Randomization Basics: rand, randc & randomize() · SystemVerilog

Part 3 · Constraint Randomization · Intermediate

4topics
4lessons

Everything in constrained-random verification starts with three primitives: the rand and randc qualifiers, the randomize solver call, and the seeding machinery. randomize is a solver call, not an assignment: it commits a full legal solution and returns 1, or commits nothing and returns 0.

One call, two outcomes: every field updated and 1, or nothing changed and 0.

Classic bug	What you see	The fix
Unchecked return	A contradiction returns 0, and the test sends the previous packet again	Check every call: `if (!obj.randomize()) $fatal(...)`
Null nested handle	A `rand` handle never built with `new()` is skipped, and the call still returns 1	Build nested objects in `new()`
Seed instability	An added fork or object shifts later seeds, so yesterday's failing seed passes	Build objects in a fixed order, or seed them by name

Lesson	Covers
rand vs randc	Uniform draws, cyclic permutations, periods and slot burning.
The randomize() Call	Return checks, solve sets, check-only mode, field lists and tool choice.
Hooks and Nested Objects	Pre and post hooks, one global solve, null handles and arrays.
Random Stability and Seeding	Stream hierarchy, creation-order shifts, reseeding and replay.

Check yourself randomize returns 0. Which fields changed, and what must the test do next?

None. A failed solve commits nothing, so every field keeps its prior value. The test must check the return value and must not drive the stale transaction into the design.

Up nextrand vs randc

Practice this lesson Report an error

Classic bug

What you see

The fix

Unchecked return

A contradiction returns 0, and the test sends the previous packet again

Check every call: if (!obj.randomize()) $fatal(...)

Null nested handle

A rand handle never built with new() is skipped, and the call still returns 1

Build nested objects in new()

Seed instability

An added fork or object shifts later seeds, so yesterday's failing seed passes

Build objects in a fixed order, or seed them by name

Lesson

Covers

rand vs randc

Uniform draws, cyclic permutations, periods and slot burning.

The randomize() Call

Return checks, solve sets, check-only mode, field lists and tool choice.

Hooks and Nested Objects

Pre and post hooks, one global solve, null handles and arrays.

Random Stability and Seeding

Stream hierarchy, creation-order shifts, reseeding and replay.