From 3be2c3b3092e934bdc2db67d5bdcabd611deca9c Mon Sep 17 00:00:00 2001 From: Ruud van Asseldonk Date: Sat, 12 Nov 2016 15:58:58 +0100 Subject: Move small-copy optimization into <&[u8] as Read> Based on the discussion in https://github.com/rust-lang/rust/pull/37573, it is likely better to keep this limited to std::io, instead of modifying a function which users expect to be a memcpy. --- src/libstd/io/impls.rs | 22 ++++++++++++++++++++-- 1 file changed, 20 insertions(+), 2 deletions(-) (limited to 'src/libstd') diff --git a/src/libstd/io/impls.rs b/src/libstd/io/impls.rs index 6b26c016638..f691289811b 100644 --- a/src/libstd/io/impls.rs +++ b/src/libstd/io/impls.rs @@ -157,7 +157,16 @@ impl<'a> Read for &'a [u8] { fn read(&mut self, buf: &mut [u8]) -> io::Result { 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) } @@ -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(()) } -- cgit 1.4.1-3-g733a5