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make memrchr use align_offset #52744

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Jul 28, 2018
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make memrchr use align_offset #52744

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make memrchr use align_offset
RalfJung Jul 26, 2018
3bc59b5
use slice::align_to
RalfJung Jul 27, 2018
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18 changes: 17 additions & 1 deletion src/libcore/slice/memchr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -102,8 +102,24 @@ pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
let ptr = text.as_ptr();
let usize_bytes = mem::size_of::<usize>();

// a version of align_offset that says how much must be *subtracted*
// from a pointer to be aligned.
#[inline(always)]
fn align_offset_down(ptr: *const u8, align: usize) -> usize {
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@nagisa nagisa Jul 26, 2018 •
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Consider slice::align_to::<usize>()? I believe that does exactly what a large part of this code does.

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Hm, plausible. Should I then also do that for memchr?

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Actually it's not so easy, we want to do the main loop on a u8 slice, not usize (or actually pairs of usize, which is the steps that the loop is taking).

let align_offset = ptr.align_offset(align);
if align_offset > align {
// Not possible to align
usize::max_value()
} else if align_offset == 0 {
0
} else {
// E.g. if align=8 and we have to add 1, then we can also subtract 7.
align - align_offset
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You probably want (align - align_offset) & (align - 1) (or % align if you're sure it will optimize) instead of the last if/else. Not sure about the first one though, seems kinda fine as-is.

}
}

// search to an aligned boundary
let end_align = (ptr as usize + len) & (usize_bytes - 1);
let end_align = align_offset_down(unsafe { ptr.offset(len as isize) }, usize_bytes);
let mut offset;
if end_align > 0 {
offset = if end_align >= len { 0 } else { len - end_align };
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