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Rollup merge of #115913 - FedericoStra:checked_ilog, r=the8472

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checked_ilog: improve performance

Addresses #115874.

(This PR replicates the original #115875, which I accidentally closed by deleting my forked repository...)
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Guillaume Gomez 2024-04-22 20:25:58 +02:00 committed by GitHub
commit 206e0df78d
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3 changed files with 89 additions and 27 deletions
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79
library/core/benches/num/int_log/mod.rs
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@ -1,7 +1,7 @@
use rand::Rng;
use test::{black_box, Bencher};
macro_rules! int_log_bench {
macro_rules! int_log10_bench {
($t:ty, $predictable:ident, $random:ident, $random_small:ident) => {
#[bench]
fn $predictable(bench: &mut Bencher) {
@ -51,8 +51,75 @@ fn $random_small(bench: &mut Bencher) {
};
}
int_log_bench! {u8, u8_log10_predictable, u8_log10_random, u8_log10_random_small}
int_log_bench! {u16, u16_log10_predictable, u16_log10_random, u16_log10_random_small}
int_log_bench! {u32, u32_log10_predictable, u32_log10_random, u32_log10_random_small}
int_log_bench! {u64, u64_log10_predictable, u64_log10_random, u64_log10_random_small}
int_log_bench! {u128, u128_log10_predictable, u128_log10_random, u128_log10_random_small}
int_log10_bench! {u8, u8_log10_predictable, u8_log10_random, u8_log10_random_small}
int_log10_bench! {u16, u16_log10_predictable, u16_log10_random, u16_log10_random_small}
int_log10_bench! {u32, u32_log10_predictable, u32_log10_random, u32_log10_random_small}
int_log10_bench! {u64, u64_log10_predictable, u64_log10_random, u64_log10_random_small}
int_log10_bench! {u128, u128_log10_predictable, u128_log10_random, u128_log10_random_small}
macro_rules! int_log_bench {
($t:ty, $random:ident, $random_small:ident, $geometric:ident) => {
#[bench]
fn $random(bench: &mut Bencher) {
let mut rng = crate::bench_rng();
/* Exponentially distributed random numbers from the whole range of the type. */
let numbers: Vec<$t> = (0..256)
.map(|_| {
let x = rng.gen::<$t>() >> rng.gen_range(0..<$t>::BITS);
if x >= 2 { x } else { 2 }
})
.collect();
bench.iter(|| {
for &b in &numbers {
for &x in &numbers {
black_box(black_box(x).ilog(b));
}
}
});
}
#[bench]
fn $random_small(bench: &mut Bencher) {
let mut rng = crate::bench_rng();
/* Exponentially distributed random numbers from the range 0..256. */
let numbers: Vec<$t> = (0..256)
.map(|_| {
let x = (rng.gen::<u8>() >> rng.gen_range(0..u8::BITS)) as $t;
if x >= 2 { x } else { 2 }
})
.collect();
bench.iter(|| {
for &b in &numbers {
for &x in &numbers {
black_box(black_box(x).ilog(b));
}
}
});
}
#[bench]
fn $geometric(bench: &mut Bencher) {
let bases: [$t; 16] = [2, 3, 4, 5, 7, 8, 9, 15, 16, 17, 31, 32, 33, 63, 64, 65];
let base_and_numbers: Vec<($t, Vec<$t>)> = bases
.iter()
.map(|&b| {
let numbers = (0..=<$t>::MAX.ilog(b)).map(|exp| b.pow(exp)).collect();
(b, numbers)
})
.collect();
bench.iter(|| {
for (b, numbers) in &base_and_numbers {
for &x in numbers {
black_box(black_box(x).ilog(black_box(*b)));
}
}
});
}
};
}
int_log_bench! {u8, u8_log_random, u8_log_random_small, u8_log_geometric}
int_log_bench! {u16, u16_log_random, u16_log_random_small, u16_log_geometric}
int_log_bench! {u32, u32_log_random, u32_log_random_small, u32_log_geometric}
int_log_bench! {u64, u64_log_random, u64_log_random_small, u64_log_geometric}
int_log_bench! {u128, u128_log_random, u128_log_random_small, u128_log_geometric}

18
library/core/src/num/int_macros.rs
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@ -3123,21 +3123,9 @@ pub const fn checked_ilog(self, base: Self) -> Option<u32> {
if self <= 0 || base <= 1 {
None
} else {
let mut n = 0;
let mut r = self;
// Optimization for 128 bit wide integers.
if Self::BITS == 128 {
let b = Self::ilog2(self) / (Self::ilog2(base) + 1);
n += b;
r /= base.pow(b as u32);
}
while r >= base {
r /= base;
n += 1;
}
Some(n)
// Delegate to the unsigned implementation.
// The condition makes sure that both casts are exact.
(self as $UnsignedT).checked_ilog(base as $UnsignedT)
}
}

19
library/core/src/num/uint_macros.rs
View File

@ -1095,18 +1095,25 @@ pub const fn checked_ilog(self, base: Self) -> Option<u32> {
None
} else {
let mut n = 0;
let mut r = self;
let mut r = 1;
// Optimization for 128 bit wide integers.
if Self::BITS == 128 {
let b = Self::ilog2(self) / (Self::ilog2(base) + 1);
n += b;
r /= base.pow(b as u32);
// The following is a correct lower bound for ⌊log(base,self)⌋ because
//
// log(base,self) = log(2,self) / log(2,base)
// ≥ ⌊log(2,self)⌋ / (⌊log(2,base)⌋ + 1)
//
// hence
//
// ⌊log(base,self)⌋ ≥ ⌊ ⌊log(2,self)⌋ / (⌊log(2,base)⌋ + 1) ⌋ .
n = self.ilog2() / (base.ilog2() + 1);
r = base.pow(n);
}
while r >= base {
r /= base;
while r <= self / base {
n += 1;
r *= base;
}
Some(n)
}