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The tests/verilog/*_if_enc.ys scripts instantiate simple encoder modules, both with and without the SystemVerilog priority/unique/unique0 keywords, and check for consistency between the two for the subset of inputs where the priority/unique/unique0 "if" result is well-defined. These tests vacuously succeed at the moment, since priority/unique keywords are silently ignored and therefore the generated logic is trivially identical. But the test cases will be capable of detecting certain types of unsound optimisation if priority/unique handling is introduced later.
41 lines
1.2 KiB
Text
41 lines
1.2 KiB
Text
logger -expect log "SAT proof finished - no model found: SUCCESS" 1
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read_verilog -sv <<EOF
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// A somewhat contrived model of an encoder, relying on SystemVerilog's
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// strong "if" semantics to guarantee priority encoder behaviour.
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module encoder( input [ 2:0 ] x, output reg [ 1:0 ] y );
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always_comb begin
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y = 2'b00;
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if( x[ 2 ] ) y = 2'b11;
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else if( x[ 1 ] ) y = 2'b10;
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else if( x[ 0 ] ) y = 2'b01;
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end
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endmodule
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// Almost the same thing, but by using "priority if" we introduce
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// "don't care" states, essentially conveying permission to synthesise
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// a simple encoder instead.
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module dut( input [ 2:0 ] x, output reg [ 1:0 ] y );
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always_comb begin
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y = 2'b00;
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priority if( x[ 2 ] ) y = 2'b11;
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else if( x[ 1 ] ) y = 2'b10;
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else if( x[ 0 ] ) y = 2'b01;
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end
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endmodule
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// A simple test bench to detect mismatches between the two encoders.
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module compare_encoders( input [ 2:0 ] x, output ok );
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wire [ 1:0 ] encout;
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wire [ 1:0 ] dutout;
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encoder e( x, encout );
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dut d( x, dutout );
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// The "priority if" above assumes $countones( x ) > 0.
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assign ok = encout == dutout || !$countones( x );
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endmodule
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EOF
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synth -flatten -top compare_encoders
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sat -prove ok 1
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