yosys/tests/opt/opt_mem_feedback.ys

# Good case: proper feedback port.

read_verilog << EOT

module top(...);

input clk;
input en;
input s;

input [3:0] ra;
output [15:0] rd;
input [3:0] wa;
input [15:0] wd;

reg [15:0] mem[0:15];

assign rd = mem[ra];

always @(posedge clk) begin
	if (en) begin
		mem[wa] <= {mem[wa][15:8], s ? wd[7:0] : mem[wa][7:0]};
	end
end

endmodule

EOT

hierarchy -auto-top
proc
opt_clean

design -save start
memory_map
design -save preopt

design -load start
opt_mem_feedback
select -assert-count 1 t:$memrd_v2
memory_map
design -save postopt

equiv_opt -assert -run prepare: :



design -reset

# Bad case: read port also used for other things.

read_verilog << EOT

module top(...);

input clk;
input en;
input s;

output [15:0] rd;
input [3:0] wa;
input [15:0] wd;

reg [15:0] mem[0:15];

assign rd = mem[wa];

always @(posedge clk) begin
	if (en) begin
		mem[wa] <= {s ? rd : wd[15:8], s ? wd[7:0] : rd};
	end
end

endmodule

EOT

hierarchy -auto-top
proc
opt_clean

design -save start
memory_map
design -save preopt

design -load start
select -assert-count 1 t:$memrd
opt_mem_feedback
select -assert-count 1 t:$memrd
memory_map
design -save postopt

equiv_opt -assert -run prepare: :



design -reset

# Bad case: another user of the mux out.

read_verilog << EOT

module top(...);

input clk;
input en;
input s;

output [15:0] rd;
input [3:0] wa;
input [15:0] wd;

reg [15:0] mem[0:15];

assign rd = s ? wd : mem[wa];

always @(posedge clk) begin
	if (en) begin
		mem[wa] <= rd;
	end
end

endmodule

EOT

hierarchy -auto-top
proc
opt_clean

design -save start
memory_map
design -save preopt

design -load start
select -assert-count 1 t:$memrd
opt_mem_feedback
select -assert-count 1 t:$memrd
memory_map
design -save postopt

equiv_opt -assert -run prepare: :



design -reset

# Tricky case: legit feedback path, but priority needs to be preserved.

read_verilog << EOT

module top(...);

input clk;
input sel;
input [3:0] wa1;
input [3:0] wa2;
input [15:0] wd1;
input [3:0] ra;
output [15:0] rd;

reg [15:0] mem [0:15];

always @(posedge clk) begin
	mem[wa1] <= sel ? wd1 : mem[wa1];
	mem[wa2] <= mem[wa2];
end

assign rd = mem[ra];

endmodule

EOT

hierarchy -auto-top
proc
opt_clean

design -save start
memory_map
design -save preopt

design -load start
opt_mem_feedback
select -assert-count 1 t:$memrd_v2
memory_map
design -save postopt

equiv_opt -assert -run prepare: :