yosys/tests/verilog/past_signedness.ys

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logger -expect-no-warnings
read_verilog -formal <<EOT
module top(input clk);
reg signed [3:0] value = -1;
reg ready = 0;
always @(posedge clk) begin
if (ready)
assert ($past(value) == -1);
ready <= 1;
end
endmodule
EOT
prep -top top
async2sync
sim -n 3 -clock clk
design -reset
read_verilog -formal <<EOT
module top(input clk);
reg signed [3:0] value = -1;
reg ready = 0;
always @(posedge clk) begin
if (ready)
assert ($past(value + 4'b0000) == 15);
ready <= 1;
end
endmodule
EOT
prep -top top
async2sync
sim -n 3 -clock clk