~ vs ! in SystemVerilog: The Bug the Compiler Won't Catch

Will this block execute? Yes or no.

logic [1:0] status;
assign status = 1;

if (~status) begin
    // Does this execute?
end

Take a moment before answering.

If you said "no" — think again.

The Setup

This is a question that separates engineers who understand what their code synthesizes to from engineers who rely on intuition. On a single-bit signal, ~ and ! give the same result. That makes the distinction easy to miss — until you hit a multi-bit signal and something breaks in a way that's nearly impossible to find in simulation.

The code above will execute the if block. Not because of a quirk or a compiler bug. Because of exactly what ~ does, and how if evaluates its condition.

Two Operators, Two Different Questions

~ — Bitwise NOT

The tilde operator asks: flip every bit in this vector. It operates on each bit independently and returns a result with the same width as the input.

logic [3:0] input_vec = 4'b1010;
logic [3:0] result;
assign result = ~input_vec;  // result = 4'b0101

If the operand contains x or z bits, those become x in the output. The result is always multi-bit if the input is multi-bit.

! — Logical NOT

The exclamation operator asks: is this entire expression zero? It evaluates the operand as a Boolean condition and returns a single bit.

logic [3:0] some_ones = 4'b0101;
logic result;
assign result = !some_ones;  // result = 1'b0 (non-zero → TRUE → !TRUE = FALSE)

Output is always 1 bit: 1'b1 if the operand is zero, 1'b0 if non-zero, 1'bx if indeterminate.

For a single-bit signal, ~ and ! give the same result. For a multi-bit signal, they can give completely different answers.

Why the Example Executes

status is 2 bits wide. The value 1 is stored as 2'b01.

With ~status:

1. Bitwise NOT flips every bit: ~(2'b01) = 2'b10
2. The if statement receives 2'b10
3. Any non-zero value in an if condition evaluates to TRUE
4. 2'b10 is not zero → the block executes

With !status:

1. 2'b01 is non-zero → logically TRUE
2. !TRUE = 1'b0 (FALSE)
3. The condition is FALSE → the block does not execute

The intent was almost certainly to check "is status zero?" That requires !. Using ~ gives the opposite answer for this value.

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