When solve-before Is Mostly Redundant

Puzzle

Difficulty: Medium · Puzzle 6 of 6 · Topic: solve...before Bias Puzzles

Does solve a before b significantly change distribution here? Explain with legal-set and weighting logic.

Code

class sb_redundant;
  rand bit a;
  rand bit [3:0] b;
  constraint c {
    a dist {0 := 1, 1 := 1};
    (a == 0) -> (b == 0);
    (a == 1) -> (b inside {[0:15]});
  }
  // Variant: solve a before b;
endclass

Hint

Separate legality from weighting intent.

Step-by-step solution

1) Legal tuples are still 17 total (same as classic puzzle).
2) Explicit dist already requests balanced a selection (0 and 1 equal weight intent).
3) Adding solve a before b does not change legal tuples and often has limited extra effect because a is already explicitly weighted.
4) Any remaining differences are solver-implementation details, not legality changes.

Answer

Answer: Legal set is unchanged (17 tuples), and both variants typically target near 50/50 on a due to explicit dist; solve-before is mostly redundant here.

Why candidates get it wrong

Using solve-before as a substitute for missing dist intent is fragile across tools/seeds.

Interviewer follow-up

If dist on a is removed, how do distributions change between variants?

Related topics

• Back to solve...before Bias Puzzles

• Randomization & Constraint Puzzles section