fn copy_from_slice(&mut self, src: &[T]) where T: Copy {
assert!(self.len() == src.len(),
"destination and source slices have different lengths");
- unsafe {
- ptr::copy_nonoverlapping(
- src.as_ptr(), self.as_mut_ptr(), self.len());
+ // First check if the amount of elements we want to copy is small:
+ // `copy_nonoverlapping` will do a memcopy, which involves an indirect
+ // function call when `memcpy` is in the dynamically-linked libc. For
+ // small elements (such as a single byte or pointer), the overhead is
+ // significant. If the element is big then the assignment is a memcopy
+ // anyway.
+ if self.len() == 1 {
+ self[0] = src[0];
+ } else {
+ unsafe {
+ ptr::copy_nonoverlapping(
+ src.as_ptr(), self.as_mut_ptr(), self.len());
+ }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Read for Cursor<T> where T: AsRef<[u8]> {
fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
- // First check if the amount of bytes we want to read is small: the read
- // in the else branch will end up calling `<&[u8] as Read>::read()`,
- // which will copy the buffer using a memcopy. If we only want to read a
- // single byte, then the overhead of the function call is significant.
- let num_read = {
- let mut inner_buf = self.fill_buf()?;
- if buf.len() == 1 && inner_buf.len() > 0 {
- buf[0] = inner_buf[0];
- 1
- } else {
- Read::read(&mut inner_buf, buf)?
- }
- };
- self.pos += num_read as u64;
- Ok(num_read)
+ let n = Read::read(&mut self.fill_buf()?, buf)?;
+ self.pos += n as u64;
+ Ok(n)
}
}