YulUtilFunctions: reject _bytes == 0 in maskLowerOrderBytesFunction#16754
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msooseth wants to merge 1 commit into
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YulUtilFunctions: reject _bytes == 0 in maskLowerOrderBytesFunction#16754msooseth wants to merge 1 commit into
msooseth wants to merge 1 commit into
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The existing assert allowed _bytes == 0, in which case the mask expression (~u256(0)) >> (256 - 8 * _bytes) becomes (~0) >> 256 — implementation-defined in C++ and only happens to yield 0 in boost::multiprecision. The generated Yul would then become `result := and(data, 0x00)`, a zeroizer, which makes no semantic sense for a "mask lower order bytes" helper. No caller passes 0 today; tighten the precondition so future callers can't accidentally trigger the degenerate case. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
maskLowerOrderBytesFunction(_bytes)asserts_bytes <= 32but allows_bytes == 0._bytes == 0, the mask expression(~u256(0)) >> (256 - 8 * _bytes)becomes~0 >> 256— implementation-defined in C++ and only happens to evaluate to0in boost'smultiprecision::number<unchecked<…>>. The emitted Yul becomesresult := and(data, 0x00), a zeroizer.0(verified via grep), so this is not a current bug. Tighten the precondition to_bytes > 0 && _bytes <= 32so future callers can't accidentally hit the degenerate case.maskLowerOrderBytesFunctionDynamicis unaffected — its Yulnot(shl(mul(8, bytes), not(0)))formulation handlesbytes == 0correctly.Test plan