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Move small-copy optimization into copy_from_slice
Ultimately copy_from_slice is being a bottleneck, not io::Cursor::read. It might be worthwhile to move the check here, so more places can benefit from it.
1 parent cd7fade commit 3418052

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2 files changed

+16
-18
lines changed

2 files changed

+16
-18
lines changed

src/libcore/slice.rs

+13-3
Original file line numberDiff line numberDiff line change
@@ -515,9 +515,19 @@ impl<T> SliceExt for [T] {
515515
fn copy_from_slice(&mut self, src: &[T]) where T: Copy {
516516
assert!(self.len() == src.len(),
517517
"destination and source slices have different lengths");
518-
unsafe {
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ptr::copy_nonoverlapping(
520-
src.as_ptr(), self.as_mut_ptr(), self.len());
518+
// First check if the amount of elements we want to copy is small:
519+
// `copy_nonoverlapping` will do a memcopy, which involves an indirect
520+
// function call when `memcpy` is in the dynamically-linked libc. For
521+
// small elements (such as a single byte or pointer), the overhead is
522+
// significant. If the element is big then the assignment is a memcopy
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// anyway.
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if self.len() == 1 {
525+
self[0] = src[0];
526+
} else {
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unsafe {
528+
ptr::copy_nonoverlapping(
529+
src.as_ptr(), self.as_mut_ptr(), self.len());
530+
}
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}
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}
523533

src/libstd/io/cursor.rs

+3-15
Original file line numberDiff line numberDiff line change
@@ -219,21 +219,9 @@ impl<T> io::Seek for Cursor<T> where T: AsRef<[u8]> {
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T> Read for Cursor<T> where T: AsRef<[u8]> {
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fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
222-
// First check if the amount of bytes we want to read is small: the read
223-
// in the else branch will end up calling `<&[u8] as Read>::read()`,
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// which will copy the buffer using a memcopy. If we only want to read a
225-
// single byte, then the overhead of the function call is significant.
226-
let num_read = {
227-
let mut inner_buf = self.fill_buf()?;
228-
if buf.len() == 1 && inner_buf.len() > 0 {
229-
buf[0] = inner_buf[0];
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1
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} else {
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Read::read(&mut inner_buf, buf)?
233-
}
234-
};
235-
self.pos += num_read as u64;
236-
Ok(num_read)
222+
let n = Read::read(&mut self.fill_buf()?, buf)?;
223+
self.pos += n as u64;
224+
Ok(n)
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}
238226
}
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