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Add small-copy optimization for copy_from_slice #37573

Merged
merged 3 commits into from
Dec 1, 2016
Merged

Add small-copy optimization for copy_from_slice #37573

merged 3 commits into from
Dec 1, 2016

Commits on Nov 30, 2016

  1. Add small-copy optimization for io::Cursor 

    During benchmarking, I found that one of my programs spent between 5 and
10 percent of the time doing memmoves. Ultimately I tracked these down
to single-byte slices being copied with a memcopy in io::Cursor::read().
Doing a manual copy if only one byte is requested can speed things up
significantly. For my program, this reduced the running time by 20%.

Why special-case only a single byte, and not a "small" slice in general?
I tried doing this for slices of at most 64 bytes and of at most 8
bytes. In both cases my test program was significantly slower.
    @ruuda
    ruuda committed Nov 30, 2016
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  2. 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.
    @ruuda
    ruuda committed Nov 30, 2016
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  3. Move small-copy optimization into <&[u8] as Read> 

    Based on the discussion in rust-lang#37573,
it is likely better to keep this limited to std::io, instead of
modifying a function which users expect to be a memcpy.
    @ruuda
    ruuda committed Nov 30, 2016
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