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Improve autovectorization of to_lowercase / to_uppercase functions

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Refactor the code in the `convert_while_ascii` helper function to make
it more suitable for auto-vectorization and also process the full ascii
prefix of the string. The generic case conversion logic will only be
invoked starting from the first non-ascii character.

The runtime on microbenchmarks with ascii-only inputs improves between
1.5x for short and 4x for long inputs on x86_64 and aarch64.

The new implementation also encapsulates all unsafe inside the
`convert_while_ascii` function.

Fixes #123712
This commit is contained in:
Jörn Horstmann 2024-04-10 22:22:18 +02:00
parent 702987f75b
commit e393f56d37
4 changed files with 105 additions and 52 deletions
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2
library/alloc/benches/str.rs
View File

@ -347,3 +347,5 @@ mod $name {
make_test!(split_space_str, s, s.split(" ").count());
make_test!(split_ad_str, s, s.split("ad").count());
make_test!(to_lowercase, s, s.to_lowercase());

129
library/alloc/src/str.rs
View File

@ -9,6 +9,7 @@
use core::borrow::{Borrow, BorrowMut};
use core::iter::FusedIterator;
use core::mem::MaybeUninit;
#[stable(feature = "encode_utf16", since = "1.8.0")]
pub use core::str::EncodeUtf16;
#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
@ -365,14 +366,9 @@ pub fn replacen<P: Pattern>(&self, pat: P, to: &str, count: usize) -> String {
without modifying the original"]
#[stable(feature = "unicode_case_mapping", since = "1.2.0")]
pub fn to_lowercase(&self) -> String {
let out = convert_while_ascii(self.as_bytes(), u8::to_ascii_lowercase);
let (mut s, rest) = convert_while_ascii(self, u8::to_ascii_lowercase);
// Safety: we know this is a valid char boundary since
// out.len() is only progressed if ascii bytes are found
let rest = unsafe { self.get_unchecked(out.len()..) };
// Safety: We have written only valid ASCII to our vec
let mut s = unsafe { String::from_utf8_unchecked(out) };
let prefix_len = s.len();
for (i, c) in rest.char_indices() {
if c == 'Σ' {
@ -381,8 +377,7 @@ pub fn to_lowercase(&self) -> String {
// in `SpecialCasing.txt`,
// so hard-code it rather than have a generic "condition" mechanism.
// See https://github.com/rust-lang/rust/issues/26035
let out_len = self.len() - rest.len();
let sigma_lowercase = map_uppercase_sigma(&self, i + out_len);
let sigma_lowercase = map_uppercase_sigma(self, prefix_len + i);
s.push(sigma_lowercase);
} else {
match conversions::to_lower(c) {
@ -458,14 +453,7 @@ fn case_ignorable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
without modifying the original"]
#[stable(feature = "unicode_case_mapping", since = "1.2.0")]
pub fn to_uppercase(&self) -> String {
let out = convert_while_ascii(self.as_bytes(), u8::to_ascii_uppercase);
// Safety: we know this is a valid char boundary since
// out.len() is only progressed if ascii bytes are found
let rest = unsafe { self.get_unchecked(out.len()..) };
// Safety: We have written only valid ASCII to our vec
let mut s = unsafe { String::from_utf8_unchecked(out) };
let (mut s, rest) = convert_while_ascii(self, u8::to_ascii_uppercase);
for c in rest.chars() {
match conversions::to_upper(c) {
@ -614,50 +602,87 @@ pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
unsafe { Box::from_raw(Box::into_raw(v) as *mut str) }
}
/// Converts the bytes while the bytes are still ascii.
/// Converts leading ascii bytes in `s` by calling the `convert` function.
///
/// For better average performance, this happens in chunks of `2*size_of::<usize>()`.
/// Returns a vec with the converted bytes.
///
/// Returns a tuple of the converted prefix and the remainder starting from
/// the first non-ascii character.
///
/// This function is only public so that it can be verified in a codegen test,
/// see `issue-123712-str-to-lower-autovectorization.rs`.
#[unstable(feature = "str_internals", issue = "none")]
#[doc(hidden)]
#[inline]
#[cfg(not(test))]
#[cfg(not(no_global_oom_handling))]
fn convert_while_ascii(b: &[u8], convert: fn(&u8) -> u8) -> Vec<u8> {
let mut out = Vec::with_capacity(b.len());
pub fn convert_while_ascii(s: &str, convert: fn(&u8) -> u8) -> (String, &str) {
// Process the input in chunks of 16 bytes to enable auto-vectorization.
// Previously the chunk size depended on the size of `usize`,
// but on 32-bit platforms with sse or neon is also the better choice.
// The only downside on other platforms would be a bit more loop-unrolling.
const N: usize = 16;
const USIZE_SIZE: usize = mem::size_of::<usize>();
const MAGIC_UNROLL: usize = 2;
const N: usize = USIZE_SIZE * MAGIC_UNROLL;
const NONASCII_MASK: usize = usize::from_ne_bytes([0x80; USIZE_SIZE]);
let mut slice = s.as_bytes();
let mut out = Vec::with_capacity(slice.len());
let mut out_slice = out.spare_capacity_mut();
let mut i = 0;
unsafe {
while i + N <= b.len() {
// Safety: we have checks the sizes `b` and `out` to know that our
let in_chunk = b.get_unchecked(i..i + N);
let out_chunk = out.spare_capacity_mut().get_unchecked_mut(i..i + N);
let mut ascii_prefix_len = 0_usize;
let mut is_ascii = [false; N];
let mut bits = 0;
for j in 0..MAGIC_UNROLL {
// read the bytes 1 usize at a time (unaligned since we haven't checked the alignment)
// safety: in_chunk is valid bytes in the range
bits |= in_chunk.as_ptr().cast::<usize>().add(j).read_unaligned();
}
// if our chunks aren't ascii, then return only the prior bytes as init
if bits & NONASCII_MASK != 0 {
break;
}
while slice.len() >= N {
// SAFETY: checked in loop condition
let chunk = unsafe { slice.get_unchecked(..N) };
// SAFETY: out_slice has at least same length as input slice and gets sliced with the same offsets
let out_chunk = unsafe { out_slice.get_unchecked_mut(..N) };
// perform the case conversions on N bytes (gets heavily autovec'd)
for j in 0..N {
// safety: in_chunk and out_chunk is valid bytes in the range
let out = out_chunk.get_unchecked_mut(j);
out.write(convert(in_chunk.get_unchecked(j)));
}
// mark these bytes as initialised
i += N;
for j in 0..N {
is_ascii[j] = chunk[j] <= 127;
}
out.set_len(i);
// Auto-vectorization for this check is a bit fragile, sum and comparing against the chunk
// size gives the best result, specifically a pmovmsk instruction on x86.
// See https://github.com/llvm/llvm-project/issues/96395 for why llvm currently does not
// currently recognize other similar idioms.
if is_ascii.iter().map(|x| *x as u8).sum::<u8>() as usize != N {
break;
}
for j in 0..N {
out_chunk[j] = MaybeUninit::new(convert(&chunk[j]));
}
ascii_prefix_len += N;
slice = unsafe { slice.get_unchecked(N..) };
out_slice = unsafe { out_slice.get_unchecked_mut(N..) };
}
out
// handle the remainder as individual bytes
while slice.len() > 0 {
let byte = slice[0];
if byte > 127 {
break;
}
// SAFETY: out_slice has at least same length as input slice
unsafe {
*out_slice.get_unchecked_mut(0) = MaybeUninit::new(convert(&byte));
}
ascii_prefix_len += 1;
slice = unsafe { slice.get_unchecked(1..) };
out_slice = unsafe { out_slice.get_unchecked_mut(1..) };
}
unsafe {
// SAFETY: ascii_prefix_len bytes have been initialized above
out.set_len(ascii_prefix_len);
// SAFETY: We have written only valid ascii to the output vec
let ascii_string = String::from_utf8_unchecked(out);
// SAFETY: we know this is a valid char boundary
// since we only skipped over leading ascii bytes
let rest = core::str::from_utf8_unchecked(slice);
(ascii_string, rest)
}
}

3
library/alloc/tests/str.rs
View File

@ -1854,7 +1854,10 @@ fn to_lowercase() {
assert_eq!("ΑΣ''Α".to_lowercase(), "ασ''α");
// https://github.com/rust-lang/rust/issues/124714
// input lengths around the boundary of the chunk size used by the ascii prefix optimization
assert_eq!("abcdefghijklmnoΣ".to_lowercase(), "abcdefghijklmnoς");
assert_eq!("abcdefghijklmnopΣ".to_lowercase(), "abcdefghijklmnopς");
assert_eq!("abcdefghijklmnopqΣ".to_lowercase(), "abcdefghijklmnopqς");
// a really long string that has it's lowercase form
// even longer. this tests that implementations don't assume

23
tests/codegen/issues/issue-123712-str-to-lower-autovectorization.rs Normal file
View File

@ -0,0 +1,23 @@
//@ only-x86_64
//@ compile-flags: -C opt-level=3
#![crate_type = "lib"]
#![no_std]
#![feature(str_internals)]
extern crate alloc;
/// Ensure that the ascii-prefix loop for `str::to_lowercase` and `str::to_uppercase` uses vector
/// instructions.
///
/// The llvm ir should be the same for all targets that support some form of simd. Only targets
/// without any simd instructions would see scalarized ir.
/// Unfortunately, there is no `only-simd` directive to only run this test on only such platforms,
/// and using test revisions would still require the core libraries for all platforms.
// CHECK-LABEL: @lower_while_ascii
// CHECK: [[A:%[0-9]]] = load <16 x i8>
// CHECK-NEXT: [[B:%[0-9]]] = icmp slt <16 x i8> [[A]], zeroinitializer
// CHECK-NEXT: [[C:%[0-9]]] = bitcast <16 x i1> [[B]] to i16
#[no_mangle]
pub fn lower_while_ascii(s: &str) -> (alloc::string::String, &str) {
alloc::str::convert_while_ascii(s, u8::to_ascii_lowercase)
}