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Differential Coverage for Debugging

Russ Cox · 25 Apr 2025 · research.swtch.com ↗

2 korrents from this piece

Russ Cox did not write this page.

Every claim below was made in this piece, quoted word for word and numbered in the order the piece makes them, so you can read it there rather than take our word for it. The sentence above each quote is our reading of the claim, not their wording. Each quote was checked against a stored copy of the page at build time; where the two differ, the quote is the fact.

  1. Our reading

    Comparing code coverage between passing and failing test runs is a cheap, effective way to localize a bug quickly.

    Their wordsDifferential coverage is cheap to compute and display, and when it's right, it can save a lot of time.

  2. Our reading

    Buggy code usually causes outright test failures rather than being silently exercised inside tests that still pass.

    Their wordsa passing test can still execute buggy code if the bug is data-dependent, or if the test is not sensitive to the specific mistake in the code. But a lot of the time, buggy code only triggers failures.