Why doesn't mul() check for b == 0 in the first if?

[jeffreydwalter]

🎉 Description

I'm just curious to know why this function:

  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0) {
      return 0;
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c;
  }

isn't written like this:

  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    if (a == 0 || b == 0) {
      return 0;
    }
    uint256 c = a * b;
    assert(c / a == b);
    return c;
  }

Why not check for b == 0 in the first if and return?

• 🐛 This is a bug report.
• 📈 This is a feature request.
• ? This is a question.

[come-maiz]

Take a look at the pull request that added this check:
#522

The comments in there discuss exactly your question.

[jeffreydwalter]

Makes sense. Thanks! Is it really that rare to have b == 0? I'd be curious (but not too curious) to see a comparison of b == 0 with an explicit check and without.

[come-maiz]

@emn178 here is assuming that most cases will be non-zero, which sounds reasonable. So this is about gas cost for this case, not about how rare is b = 0 compared to a = 0.
You can use remix as they did in the PR to satisfy your curiousity :)