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Small optimization for integers Display implementation
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@GuillaumeGomez
GuillaumeGomez committed Oct 1, 2024
1 parent 098ada1 commit c4be3da
Showing 1 changed file with 92 additions and 46 deletions.
138 changes: 92 additions & 46 deletions core/src/fmt/num.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -208,11 +208,46 @@ static DEC_DIGITS_LUT: &[u8; 200] = b"0001020304050607080910111213141516171819\
8081828384858687888990919293949596979899";

macro_rules! impl_Display {
($($t:ident),* as $u:ident via $conv_fn:ident named $name:ident) => {
($($t:ident => $size:literal $(as $positive:ident in $other:ident)? => named $name:ident,)* ; as $u:ident via $conv_fn:ident named $gen_name:ident) => {

$(
#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for $t {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
// If it's a signed integer.
$(
let is_nonnegative = *self >= 0;

#[cfg(not(feature = "optimize_for_size"))]
{
if !is_nonnegative {
// convert the negative num to positive by summing 1 to its 2s complement
return $other((!self as $positive).wrapping_add(1), false, f);
}
}
#[cfg(feature = "optimize_for_size")]
{
if !is_nonnegative {
// convert the negative num to positive by summing 1 to its 2s complement
return $other((!self.$conv_fn()).wrapping_add(1), false, f);
}
}
)?
// If it's a positive integer.
#[cfg(not(feature = "optimize_for_size"))]
{
$name(*self, true, f)
}
#[cfg(feature = "optimize_for_size")]
{
$gen_name(*self, true, f)
}
}
}

#[cfg(not(feature = "optimize_for_size"))]
fn $name(mut n: $u, is_nonnegative: bool, f: &mut fmt::Formatter<'_>) -> fmt::Result {
// 2^128 is about 3*10^38, so 39 gives an extra byte of space
let mut buf = [MaybeUninit::<u8>::uninit(); 39];
fn $name(mut n: $t, is_nonnegative: bool, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut buf = [MaybeUninit::<u8>::uninit(); $size];
let mut curr = buf.len();
let buf_ptr = MaybeUninit::slice_as_mut_ptr(&mut buf);
let lut_ptr = DEC_DIGITS_LUT.as_ptr();
Expand All @@ -226,22 +261,26 @@ macro_rules! impl_Display {
// is safe to access.
unsafe {
// need at least 16 bits for the 4-characters-at-a-time to work.
assert!(crate::mem::size_of::<$u>() >= 2);

// eagerly decode 4 characters at a time
while n >= 10000 {
let rem = (n % 10000) as usize;
n /= 10000;

let d1 = (rem / 100) << 1;
let d2 = (rem % 100) << 1;
curr -= 4;

// We are allowed to copy to `buf_ptr[curr..curr + 3]` here since
// otherwise `curr < 0`. But then `n` was originally at least `10000^10`
// which is `10^40 > 2^128 > n`.
ptr::copy_nonoverlapping(lut_ptr.add(d1), buf_ptr.add(curr), 2);
ptr::copy_nonoverlapping(lut_ptr.add(d2), buf_ptr.add(curr + 2), 2);
#[allow(overflowing_literals)]
#[allow(unused_comparisons)]
// This block will be removed for smaller types at compile time and in the worst
// case, it will prevent to have the `10000` literal to overflow for `i8` and `u8`.
if core::mem::size_of::<$t>() >= 2 {
// eagerly decode 4 characters at a time
while n >= 10000 {
let rem = (n % 10000) as usize;
n /= 10000;

let d1 = (rem / 100) << 1;
let d2 = (rem % 100) << 1;
curr -= 4;

// We are allowed to copy to `buf_ptr[curr..curr + 3]` here since
// otherwise `curr < 0`. But then `n` was originally at least `10000^10`
// which is `10^40 > 2^128 > n`.
ptr::copy_nonoverlapping(lut_ptr.add(d1 as usize), buf_ptr.add(curr), 2);
ptr::copy_nonoverlapping(lut_ptr.add(d2 as usize), buf_ptr.add(curr + 2), 2);
}
}

// if we reach here numbers are <= 9999, so at most 4 chars long
Expand All @@ -255,6 +294,8 @@ macro_rules! impl_Display {
ptr::copy_nonoverlapping(lut_ptr.add(d1), buf_ptr.add(curr), 2);
}

// if we reach here numbers are <= 100, so at most 2 chars long
// The biggest it can be is 99, and 99 << 1 == 198, so a `u8` is enough.
// decode last 1 or 2 chars
if n < 10 {
curr -= 1;
Expand All @@ -273,11 +314,10 @@ macro_rules! impl_Display {
slice::from_raw_parts(buf_ptr.add(curr), buf.len() - curr))
};
f.pad_integral(is_nonnegative, "", buf_slice)
}
})*

#[cfg(feature = "optimize_for_size")]
fn $name(mut n: $u, is_nonnegative: bool, f: &mut fmt::Formatter<'_>) -> fmt::Result {
// 2^128 is about 3*10^38, so 39 gives an extra byte of space
fn $gen_name(mut n: $u, is_nonnegative: bool, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut buf = [MaybeUninit::<u8>::uninit(); 39];
let mut curr = buf.len();
let buf_ptr = MaybeUninit::slice_as_mut_ptr(&mut buf);
Expand Down Expand Up @@ -306,21 +346,6 @@ macro_rules! impl_Display {
};
f.pad_integral(is_nonnegative, "", buf_slice)
}

$(#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for $t {
#[allow(unused_comparisons)]
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let is_nonnegative = *self >= 0;
let n = if is_nonnegative {
self.$conv_fn()
} else {
// convert the negative num to positive by summing 1 to it's 2 complement
(!self.$conv_fn()).wrapping_add(1)
};
$name(n, is_nonnegative, f)
}
})*
};
}

Expand Down Expand Up @@ -374,7 +399,6 @@ macro_rules! impl_Exp {
(n, exponent, exponent, added_precision)
};

// 39 digits (worst case u128) + . = 40
// Since `curr` always decreases by the number of digits copied, this means
// that `curr >= 0`.
let mut buf = [MaybeUninit::<u8>::uninit(); 40];
Expand Down Expand Up @@ -469,7 +493,7 @@ macro_rules! impl_Exp {
let n = if is_nonnegative {
self.$conv_fn()
} else {
// convert the negative num to positive by summing 1 to it's 2 complement
// convert the negative num to positive by summing 1 to its 2s complement
(!self.$conv_fn()).wrapping_add(1)
};
$name(n, is_nonnegative, false, f)
Expand All @@ -484,7 +508,7 @@ macro_rules! impl_Exp {
let n = if is_nonnegative {
self.$conv_fn()
} else {
// convert the negative num to positive by summing 1 to it's 2 complement
// convert the negative num to positive by summing 1 to its 2s complement
(!self.$conv_fn()).wrapping_add(1)
};
$name(n, is_nonnegative, true, f)
Expand All @@ -499,8 +523,17 @@ macro_rules! impl_Exp {
mod imp {
use super::*;
impl_Display!(
i8, u8, i16, u16, i32, u32, i64, u64, usize, isize
as u64 via to_u64 named fmt_u64
i8 => 3 as u8 in fmt_u8 => named fmt_i8,
u8 => 3 => named fmt_u8,
i16 => 5 as u16 in fmt_u16 => named fmt_i16,
u16 => 5 => named fmt_u16,
i32 => 10 as u32 in fmt_u32 => named fmt_i32,
u32 => 10 => named fmt_u32,
i64 => 19 as u64 in fmt_u64 => named fmt_i64,
u64 => 20 => named fmt_u64,
isize => 19 as usize in fmt_usize => named fmt_isize,
usize => 20 => named fmt_usize,
; as u64 via to_u64 named fmt_u64
);
impl_Exp!(
i8, u8, i16, u16, i32, u32, i64, u64, usize, isize
Expand All @@ -511,8 +544,21 @@ mod imp {
#[cfg(not(any(target_pointer_width = "64", target_arch = "wasm32")))]
mod imp {
use super::*;
impl_Display!(i8, u8, i16, u16, i32, u32, isize, usize as u32 via to_u32 named fmt_u32);
impl_Display!(i64, u64 as u64 via to_u64 named fmt_u64);
impl_Display!(
i8 => 3 as u8 in fmt_u8 => named fmt_i8,
u8 => 3 => named fmt_u8,
i16 => 5 as u16 in fmt_u16 => named fmt_i16,
u16 => 5 => named fmt_u16,
i32 => 10 as u32 in fmt_u32 => named fmt_i32,
u32 => 10 => named fmt_u32,
isize => 10 as usize in fmt_usize => named fmt_isize,
usize => 10 => named fmt_usize,
; as u32 via to_u32 named fmt_u32);
impl_Display!(
i64 => 19 as u64 in fmt_u64 => named fmt_i64,
u64 => 20 => named fmt_u64,
; as u64 via to_u64 named fmt_u64);

impl_Exp!(i8, u8, i16, u16, i32, u32, isize, usize as u32 via to_u32 named exp_u32);
impl_Exp!(i64, u64 as u64 via to_u64 named exp_u64);
}
Expand Down Expand Up @@ -619,7 +665,7 @@ impl fmt::Display for i128 {
let n = if is_nonnegative {
self.to_u128()
} else {
// convert the negative num to positive by summing 1 to it's 2 complement
// convert the negative num to positive by summing 1 to its 2s complement
(!self.to_u128()).wrapping_add(1)
};
fmt_u128(n, is_nonnegative, f)
Expand Down

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