Dynamic Constraint Control · SystemVerilog

Part 3 · Constraint Randomization · Intermediate

5topics
5lessons

One transaction class serves many tests. Each test freezes fields, drops rules, pins values, or reshapes the whole space, and each change belongs to one layer.

Each kind of change belongs to one layer.

You need to	Use	Scope
Tighten one call	`randomize() with { ... }`	One call
Drop a rule for a while, on one object	`constraint_mode(0)`, then restore	One object until restored
Replace a rule for a whole test	A subclass with the same block name, installed by a factory	Every object of the type
Pin a field across many calls	Set it, then `rand_mode(0)`	One field until restored
Read a range from configuration	A non-`rand` knob inside the constraint	Every call that reads it

Lesson	Control question it answers
rand_mode(): Freezing Fields	Which fields does the solver assign, and which does it read as state?
constraint_mode(): Toggling Blocks	Which named rule blocks stay active on this object?
Inline Constraints: randomize() with	What extra rules apply to this one call only?
Constraint Override via Inheritance	What standing personality does this test type carry?
State Variables and Cross-Object	What configuration does the solve read as constants?

Check yourself One phase needs illegal lengths while the rest of the test needs short bursts. Which two mechanisms combine, and why can inline constraints not do it alone?

Disable the legality block with constraint_mode(0), then steer with an inline with toward the illegal corner. Inline expressions only intersect with class constraints, so they can never relax the legal range by themselves.

Up nextrand_mode(): Freezing Fields

Practice this lesson Report an error

You need to

Use

Scope

Tighten one call

randomize() with { ... }

One call

Drop a rule for a while, on one object

constraint_mode(0), then restore

One object until restored

Replace a rule for a whole test

A subclass with the same block name, installed by a factory

Every object of the type

Pin a field across many calls

Set it, then rand_mode(0)

One field until restored

Read a range from configuration

A non-rand knob inside the constraint

Every call that reads it

Lesson

Control question it answers

rand_mode(): Freezing Fields

Which fields does the solver assign, and which does it read as state?

constraint_mode(): Toggling Blocks

Which named rule blocks stay active on this object?

Inline Constraints: randomize() with

What extra rules apply to this one call only?

Constraint Override via Inheritance

What standing personality does this test type carry?

State Variables and Cross-Object

What configuration does the solve read as constants?