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Add small-copy optimization for copy_from_slice #37573

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Merged
merged 3 commits into from
Dec 1, 2016
Merged

Add small-copy optimization for copy_from_slice #37573

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cd7fade
Add small-copy optimization for io::Cursor
ruuda Nov 2, 2016
3418052
Move small-copy optimization into copy_from_slice
ruuda Nov 3, 2016
3be2c3b
Move small-copy optimization into <&[u8] as Read>
ruuda Nov 12, 2016
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22 changes: 20 additions & 2 deletions src/libstd/io/impls.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -157,7 +157,16 @@ impl<'a> Read for &'a [u8] {
fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
let amt = cmp::min(buf.len(), self.len());
let (a, b) = self.split_at(amt);
buf[..amt].copy_from_slice(a);

// First check if the amount of bytes we want to read is small:
// `copy_from_slice` will generally expand to a call to `memcpy`, and
// for a single byte the overhead is significant.
if amt == 1 {
buf[0] = a[0];
} else {
buf[..amt].copy_from_slice(a);
}

*self = b;
Ok(amt)
}
Expand All @@ -169,7 +178,16 @@ impl<'a> Read for &'a [u8] {
"failed to fill whole buffer"));
}
let (a, b) = self.split_at(buf.len());
buf.copy_from_slice(a);

// First check if the amount of bytes we want to read is small:
// `copy_from_slice` will generally expand to a call to `memcpy`, and
// for a single byte the overhead is significant.
if buf.len() == 1 {
buf[0] = a[0];
} else {
buf.copy_from_slice(a);
}

*self = b;
Ok(())
}
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