libm: define and implement trait NarrowingDiv for unsigned integer division
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This was referenced Aug 12, 2025
tgross35
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| /// | ||
| /// # Safety | ||
| /// Requires that `n.leading_zeros() == 0` and `a < n`. | ||
| unsafe fn div_three_digits_by_two<U>(a0: U, a: U::D, n: U::D) -> (U, U::D) |
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Maybe div_three_3x_by_2x or so? Just thinking that the inputs aren't really digits
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They are digits in base R = 2.pow(U::BITS), and this function does division of a 3-digit number by a 2-digit number in that base. Admittedly the signature is rather weird, passing the "high two thirds" of the 3-digit number as one U::D and the least significant third as a U.
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New utility in
libm::support:trait NarrowingDivfor dividingu2N / uNwhen the quotient fits inuNu256 / u128This is the inverse operation of unsigned widening multiplication:
The trait API is based on x86's
div-instruction: quotient overflow happens exactly when the high half of the dividend is greater or equal to the divisor, which includes division by zero.Split from #1002