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BufWriter: avoid using expensive Vec methods

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We use a Vec as our internal, constant-sized buffer, but the overhead of
using methods like `extend_from_slice` can be enormous, likely because
they don't get inlined, because `Vec` has to repeat bounds checks that
we've already done, and because it makes considerations for things like
reallocating, even though they should never happen.
This commit is contained in:
Tyson Nottingham 2020-12-10 21:34:28 -08:00
parent 1f32d40ac3
commit b43e8e248b
1 changed files with 75 additions and 12 deletions
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87
library/std/src/io/buffered/bufwriter.rs
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@ -4,6 +4,7 @@ use crate::io::{
self, Error, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
};
use crate::mem;
use crate::ptr;
/// Wraps a writer and buffers its output.
///
@ -68,6 +69,10 @@ use crate::mem;
#[stable(feature = "rust1", since = "1.0.0")]
pub struct BufWriter<W: Write> {
inner: Option<W>,
// The buffer. Avoid using this like a normal `Vec` in common code paths.
// That is, don't use `buf.push`, `buf.extend_from_slice`, or any other
// methods that require bounds checking or the like. This makes an enormous
// difference to performance (we may want to stop using a `Vec` entirely).
buf: Vec<u8>,
// #30888: If the inner writer panics in a call to write, we don't want to
// write the buffered data a second time in BufWriter's destructor. This
@ -150,7 +155,11 @@ impl<W: Write> BufWriter<W> {
impl Drop for BufGuard<'_> {
fn drop(&mut self) {
if self.written > 0 {
self.buffer.drain(..self.written);
if self.done() {
self.buffer.clear();
} else {
self.buffer.drain(..self.written);
}
}
}
}
@ -183,7 +192,12 @@ impl<W: Write> BufWriter<W> {
pub(super) fn write_to_buf(&mut self, buf: &[u8]) -> usize {
let available = self.buf.capacity() - self.buf.len();
let amt_to_buffer = available.min(buf.len());
self.buf.extend_from_slice(&buf[..amt_to_buffer]);
// SAFETY: `amt_to_buffer` is <= buffer's spare capacity by construction.
unsafe {
self.write_to_buffer_unchecked(&buf[..amt_to_buffer]);
}
amt_to_buffer
}
@ -348,7 +362,13 @@ impl<W: Write> BufWriter<W> {
self.panicked = false;
r
} else {
self.buf.extend_from_slice(buf);
// SAFETY: We just called `self.flush_buf()`, so `self.buf.len()` is 0, and
// we entered this else block because `buf.len() < self.buf.capacity()`.
// Therefore, `self.buf.len() + buf.len() <= self.buf.capacity()`.
unsafe {
self.write_to_buffer_unchecked(buf);
}
Ok(buf.len())
}
}
@ -373,10 +393,29 @@ impl<W: Write> BufWriter<W> {
self.panicked = false;
r
} else {
self.buf.extend_from_slice(buf);
// SAFETY: We just called `self.flush_buf()`, so `self.buf.len()` is 0, and
// we entered this else block because `buf.len() < self.buf.capacity()`.
// Therefore, `self.buf.len() + buf.len() <= self.buf.capacity()`.
unsafe {
self.write_to_buffer_unchecked(buf);
}
Ok(())
}
}
// SAFETY: Requires `self.buf.len() + buf.len() <= self.buf.capacity()`,
// i.e., that input buffer length is less than or equal to spare capacity.
#[inline(always)]
unsafe fn write_to_buffer_unchecked(&mut self, buf: &[u8]) {
debug_assert!(self.buf.len() + buf.len() <= self.buf.capacity());
let old_len = self.buf.len();
let buf_len = buf.len();
let src = buf.as_ptr();
let dst = self.buf.as_mut_ptr().add(old_len);
ptr::copy_nonoverlapping(src, dst, buf_len);
self.buf.set_len(old_len + buf_len);
}
}
#[unstable(feature = "bufwriter_into_raw_parts", issue = "80690")]
@ -456,7 +495,11 @@ impl<W: Write> Write for BufWriter<W> {
// prevents pathological cases for other clients which *always* make writes of this size.
// See #72919 and #79930 for more info and a breadcrumb trail.
if self.buf.len() + buf.len() <= self.buf.capacity() && buf.len() != self.buf.capacity() {
self.buf.extend_from_slice(buf);
// SAFETY: safe by above conditional.
unsafe {
self.write_to_buffer_unchecked(buf);
}
Ok(buf.len())
} else {
self.flush_and_write(buf)
@ -471,7 +514,11 @@ impl<W: Write> Write for BufWriter<W> {
// prevents pathological cases for other clients which *always* make writes of this size.
// See #72919 and #79930 for more info and a breadcrumb trail.
if self.buf.len() + buf.len() <= self.buf.capacity() && buf.len() != self.buf.capacity() {
self.buf.extend_from_slice(buf);
// SAFETY: safe by above conditional.
unsafe {
self.write_to_buffer_unchecked(buf);
}
Ok(())
} else {
self.flush_and_write_all(buf)
@ -492,7 +539,13 @@ impl<W: Write> Write for BufWriter<W> {
self.panicked = false;
r
} else {
bufs.iter().for_each(|b| self.buf.extend_from_slice(b));
// SAFETY: We checked whether or not the spare capacity was large enough above. If
// it was, then we're safe already. If it wasn't, we flushed, making sufficient
// room for any input <= the buffer size, which includes this input.
unsafe {
bufs.iter().for_each(|b| self.write_to_buffer_unchecked(b));
};
Ok(total_len)
}
} else {
@ -511,7 +564,13 @@ impl<W: Write> Write for BufWriter<W> {
self.panicked = false;
return r;
} else {
self.buf.extend_from_slice(buf);
// SAFETY: We checked whether or not the spare capacity was large enough above.
// If it was, then we're safe already. If it wasn't, we flushed, making
// sufficient room for any input <= the buffer size, which includes this input.
unsafe {
self.write_to_buffer_unchecked(buf);
}
buf.len()
}
} else {
@ -519,11 +578,15 @@ impl<W: Write> Write for BufWriter<W> {
};
debug_assert!(total_written != 0);
for buf in iter {
if self.buf.len() + buf.len() > self.buf.capacity() {
break;
} else {
self.buf.extend_from_slice(buf);
if self.buf.len() + buf.len() <= self.buf.capacity() {
// SAFETY: safe by above conditional.
unsafe {
self.write_to_buffer_unchecked(buf);
}
total_written += buf.len();
} else {
break;
}
}
Ok(total_written)