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switch to using safe optimisations
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@alindima
alindima committed Jan 11, 2024
1 parent 748bdb9 commit 8d0ade9
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118 changes: 21 additions & 97 deletions reed-solomon-novelpoly/src/field/inc_afft.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -88,27 +88,25 @@ fn b_is_one() {
// We're hunting for the differences and trying to undersrtand the algorithm.

/// Inverse additive FFT in the "novel polynomial basis"
///
/// # Safety
/// See safety section of `AdditiveFFT::inverse_afft`.
pub unsafe fn inverse_afft(data: &mut [Additive], size: usize, index: usize) {
#[inline(always)]
pub fn inverse_afft(data: &mut [Additive], size: usize, index: usize) {
unsafe { &AFFT }.inverse_afft(data, size, index)
}

#[cfg(all(target_feature = "avx", feature = "avx"))]
#[inline(always)]
pub fn inverse_afft_faster8(data: &mut [Additive], size: usize, index: usize) {
unsafe { &AFFT }.inverse_afft_faster8(data, size, index)
}

/// Additive FFT in the "novel polynomial basis"
///
/// # Safety
/// See safety section of `AdditiveFFT::afft`.
pub unsafe fn afft(data: &mut [Additive], size: usize, index: usize) {
#[inline(always)]
pub fn afft(data: &mut [Additive], size: usize, index: usize) {
unsafe { &AFFT }.afft(data, size, index)
}

#[cfg(all(target_feature = "avx", feature = "avx"))]
#[inline(always)]
/// Additive FFT in the "novel polynomial basis"
pub fn afft_faster8(data: &mut [Additive], size: usize, index: usize) {
unsafe { &AFFT }.afft_faster8(data, size, index)
Expand Down Expand Up @@ -138,12 +136,7 @@ impl AdditiveFFT {
}

/// Inverse additive FFT in the "novel polynomial basis"
///
/// # Safety
///
/// - caller must ensure than `size` is a power of two and that the length of the `data` slice is at least equal to `size`.
/// - caller must ensure that `index + size - 2` is less than or equal to 65534.
pub unsafe fn inverse_afft(&self, data: &mut [Additive], size: usize, index: usize) {
pub fn inverse_afft(&self, data: &mut [Additive], size: usize, index: usize) {
// All line references to Algorithm 2 page 6288 of
// https://www.citi.sinica.edu.tw/papers/whc/5524-F.pdf

Expand All @@ -154,6 +147,8 @@ impl AdditiveFFT {
// After this, we start at depth (i of Algorithm 2) = (k of Algorithm 2) - 1
// and progress through FIELD_BITS-1 steps, obtaining \Psi_\beta(0,0).
let mut depart_no = 1_usize;
assert!(data.len() >= size);

while depart_no < size {
// if depart_no >= 8 {
// println!("\n\n\nplain/Round depart_no={depart_no}");
Expand All @@ -180,42 +175,16 @@ impl AdditiveFFT {
// if depart_no >= 8 && false{
// data[i + depart_no] ^= dbg!(data[dbg!(i)]);
// } else {
#[cfg(debug)]
{
data[i + depart_no] ^= data[i];
}

#[cfg(not(debug))]
{
// SAFETY
//
// j is smaller than size. depart_no is smaller than size.
// depart_no is always doubled, so it's always a power of two smaller than size.
// this means that depart_no is at most half of size, assuming size is a power of two.
//
// i is at most j - 1. j is greater than depart_no but is incremented by double of depart_no.
// for the max depart_no value of size/2, j will only have the one value of size/2,
// so the index will be size/2 - 1 + size/2, which is equal to size - 1, which is safe.
// i will always be smaller than i + depart_no, since they're positive integers. qed.
let local = unsafe { *data.get_unchecked(i) };
unsafe { *data.get_unchecked_mut(i + depart_no) ^= local };
}
// }
// TODO: Optimising bounds checks on this line will yield a great performance improvement.
data[i + depart_no] ^= data[i];
}

// Algorithm 2 indexs the skew factor in line 5 page 6288
// by i and \omega_{j 2^{i+1}}, but not by r explicitly.
// We further explore this confusion below. (TODO)
#[cfg(debug)]
let skew = self.skews[j + index - 1];

#[cfg(not(debug))]
// SAFETY:
//
// Safe because caller ensured that index + size - 2 is less than or equal to 65534 (the skew vector len).
// Since, j is at most size - 1, this is safe.
let skew = unsafe { *self.skews.get_unchecked(j + index - 1) };

// It's reasonale to skip the loop if skew is zero, but doing so with
// all bits set requires justification. (TODO)
if skew.0 != ONEMASK {
Expand All @@ -226,18 +195,9 @@ impl AdditiveFFT {
// if depart_no >= 8 && false{
// data[i] ^= dbg!(dbg!(data[dbg!(i + depart_no)]).mul(skew));
// } else {
#[cfg(debug)]
{
data[i] ^= data[i + depart_no].mul(skew);
}

#[cfg(not(debug))]
// Same safety princicples as the first `for i in (j - depart_no)..j` loop.
{
let local = unsafe { *data.get_unchecked(i + depart_no) };
unsafe { *data.get_unchecked_mut(i) ^= local.mul(skew) };
}
// }

// TODO: Optimising bounds checks on this line will yield a great performance improvement.
data[i] ^= data[i + depart_no].mul(skew);
}
}

Expand Down Expand Up @@ -304,12 +264,7 @@ impl AdditiveFFT {
}

/// Additive FFT in the "novel polynomial basis"
///
/// # Safety
///
/// - caller must ensure than `size` is a power of two and that the length of the `data` slice is at least equal to `size`.
/// - caller must ensure that `index + size - 2` is less than or equal to 65534.
pub unsafe fn afft(&self, data: &mut [Additive], size: usize, index: usize) {
pub fn afft(&self, data: &mut [Additive], size: usize, index: usize) {
// All line references to Algorithm 1 page 6287 of
// https://www.citi.sinica.edu.tw/papers/whc/5524-F.pdf

Expand All @@ -320,6 +275,8 @@ impl AdditiveFFT {
// After this, we start at depth (i of Algorithm 1) = (k of Algorithm 1) - 1
// and progress through FIELD_BITS-1 steps, obtaining \Psi_\beta(0,0).
let mut depart_no = size >> 1_usize;
assert!(data.len() >= size);

while depart_no > 0 {
// Agrees with for loop (j of Algorithm 1) in (0..2^{k-i-1}) from line 5,
// except we've j in (depart_no..size).step_by(2*depart_no), meaning
Expand All @@ -342,16 +299,8 @@ impl AdditiveFFT {
// like in (19) in the proof of Lemma 4. (TODO)
// We should understand the rest of this basis story, like (8) too. (TODO)

#[cfg(debug)]
let skew = self.skews[j + index - 1];

#[cfg(not(debug))]
// SAFETY:
//
// Safe because caller ensured that index + size - 2 is less than or equal to 65534 (the skew vector len).
// Since, j is at most size - 1, this is safe.
let skew = unsafe { *self.skews.get_unchecked(j + index - 1) };

// It's reasonale to skip the loop if skew is zero, but doing so with
// all bits set requires justification. (TODO)
if skew.0 != ONEMASK {
Expand All @@ -360,43 +309,18 @@ impl AdditiveFFT {
// Line 6, explained by (28) page 6287, but
// adding depart_no acts like the r+2^i superscript.

#[cfg(debug)]
{
data[i] ^= data[i + depart_no].mul(skew);
}

#[cfg(not(debug))]
{
// SAFETY
//
// j is smaller than size. depart_no is smaller than size/2 and it's always halved.
// this means that depart_no is at most half of size, assuming size is a power of two.
//
// i is at most j - 1. j is greater than depart_no but is incremented by double of depart_no.
// for the max depart_no value of size/2, j will only have the one value of size/2,
// so the index will be size/2 - 1 + size/2, which is equal to size - 1, which is safe.
// i will always be smaller than i + depart_no, since they're positive integers. qed.
let local = unsafe { *data.get_unchecked(i + depart_no) };
unsafe { *data.get_unchecked_mut(i) ^= local.mul(skew) };
}
// TODO: Optimising bounds checks on this line will yield a great performance improvement.
data[i] ^= data[i + depart_no].mul(skew);
}
}

// Again loop on line 5, so i corresponds to j in Algorithm 1
for i in (j - depart_no)..j {
// Line 7, explained by (31) page 6287, but
// adding depart_no acts like the r+2^i superscript.
#[cfg(debug)]
{
data[i + depart_no] ^= data[i];
}

#[cfg(not(debug))]
{
// Same safety princicples as the first `for i in (j - depart_no)..j` loop.
let local = unsafe { *data.get_unchecked(i) };
unsafe { *data.get_unchecked_mut(i + depart_no) ^= local };
}
// TODO: Optimising bounds checks on this line will yield a great performance improvement.
data[i + depart_no] ^= data[i];
}

// Increment by double depart_no in agreement with
Expand Down
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