Handle wide power-of-two divisors in exact division#1271
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Avoid indexing past the dividend when checking whether it has at least as many trailing zeros as a wider divisor. Use an unbounded right shift after the trailing-zero exactness check so shifting by the full dividend precision produces zero instead of panicking. Add a regression test for exact division by a wider power-of-two divisor. Co-authored-by: GPT 5.5 <gpt-5.5@openai.com>
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Summary
UintRef::div_exactcould panic when the divisor was wider than the dividend and had exactly as many trailing zeros as the dividend precision. In that case, exact division should report inexact division rather than indexing or shifting past the dividend.Fix
ensure_trailing_zerosnow handles zero-count requests that cover the full dividend width by checking whether the whole dividend is zero, instead of indexing the limb atnlimbs.After the trailing-zero exactness check,
div_exactnow uses an unbounded right shift so shifting by the full dividend precision produces zero instead of panicking.Tests
Added a regression test covering exact division of
U128::ONEby aU256power-of-two divisor at bit positionU128::BITS.Verified with:
This work was completed by Trail of Bits as part of the Patch The Planet project in collaboration with OpenAI. The issue was identified primarily by the Codex coding agent, and manually reviewed before submission.