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memrchr: Correct aligned offset computation #35969

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merged 2 commits into from
Aug 27, 2016
Merged

memrchr: Correct aligned offset computation #35969

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d1ecee9
memrchr: Correct aligned offset computation
bluss Aug 24, 2016
8295c50
memrchr: Use a conditional instead of subtracting a complicated min
bluss Aug 24, 2016
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24 changes: 23 additions & 1 deletion src/libstd/memchr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -209,7 +209,7 @@ mod fallback {
let end_align = (ptr as usize + len) & (usize_bytes - 1);
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Preexisting, but this uses size_of to calculate usize_bytes, but alignment of usize on a system is not size_of::<usize> but rather align_of::<usize>? Seems like a pessimisation?

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It doesn't seem like a big deal. I prefer a separate PR to deal with this. Let's have the fix here be minimal, especially if we backport it.

let mut offset;
if end_align > 0 {
offset = len - cmp::min(usize_bytes - end_align, len);
offset = if end_align >= len { 0 } else { len - end_align };
if let Some(index) = text[offset..].iter().rposition(|elt| *elt == x) {
return Some(offset + index);
}
Expand Down Expand Up @@ -309,6 +309,17 @@ mod fallback {
fn no_match_reversed() {
assert_eq!(None, memrchr(b'a', b"xyz"));
}

#[test]
fn each_alignment_reversed() {
let mut data = [1u8; 64];
let needle = 2;
let pos = 40;
data[pos] = needle;
for start in 0..16 {
assert_eq!(Some(pos - start), memrchr(needle, &data[start..]));
}
}
}

#[cfg(test)]
Expand Down Expand Up @@ -385,4 +396,15 @@ mod tests {
fn no_match_reversed() {
assert_eq!(None, memrchr(b'a', b"xyz"));
}

#[test]
fn each_alignment() {
let mut data = [1u8; 64];
let needle = 2;
let pos = 40;
data[pos] = needle;
for start in 0..16 {
assert_eq!(Some(pos - start), memchr(needle, &data[start..]));
}
}
}