
index 000000000..2aa3b7a26
--- /dev/null
+++ b/tests/opt/opt_dff_arst.ys
@@ -0,0 +1,101 @@
+### Always-active ARST removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+output [11:0] Q;
+input ARST;
+input EN;
+
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff0 (.CLK(CLK), .ARST(1'b1), .D(D), .Q(Q[1:0]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .ARST_POLARITY(1'b0), .ARST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .ARST(1'b0), .EN(EN), .D(D), .Q(Q[3:2]));
+$adlatch #(.EN_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.EN(EN), .ARST(1'b1), .D(D), .Q(Q[5:4]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .ARST(1'bx), .D(D), .Q(Q[7:6]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .ARST_POLARITY(1'b0), .ARST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(CLK), .ARST(1'bx), .EN(EN), .D(D), .Q(Q[9:8]));
+$adlatch #(.EN_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff5 (.EN(EN), .ARST(1'bx), .D(D), .Q(Q[11:10]));
+
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:*
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$adlatch
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:*
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 2 t:$_DFF_???_
+select -assert-count 2 t:$_DFFE_????_
+select -assert-count 2 t:$_DLATCH_???_
+
+design -reset
+
+
+### Never-active ARST removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+output [5:0] Q;
+input ARST;
+input EN;
+
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff0 (.CLK(CLK), .ARST(1'b0), .D(D), .Q(Q[1:0]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .ARST_POLARITY(1'b0), .ARST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .ARST(1'b1), .EN(EN), .D(D), .Q(Q[3:2]));
+$adlatch #(.EN_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.EN(EN), .ARST(1'b0), .D(D), .Q(Q[5:4]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$adff
+select -assert-none t:$adffe
+select -assert-none t:$adlatch
+select -assert-count 1 t:$dff
+select -assert-count 1 t:$dffe
+select -assert-count 1 t:$dlatch
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$_DFF_???_
+select -assert-none t:$_DFFE_????_
+select -assert-none t:$_DLATCH_???_
+select -assert-count 2 t:$_DFF_P_
+select -assert-count 2 t:$_DFFE_PP_
+select -assert-count 2 t:$_DLATCH_P_
+
+design -reset
diff --git a/tests/opt/opt_dff_clk.ys b/tests/opt/opt_dff_clk.ys
new file mode 100644
index 000000000..f3aefa406
--- /dev/null
+++ b/tests/opt/opt_dff_clk.ys
@@ -0,0 +1,45 @@
+### Never-toggling CLK removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input EN;
+input [1:0] D;
+(* init = 18'h15555 *)
+output [17:0] Q;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dff #(.CLK_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(1'b0), .D(D), .Q(Q[1:0]));
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff1 (.CLK(1'b1), .EN(EN), .D(D), .Q(Q[3:2]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(1'bx), .ARST(ARST), .D(D), .Q(Q[5:4]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(1'b0), .EN(EN), .ARST(ARST), .D(D), .Q(Q[7:6]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(1'b1), .SRST(SRST), .D(D), .Q(Q[9:8]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff5 (.CLK(1'bx), .EN(EN), .SRST(SRST), .D(D), .Q(Q[11:10]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff6 (.CLK(1'bx), .EN(EN), .SRST(SRST), .D(D), .Q(Q[13:12]));
+$dffsr #(.CLK_POLARITY(1'b1), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.CLK(1'b1), .SET(SET), .CLR(CLR), .D(D), .Q(Q[15:14]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff8 (.CLK(1'bx), .EN(EN), .SET(SET), .CLR(CLR), .D(D), .Q(Q[17:16]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 2 t:$dlatch
+select -assert-count 2 t:$sr
+select -assert-none t:$dlatch t:$sr %% %n t:* %i
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 4 t:$_DLATCH_?_
+select -assert-count 4 t:$_SR_??_
+select -assert-none t:$_DLATCH_?_ t:$_SR_??_ %% %n t:* %i
diff --git a/tests/opt/opt_dff_const.ys b/tests/opt/opt_dff_const.ys
new file mode 100644
index 000000000..6a7dec7fa
--- /dev/null
+++ b/tests/opt/opt_dff_const.ys
@@ -0,0 +1,49 @@
+### Replace FFs with a const.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input EN;
+(* init=84'haaaaaaaaaaaaaaaaaaaaa *)
+output [83:0] Q;
+input SRST;
+input ARST;
+input [3:0] CLR;
+input [3:0] SET;
+
+$dff #(.CLK_POLARITY(1'b1), .WIDTH(4)) ff0 (.CLK(CLK), .D(4'hc), .Q(Q[3:0]));
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(4)) ff1 (.CLK(CLK), .EN(EN), .D(4'hc), .Q(Q[7:4]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(8'hf0), .WIDTH(8)) ff2 (.CLK(CLK), .ARST(ARST), .D(8'hcc), .Q(Q[15:8]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(8'hf0), .WIDTH(8)) ff3 (.CLK(CLK), .EN(EN), .ARST(ARST), .D(8'hcc), .Q(Q[23:16]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(8'hf0), .WIDTH(8)) ff4 (.CLK(CLK), .SRST(SRST), .D(8'hcc), .Q(Q[31:24]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(8'hf0), .WIDTH(8)) ff5 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(8'hcc), .Q(Q[39:32]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(8'hf0), .WIDTH(8)) ff6 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(8'hcc), .Q(Q[47:40]));
+$dffsr #(.CLK_POLARITY(1'b1), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(8)) ff7 (.CLK(CLK), .SET({SET, 4'hf}), .CLR({4'h0, CLR}), .D(8'hcc), .Q(Q[55:48]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b0), .SET_POLARITY(1'b1), .WIDTH(8)) ff8 (.CLK(CLK), .EN(EN), .SET({SET, 4'h0}), .CLR({4'hf, CLR}), .D(8'hcc), .Q(Q[63:56]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(4)) ff9 (.EN(EN), .D(4'hc), .Q(Q[67:64]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(8'hf0), .WIDTH(8)) ff10 (.EN(EN), .ARST(ARST), .D(8'hcc), .Q(Q[75:68]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b1), .WIDTH(8)) ff11 (.EN(EN), .SET({SET, 4'h0}), .CLR({4'h0, CLR}), .D(8'hcc), .Q(Q[83:76]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 1 t:$dff r:WIDTH=2 %i
+select -assert-count 1 t:$dffe r:WIDTH=2 %i
+select -assert-count 1 t:$adff r:WIDTH=6 %i
+select -assert-count 1 t:$adffe r:WIDTH=6 %i
+select -assert-count 1 t:$sdff r:WIDTH=6 %i
+select -assert-count 1 t:$sdffe r:WIDTH=6 %i
+select -assert-count 1 t:$sdffce r:WIDTH=6 %i
+select -assert-count 1 t:$dffsr r:WIDTH=6 %i
+select -assert-count 1 t:$dffsre r:WIDTH=6 %i
+select -assert-count 1 t:$dlatch r:WIDTH=2 %i
+select -assert-count 1 t:$adlatch r:WIDTH=6 %i
+select -assert-count 1 t:$dlatchsr r:WIDTH=6 %i
diff --git a/tests/opt/opt_dff_en.ys b/tests/opt/opt_dff_en.ys
new file mode 100644
index 000000000..06ee6c63d
--- /dev/null
+++ b/tests/opt/opt_dff_en.ys
@@ -0,0 +1,157 @@
+### Always-active EN removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+output [15:0] Q;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(CLK), .EN(1'b1), .D(D), .Q(Q[1:0]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .EN(1'b0), .ARST(ARST), .D(D), .Q(Q[3:2]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .EN(1'b1), .SRST(SRST), .D(D), .Q(Q[5:4]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .EN(1'b1), .SRST(SRST), .D(D), .Q(Q[7:6]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff4 (.CLK(CLK), .EN(1'b0), .SET(SET), .CLR(CLR), .D(D), .Q(Q[9:8]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(2)) ff5 (.EN(1'b1), .D(D), .Q(Q[11:10]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff6 (.EN(1'b0), .ARST(ARST), .D(D), .Q(Q[13:12]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.EN(1'b0), .SET(SET), .CLR(CLR), .D(D), .Q(Q[15:14]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+# Equivalence check will fail for unmapped adlatch and dlatchsr due to negative hold hack.
+delete top/ff6 top/ff7
+equiv_opt -undef -assert -multiclock opt_dff
+
+design -load orig
+delete top/ff6 top/ff7
+simplemap
+equiv_opt -undef -assert -multiclock opt_dff
+
+design -load orig
+opt_dff
+select -assert-count 0 t:$dffe
+select -assert-count 0 t:$adffe
+select -assert-count 0 t:$sdffe
+select -assert-count 0 t:$sdffce
+select -assert-count 0 t:$dffsre
+select -assert-count 0 t:$dlatch
+select -assert-count 0 t:$adlatch
+select -assert-count 0 t:$dlatchsr
+select -assert-count 1 t:$dff
+select -assert-count 2 t:$sdff
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$dffsr
+
+design -load orig
+simplemap
+opt_dff
+select -assert-count 0 t:$_DFFE_*
+select -assert-count 0 t:$_SDFFE_*
+select -assert-count 0 t:$_SDFFCE_*
+select -assert-count 0 t:$_DFFSRE_*
+select -assert-count 0 t:$_DLATCH*
+select -assert-count 2 t:$_DFF_P_
+select -assert-count 4 t:$_SDFF_PP?_
+select -assert-count 2 t:$_DFF_PP?_
+select -assert-count 2 t:$_DFFSR_PNP_
+
+design -reset
+
+
+
+### Never-active EN removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+(* init = 32'h55555555 *)
+output [31:0] Q;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(CLK), .EN(1'b0), .D(D), .Q(Q[1:0]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .EN(1'b1), .ARST(ARST), .D(D), .Q(Q[3:2]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .EN(1'b0), .SRST(SRST), .D(D), .Q(Q[5:4]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .EN(1'b0), .SRST(SRST), .D(D), .Q(Q[7:6]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff4 (.CLK(CLK), .EN(1'b1), .SET(SET), .CLR(CLR), .D(D), .Q(Q[9:8]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(2)) ff5 (.EN(1'b0), .D(D), .Q(Q[11:10]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff6 (.EN(1'b1), .ARST(ARST), .D(D), .Q(Q[13:12]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.EN(1'b1), .SET(SET), .CLR(CLR), .D(D), .Q(Q[15:14]));
+
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff8 (.CLK(CLK), .EN(1'bx), .D(D), .Q(Q[17:16]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff9 (.CLK(CLK), .EN(1'bx), .ARST(ARST), .D(D), .Q(Q[19:18]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff10 (.CLK(CLK), .EN(1'bx), .SRST(SRST), .D(D), .Q(Q[21:20]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff11 (.CLK(CLK), .EN(1'bx), .SRST(SRST), .D(D), .Q(Q[23:22]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff12 (.CLK(CLK), .EN(1'bx), .SET(SET), .CLR(CLR), .D(D), .Q(Q[25:24]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(2)) ff13 (.EN(1'bx), .D(D), .Q(Q[27:26]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff14 (.EN(1'bx), .ARST(ARST), .D(D), .Q(Q[29:28]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff15 (.EN(1'bx), .SET(SET), .CLR(CLR), .D(D), .Q(Q[31:30]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 2 t:$dffe
+select -assert-count 4 t:$dlatch
+select -assert-count 4 t:$sr
+select -assert-none t:$dffe t:$dlatch t:$sr %% %n t:* %i
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 2 t:$dffe
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$sdffe
+select -assert-count 1 t:$sdffce
+select -assert-count 1 t:$dffsre
+select -assert-count 3 t:$dlatch
+select -assert-count 1 t:$adlatch
+select -assert-count 1 t:$dlatchsr
+select -assert-count 2 t:$sr
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 4 t:$_DFFE_??_
+select -assert-count 8 t:$_DLATCH_?_
+select -assert-count 8 t:$_SR_??_
+select -assert-none t:$_DFFE_??_ t:$_DLATCH_?_ t:$_SR_??_ %% %n t:* %i
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 4 t:$_DFFE_??_
+select -assert-count 2 t:$_DFFE_????_
+select -assert-count 2 t:$_SDFFE_????_
+select -assert-count 2 t:$_SDFFCE_????_
+select -assert-count 2 t:$_DFFSRE_????_
+select -assert-count 6 t:$_DLATCH_?_
+select -assert-count 2 t:$_DLATCH_???_
+select -assert-count 2 t:$_DLATCHSR_???_
+select -assert-count 4 t:$_SR_??_
diff --git a/tests/opt/opt_dff_mux.ys b/tests/opt/opt_dff_mux.ys
new file mode 100644
index 000000000..ed01bed59
--- /dev/null
+++ b/tests/opt/opt_dff_mux.ys
@@ -0,0 +1,86 @@
+### CE and SRST matching.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input NE, NS;
+input EN;
+output [23:0] Q;
+input [23:0] D;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dff #(.CLK_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(CLK), .D(NS ? 2'h2 : NE ? D[1:0] : Q[1:0]), .Q(Q[1:0]));
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff1 (.CLK(CLK), .EN(EN), .D(NS ? 2'h2 : NE ? D[3:2] : Q[3:2]), .Q(Q[3:2]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .ARST(ARST), .D(NS ? 2'h2 : NE ? D[5:4] : Q[5:4]), .Q(Q[5:4]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .EN(EN), .ARST(ARST), .D(NS ? 2'h2 : NE ? D[7:6] : Q[7:6]), .Q(Q[7:6]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(CLK), .SRST(SRST), .D(NS ? 2'h2 : NE ? D[9:8] : Q[9:8]), .Q(Q[9:8]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff5 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(NS ? 2'h2 : NE ? D[11:10] : Q[11:10]), .Q(Q[11:10]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff6 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(NS ? 2'h2 : NE ? D[13:12] : Q[13:12]), .Q(Q[13:12]));
+$dffsr #(.CLK_POLARITY(1'b1), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.CLK(CLK), .SET(SET), .CLR(CLR), .D(NS ? 2'h2 : NE ? D[15:14] : Q[15:14]), .Q(Q[15:14]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff8 (.CLK(CLK), .EN(EN), .SET(SET), .CLR(CLR), .D(NS ? 2'h2 : NE ? D[17:16] : Q[17:16]), .Q(Q[17:16]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+clean
+select -assert-count 0 t:$dff
+select -assert-count 0 t:$dffe
+select -assert-count 0 t:$adff
+select -assert-count 2 t:$adffe
+select -assert-count 0 t:$dffsr
+select -assert-count 2 t:$dffsre
+select -assert-count 0 t:$sdff
+select -assert-count 3 t:$sdffe
+select -assert-count 2 t:$sdffce
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -nodffe -nosdff
+design -load postopt
+clean
+select -assert-count 1 t:$dff
+select -assert-count 1 t:$dffe
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$sdff
+select -assert-count 1 t:$sdffe
+select -assert-count 1 t:$sdffce
+equiv_opt -undef -assert -multiclock opt_dff -nodffe
+design -load postopt
+clean
+select -assert-count 0 t:$dff
+select -assert-count 0 t:$dffe
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsre
+select -assert-count 2 t:$sdff
+select -assert-count 1 t:$sdffe
+select -assert-count 2 t:$sdffce
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -nosdff
+design -load postopt
+clean
+select -assert-count 0 t:$dff
+select -assert-count 2 t:$dffe
+select -assert-count 0 t:$adff
+select -assert-count 2 t:$adffe
+select -assert-count 0 t:$dffsr
+select -assert-count 2 t:$dffsre
+select -assert-count 0 t:$sdff
+select -assert-count 2 t:$sdffe
+select -assert-count 1 t:$sdffce
diff --git a/tests/opt/opt_dff_qd.ys b/tests/opt/opt_dff_qd.ys
new file mode 100644
index 000000000..afc96c42f
--- /dev/null
+++ b/tests/opt/opt_dff_qd.ys
@@ -0,0 +1,56 @@
+### Q = D case.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input EN;
+(* init = 24'h555555 *)
+output [23:0] Q;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dff #(.CLK_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(CLK), .D(Q[1:0]), .Q(Q[1:0]));
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff1 (.CLK(CLK), .EN(EN), .D(Q[3:2]), .Q(Q[3:2]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .ARST(ARST), .D(Q[5:4]), .Q(Q[5:4]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .EN(EN), .ARST(ARST), .D(Q[7:6]), .Q(Q[7:6]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(CLK), .SRST(SRST), .D(Q[9:8]), .Q(Q[9:8]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff5 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(Q[11:10]), .Q(Q[11:10]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff6 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(Q[13:12]), .Q(Q[13:12]));
+$dffsr #(.CLK_POLARITY(1'b1), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.CLK(CLK), .SET(SET), .CLR(CLR), .D(Q[15:14]), .Q(Q[15:14]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff8 (.CLK(CLK), .EN(EN), .SET(SET), .CLR(CLR), .D(Q[17:16]), .Q(Q[17:16]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(2)) ff9 (.EN(EN), .D(Q[19:18]), .Q(Q[19:18]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff10 (.EN(EN), .ARST(ARST), .D(Q[21:20]), .Q(Q[21:20]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff11 (.EN(EN), .SET(SET), .CLR(CLR), .D(Q[23:22]), .Q(Q[23:22]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+# Equivalence check will fail for unmapped adlatch and dlatchsr due to negative hold hack.
+delete top/ff10 top/ff11
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+
+design -load orig
+opt_dff -keepdc
+select -assert-count 1 t:$and
+select -assert-count 3 t:$dffe
+select -assert-count 3 t:$dlatch
+select -assert-count 3 t:$sr
+select -assert-none t:$and t:$dffe t:$dlatch t:$sr %% %n t:* %i
+
+design -load orig
+simplemap
+opt_dff -keepdc
+select -assert-count 2 t:$_AND_
+select -assert-count 6 t:$_DFFE_??_
+select -assert-count 6 t:$_DLATCH_?_
+select -assert-count 6 t:$_SR_??_
+select -assert-none t:$_AND_ t:$_DFFE_??_ t:$_DLATCH_?_ t:$_SR_??_ %% %n t:* %i
+
diff --git a/tests/opt/opt_dff_sr.ys b/tests/opt/opt_dff_sr.ys
new file mode 100644
index 000000000..daedb115c
--- /dev/null
+++ b/tests/opt/opt_dff_sr.ys
@@ -0,0 +1,304 @@
+### Always-active SET/CLR removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR({CLR, CLR, CLR, 1'b1, 1'b0, 1'bx}), .SET({1'b1, 1'b0, 1'bx, SET, SET, SET}), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .CLR({CLR, CLR, CLR, 1'b1, 1'b0, 1'bx}), .SET({1'b1, 1'b0, 1'bx, SET, SET, SET}), .D(D), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .CLR({CLR, CLR, CLR, 1'b1, 1'b0, 1'bx}), .SET({1'b1, 1'b0, 1'bx, SET, SET, SET}), .D(D), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR({CLR, CLR, CLR, 1'b1, 1'b0, 1'bx}), .SET({1'b1, 1'b0, 1'bx, SET, SET, SET}), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsr r:WIDTH=2 %i
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$dffsre r:WIDTH=2 %i
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$dlatchsr r:WIDTH=2 %i
+select -assert-none t:$sr
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsr r:WIDTH=4 %i
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$dffsre r:WIDTH=4 %i
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$dlatchsr r:WIDTH=4 %i
+select -assert-count 1 t:$sr
+select -assert-count 1 t:$sr r:WIDTH=4 %i
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 1 t:$_DFF_PP0_
+select -assert-count 1 t:$_DFF_PP1_
+select -assert-count 1 t:$_DFFE_PN0P_
+select -assert-count 1 t:$_DFFE_PN1P_
+select -assert-count 1 t:$_DLATCH_PP0_
+select -assert-count 1 t:$_DLATCH_PN1_
+select -assert-none t:$_DFF_PP0_ t:$_DFF_PP1_ t:$_DFFE_PN0P_ t:$_DFFE_PN1P_ t:$_DLATCH_PP0_ t:$_DLATCH_PN1_ t:$_NOT_ %% %n t:* %i
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$_DFF_PP0_
+select -assert-count 1 t:$_DFF_PP1_
+select -assert-count 2 t:$_DFFSR_PPP_
+select -assert-count 1 t:$_DFFE_PN0P_
+select -assert-count 1 t:$_DFFE_PN1P_
+select -assert-count 2 t:$_DFFSRE_PNNP_
+select -assert-count 1 t:$_DLATCH_PP0_
+select -assert-count 1 t:$_DLATCH_PN1_
+select -assert-count 2 t:$_DLATCHSR_PNP_
+select -assert-count 1 t:$_DLATCH_P_
+select -assert-count 1 t:$_DLATCH_N_
+select -assert-count 2 t:$_SR_PN_
+select -assert-none t:$_DFF_PP0_ t:$_DFF_PP1_ t:$_DFFSR_PPP_ t:$_DFFE_PN0P_ t:$_DFFE_PN1P_ t:$_DFFSRE_PNNP_ t:$_DLATCH_PP0_ t:$_DLATCH_PN1_ t:$_DLATCHSR_PNP_ t:$_NOT_ t:$_DLATCH_N_ t:$_DLATCH_P_ t:$_SR_PN_ %% %n t:* %i
+
+design -reset
+
+
+
+### Never-active CLR removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR(6'h00), .SET({6{SET}}), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR(6'h3f), .SET({6{SET}}), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR(6'h00), .SET({6{SET}}), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR(6'h3f), .SET({6{SET}}), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$dffsr
+select -assert-count 0 t:$dffsre
+select -assert-count 0 t:$dlatchsr
+select -assert-count 0 t:$sr
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$adlatch
+select -assert-count 1 t:$dlatch
+
+design -reset
+
+
+
+### Never-active CLR removal (not applicable).
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input ALT;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR(6'h00), .SET({{5{SET}}, ALT}), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR(6'h3f), .SET({{5{SET}}, ALT}), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR(6'h00), .SET({{5{SET}}, ALT}), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR(6'h3f), .SET({{5{SET}}, ALT}), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$sr
+select -assert-count 0 t:$adff
+select -assert-count 0 t:$adffe
+select -assert-count 0 t:$adlatch
+select -assert-count 0 t:$dlatch
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$_DFFSR_*
+select -assert-count 0 t:$_DFFSRE_*
+select -assert-count 0 t:$_DLATCHSR_*
+select -assert-count 0 t:$_SR_*
+select -assert-count 6 t:$_DFF_PP1_
+select -assert-count 6 t:$_DFFE_PN1P_
+select -assert-count 6 t:$_DLATCH_PN1_
+select -assert-count 6 t:$_DLATCH_P_
+
+design -reset
+
+
+
+### Never-active SET removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR({6{CLR}}), .SET(6'h00), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR({6{CLR}}), .SET(6'h3f), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR({6{CLR}}), .SET(6'h3f), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR({6{CLR}}), .SET(6'h00), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$dffsr
+select -assert-count 0 t:$dffsre
+select -assert-count 0 t:$dlatchsr
+select -assert-count 0 t:$sr
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$adlatch
+select -assert-count 1 t:$dlatch
+
+design -reset
+
+
+
+### Never-active CLR removal (not applicable).
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input ALT;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR({{5{CLR}}, ALT}), .SET(6'h00), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR({{5{CLR}}, ALT}), .SET(6'h3f), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR({{5{CLR}}, ALT}), .SET(6'h3f), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR({{5{CLR}}, ALT}), .SET(6'h00), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$sr
+select -assert-count 0 t:$adff
+select -assert-count 0 t:$adffe
+select -assert-count 0 t:$adlatch
+select -assert-count 0 t:$dlatch
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$_DFFSR_*
+select -assert-count 0 t:$_DFFSRE_*
+select -assert-count 0 t:$_DLATCHSR_*
+select -assert-count 0 t:$_SR_*
+select -assert-count 6 t:$_DFF_PP0_
+select -assert-count 6 t:$_DFFE_PN0P_
+select -assert-count 6 t:$_DLATCH_PP0_
+select -assert-count 6 t:$_DLATCH_N_
+
+design -reset
+
+
+
+### SET/CLR merge into ARST.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input ARST;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR({ARST, 5'h00}), .SET({1'b0, {5{ARST}}}), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR({ARST, 5'h1f}), .SET({1'b1, {5{ARST}}}), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR({ARST, 5'h00}), .SET({1'b1, {5{ARST}}}), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR({ARST, 5'h1f}), .SET({1'b0, {5{ARST}}}), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$dffsr
+select -assert-count 0 t:$dffsre
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$sr
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adff r:ARST_VALUE=6'h1f %i
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$adffe r:ARST_VALUE=6'h1f %i
+select -assert-count 0 t:$adlatch
+select -assert-count 0 t:$dlatch
diff --git a/tests/opt/opt_dff_srst.ys b/tests/opt/opt_dff_srst.ys
new file mode 100644
index 000000000..4a77de0b8
--- /dev/null
+++ b/tests/opt/opt_dff_srst.ys
@@ -0,0 +1,113 @@
+### Always-active SRST removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+(* init=12'h555 *)
+output [11:0] Q;
+input SRST;
+input EN;
+
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff0 (.CLK(CLK), .SRST(1'b1), .D(D), .Q(Q[1:0]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .SRST(1'b0), .EN(EN), .D(D), .Q(Q[3:2]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .SRST(1'b0), .EN(EN), .D(D), .Q(Q[5:4]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .SRST(1'bx), .D(D), .Q(Q[7:6]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(CLK), .SRST(1'bx), .EN(EN), .D(D), .Q(Q[9:8]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff5 (.CLK(CLK), .SRST(1'bx), .EN(EN), .D(D), .Q(Q[11:10]));
+
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 0 t:$sdff
+select -assert-count 0 t:$sdffe
+select -assert-count 0 t:$sdffce
+select -assert-count 4 t:$dff
+select -assert-count 2 t:$dffe
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$sdff
+select -assert-count 1 t:$sdffe
+select -assert-count 1 t:$sdffce
+select -assert-count 2 t:$dff
+select -assert-count 1 t:$dffe
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$_SDFF_???_
+select -assert-none t:$_SDFFE_????_
+select -assert-none t:$_SDFFCE_????_
+select -assert-count 8 t:$_DFF_?_
+select -assert-count 4 t:$_DFFE_??_
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 2 t:$_SDFF_???_
+select -assert-count 2 t:$_SDFFE_????_
+select -assert-count 2 t:$_SDFFCE_????_
+select -assert-count 4 t:$_DFF_?_
+select -assert-count 2 t:$_DFFE_??_
+
+design -reset
+
+
+### Never-active SRST removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+output [5:0] Q;
+input SRST;
+input EN;
+
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff0 (.CLK(CLK), .SRST(1'b0), .D(D), .Q(Q[1:0]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .SRST(1'b1), .EN(EN), .D(D), .Q(Q[3:2]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .SRST(1'b1), .EN(EN), .D(D), .Q(Q[5:4]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$sdff
+select -assert-none t:$sdffe
+select -assert-none t:$sdffce
+select -assert-count 1 t:$dff
+select -assert-count 2 t:$dffe
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$_SDFF_???_
+select -assert-none t:$_SDFFE_????_
+select -assert-none t:$_SDFFCE_????_
+select -assert-count 2 t:$_DFF_P_
+select -assert-count 4 t:$_DFFE_PP_
+
+design -reset
+
diff --git a/tests/opt/opt_rmdff.ys b/tests/opt/opt_rmdff.ys
index 6fb24fd66..998414597 100644
--- a/tests/opt/opt_rmdff.ys
+++ b/tests/opt/opt_rmdff.ys
@@ -4,7 +4,7 @@ design -stash gold
 
 read_verilog -icells opt_rmdff.v
 proc
-opt_rmdff
+opt_dff
 
 select -assert-count 0 c:remove*
 select -assert-min 7 c:keep*
diff --git a/tests/opt/opt_rmdff_sat.ys b/tests/opt/opt_rmdff_sat.ys
index 1c3dd9c05..231c43ecb 100644
--- a/tests/opt/opt_rmdff_sat.ys
+++ b/tests/opt/opt_rmdff_sat.ys
@@ -1,5 +1,5 @@
 read_verilog opt_rmdff_sat.v
 prep -flatten
-opt_rmdff -sat
-synth
+opt_dff -sat -nosdff
+simplemap
 select -assert-count 5 t:$_DFF_P_