SystemVerilog Lint Fixing

SystemVerilog Lint Fix Patterns

Error-specific fix patterns for common SystemVerilog lint issues.

WIDTHTRUNC / WIDTHEXPAND

Message: Width mismatch in assignment

Diagnosis: Signal widths don't match between source and destination.

Fixes:

// Truncation (wide to narrow) - explicit slice
logic [15:0] wide;
logic [7:0]  narrow;
assign narrow = wide[7:0];

// Extension (narrow to wide) - zero extend
assign wide = {8'b0, narrow};

// Sign extension
assign wide = {{8{narrow[7]}}, narrow};

// Using $bits for dynamic width
assign narrow = wide[$bits(narrow)-1:0];

LATCH

Message: Latch inferred for signal

Diagnosis: Combinational block doesn't assign signal in all paths.

Fixes:

// Add default assignment at block start
always_comb begin
    out = '0;  // Default prevents latch
    if (sel) begin
        out = in_a;
    end
    // No else needed - default covers it
end

// Complete case statement
always_comb begin
    unique case (sel)
        2'b00: out = a;
        2'b01: out = b;
        2'b10: out = c;
        default: out = '0;  // Explicit default
    endcase
end

UNDRIVEN

Message: Signal is not driven

Diagnosis: Signal declared but never assigned a value.

Fixes:

// Option 1: Connect to appropriate source
assign undriven_sig = source_sig;

// Option 2: Remove if truly unused
// Delete the declaration

// Option 3: Tie off if intentionally unused
assign unused_input = '0;

UNUSED

Message: Signal/variable is unused

Fixes:

// Option 1: Remove declaration entirely

// Option 2: Use lint pragma for intentional cases
/* verilator lint_off UNUSED */
logic debug_signal;
/* verilator lint_on UNUSED */

// Option 3: Prefix with underscore (convention)
logic _unused_port;

CASEINCOMPLETE

Message: Case statement does not cover all values

Fix: Add default case

unique case (state)
    IDLE:   next_state = start ? ACTIVE : IDLE;
    ACTIVE: next_state = done ? DONE : ACTIVE;
    DONE:   next_state = IDLE;
    default: next_state = IDLE;  // Required
endcase

BLKSEQ

Message: Blocking assignment in sequential block

Fix: Use non-blocking for sequential logic

// Wrong
always_ff @(posedge clk) begin
    data = new_data;  // Blocking - bad
end

// Correct
always_ff @(posedge clk) begin
    data <= new_data;  // Non-blocking - good
end

COMBDLY

Message: Delayed assignment in combinational block

Fix: Remove delays from combinational logic

// Wrong
always_comb begin
    out = #1 in_a & in_b;  // Delay - not synthesizable
end

// Correct
always_comb begin
    out = in_a & in_b;
end

MULTIDRIVEN

Message: Signal driven from multiple always blocks

Fix: Consolidate drivers or use separate signals

// Wrong - multiple drivers
always_ff @(posedge clk) data <= a;
always_ff @(posedge clk) data <= b;

// Correct - single driver with mux
always_ff @(posedge clk) begin
    data <= sel ? a : b;
end

Fix Workflow

1. Read error message and line number
2. Identify error category from above
3. Apply appropriate fix pattern
4. Re-run lint to verify
5. Check for cascading errors from the fix