Compile-time Assertions (@check)

Purpose

Validate compile-time constraints (widths, config sanity, address ranges, parameter compatibility) during elaboration. A @check failure aborts compilation. @check never generates hardware, simulation logic, or runtime behavior.

Syntax

@check (<constant_expression>, <string_message>);

Parentheses are required.

Semantics

• If the expression is true (nonzero integer): compilation continues.
• If the expression is false (zero): compile error with message "CHECK FAILED: message".
• If the expression cannot be evaluated as a constant: compile error (non-constant expression).

Placement rules

@check may appear in:

• Inside @project
• Inside @module

@check may not appear inside:

• Blocks (ASYNCHRONOUS, SYNCHRONOUS, PORT, WIRE, REGISTER, MEM, etc.)
• Other directives

Expression rules

The expression must evaluate to a constant, nonnegative integer at compile time.

Allowed operands:

• Integer literals
• CONST identifiers
• CONFIG.NAME identifiers
• Compile-time integer operators
• clog2()
• Parentheses
• Comparisons (==, !=, <, <=, >, >=)
• Logical operators (&&, ||, !)

Forbidden operands:

• Module ports, wires, registers
• Memory ports
• Signal slices
• Any runtime expression

Evaluation order

@check is evaluated after:

1. Resolution of all preceding CONST definitions
2. Resolution of all preceding CONFIG entries
3. OVERRIDE evaluation (if applicable)

@check can reference CONFIG from the same project and CONST from the same module, including values after any OVERRIDE has been applied.

Examples

// Width constraint
CONST { WIDTH = 32; }
@check (WIDTH % 8 == 0, "Width must be a multiple of 8.");

// Project config constraint
@check (CONFIG.DATA_WIDTH >= 8, "Data width must be at least 8.");

// Address width sanity
CONST { DEPTH = 256; ADDR_W = 8; }
@check (ADDR_W == clog2(DEPTH), "Address width does not match depth.");

// Invalid: runtime signal
@check (select == 3, "...");
// ERROR: non-constant expression in @check

Error conditions

• Expression evaluates to zero
• Expression contains any runtime signal
• Expression contains undefined identifiers
• Expression produces a non-integer
• Expression uses operators disallowed in constant expressions