Write u128 using only two divisions
This commit is contained in:
David Tolnay
parent
23d280dc7a
commit
3e47651d90
88
src/lib.rs
88
src/lib.rs
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@ -40,13 +40,7 @@ const MAX_LEN: usize = 40; // i128::MIN (including minus sign)
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// Adaptation of the original implementation at
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// https://github.com/rust-lang/rust/blob/b8214dc6c6fc20d0a660fb5700dca9ebf51ebe89/src/libcore/fmt/num.rs#L188-L266
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macro_rules! impl_Integer {
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($($t:ident),* as $conv_fn:ident) =>
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(impl_Integer!(
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$($t),* as $conv_fn,
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|n:$conv_fn| (n / 10000, (n % 10000) as isize)
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););
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($($t:ident),* as $conv_fn:ident, $divmod_10000:expr) => ($(
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($($t:ident),* as $conv_fn:ident) => ($(
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impl Integer for $t {
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fn write<W: io::Write>(self, mut wr: W) -> io::Result<usize> {
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let mut buf = unsafe { mem::uninitialized() };
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@ -74,13 +68,11 @@ macro_rules! impl_Integer {
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// eagerly decode 4 characters at a time
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if <$t>::max_value() as u64 >= 10000 {
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while n >= 10000 {
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// division with remainder on u128 is badly optimized by LLVM.
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// see “udiv128.rs” for more info.
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let (q, r) = $divmod_10000(n);
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n = q;
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let rem = (n % 10000) as isize;
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n /= 10000;
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let d1 = (r / 100) << 1;
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let d2 = (r % 100) << 1;
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let d1 = (rem / 100) << 1;
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let d2 = (rem % 100) << 1;
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curr -= 4;
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ptr::copy_nonoverlapping(lut_ptr.offset(d1), buf_ptr.offset(curr), 2);
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ptr::copy_nonoverlapping(lut_ptr.offset(d2), buf_ptr.offset(curr + 2), 2);
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@ -128,5 +120,73 @@ impl_Integer!(isize, usize as u16);
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impl_Integer!(isize, usize as u32);
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#[cfg(target_pointer_width = "64")]
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impl_Integer!(isize, usize as u64);
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#[cfg(all(feature = "i128"))]
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impl_Integer!(i128, u128 as u128, udiv128::udivmod_10000);
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macro_rules! impl_Integer128 {
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($($t:ident),*) => {$(
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impl Integer for $t {
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fn write<W: io::Write>(self, mut wr: W) -> io::Result<usize> {
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let mut buf = unsafe { mem::uninitialized() };
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let bytes = self.write_to(&mut buf);
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try!(wr.write_all(bytes));
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Ok(bytes.len())
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}
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}
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impl IntegerPrivate for $t {
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#[allow(unused_comparisons)]
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fn write_to(self, buf: &mut [u8; MAX_LEN]) -> &[u8] {
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let is_nonnegative = self >= 0;
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let n = if is_nonnegative {
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self as u128
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} else {
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// convert the negative num to positive by summing 1 to it's 2 complement
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(!(self as u128)).wrapping_add(1)
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};
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let mut curr = buf.len() as isize;
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let buf_ptr = buf.as_mut_ptr();
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unsafe {
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// Divide by 10^19 which is the highest power less than 2^64.
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let (n, rem) = udiv128::udivmod_1e19(n);
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curr -= rem.write_to(buf).len() as isize;
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if n != 0 {
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// Memset the base10 leading zeros of rem.
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let target = buf.len() as isize - 19;
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ptr::write_bytes(buf_ptr.offset(target), b'0', (curr - target) as usize);
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curr = target;
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// Divide by 10^19 again.
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let (n, rem) = udiv128::udivmod_1e19(n);
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let buf2 = buf_ptr.offset(curr - buf.len() as isize) as *mut _;
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curr -= rem.write_to(&mut *buf2).len() as isize;
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if n != 0 {
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// Memset the leading zeros.
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let target = buf.len() as isize - 38;
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ptr::write_bytes(buf_ptr.offset(target), b'0', (curr - target) as usize);
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curr = target;
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// There is at most one digit left
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// because u128::max / 10^19 / 10^19 is 3.
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curr -= 1;
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*buf_ptr.offset(curr) = (n as u8) + b'0';
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}
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}
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if !is_nonnegative {
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curr -= 1;
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*buf_ptr.offset(curr) = b'-';
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}
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let len = buf.len() - curr as usize;
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slice::from_raw_parts(buf_ptr.offset(curr), len)
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}
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}
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}
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)*};
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}
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#[cfg(all(feature = "i128"))]
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impl_Integer128!(i128, u128);
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@ -21,25 +21,25 @@
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// (https://github.com/rust-lang/rust/issues/44545) and to allow function
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// inlining which doesn’t happen with the intrinsic.
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pub fn udivmod_10000(n: u128) -> (u128, isize) {
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pub fn udivmod_1e19(n: u128) -> (u128, u64) {
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let d = 10_000_000_000_000_000_000_u64; // 10^19
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let high = (n >> 64) as u64;
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if high == 0 {
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let low = n as u64;
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return ((low / 10000) as u128, (low % 10000) as isize);
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return ((low / d) as u128, low % d);
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}
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let leading_zeros_10000 = 114;
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debug_assert_eq!(leading_zeros_10000, 10000u128.leading_zeros());
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let sr = 1 + leading_zeros_10000 - high.leading_zeros();
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let sr = 65 - high.leading_zeros();
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// 52 <= sr <= 115
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// 2 <= sr <= 65
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let mut q: u128 = n << (128 - sr);
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let mut r: u128 = n >> sr;
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let mut carry: u64 = 0;
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// Don't use a range because they may generate references to memcpy in unoptimized code
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//
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// Loop invariants: r < 10000; carry is 0 or 1
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// Loop invariants: r < d; carry is 0 or 1
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let mut i = 0;
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while i < sr {
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i += 1;
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@ -49,14 +49,14 @@ pub fn udivmod_10000(n: u128) -> (u128, isize) {
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q = (q << 1) | carry as u128;
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// carry = 0
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// if r >= 10000 {
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// r -= 10000;
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// if r >= d {
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// r -= d;
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// carry = 1;
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// }
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let s = 10000u128.wrapping_sub(r).wrapping_sub(1) as i128 >> 127;
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let s = (d as u128).wrapping_sub(r).wrapping_sub(1) as i128 >> 127;
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carry = (s & 1) as u64;
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r -= 10000u128 & s as u128;
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r -= (d as u128) & s as u128;
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}
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((q << 1) | carry as u128, r as isize)
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((q << 1) | carry as u128, r as u64)
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}
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