Handle remainder as a wide word in div3by2#647
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tarcieri merged 1 commit intoRustCrypto:masterfrom Aug 15, 2024
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Signed-off-by: Andrew Whitehead <cywolf@gmail.com>
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Aug 15, 2024
fjarri
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Aug 16, 2024
| // If r < b and q*y[-2] > r*x[-1], then set q = q - 1 and r = r + v1 | ||
| let done = ConstChoice::from_word_nonzero((rem >> Word::BITS) as Word) | ||
| .or(ConstChoice::from_wide_word_le(qy, rx)); | ||
| quo = done.select_word(quo.saturating_sub(1), quo); |
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I would rather have wrapping_sub here, I'm not sure if saturating_sub is constant-time
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Spot checking it in Godbolt saturating_sub uses cmovae on x86 and csel on aarch64, which (for now, until the optimizer decides differently) seem pretty ideal
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In regards comments on #646, this handles potential overflow in the remainder value.