Use multiplication by reciprocal to speed up divide #84
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Co-authored-by: Ivan Siutsou <47280527+Melirius@users.noreply.github.com> Signed-off-by: Kristof Roomp <kristofr@gmail.com>
…also provides typesafe casts
Initial Lepton implementation tests all the DC coefficients on the edges to be nonzero
Melirius
reviewed
May 29, 2024
| let orig_lt_zero = 1 - pred.cmp_lt(i32x8::ZERO); | ||
| let best_prior_is_zero: i32x8 = cast(!best_prior_abs.cmp_eq(u32x8::ZERO)); | ||
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| let best_prior_sign = best_prior_is_zero & orig_lt_zero; |
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Here also can be a corner case: sign of prior as i16 is used in initial implementation, not of i32 prior.
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Hmm, and what about using libdivide only for 16-bit quantization table (or, better, for table with values larger than 8355)? Normal JPEGs will use then this fast method, and libdivide will be only a fallback for bad tables. Branch predictor should be OK with that. See https://github.com/Melirius/lepton_jpeg_rust/tree/recip-var |
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Current approach doesn't work due to rounding issues.
Looking at using libdivide no-branch, since we never divide by 1 (because of the 1 << 13)