Rollup merge of rust-lang#52854 - RalfJung:memrchr, r=Kimundi

fix memrchr in miri

The previous PR rust-lang#52744 was not enough because it assumed that the split between the `mid` and `end` parts returned by `align_to` was aligned. But really the only guarantee we have is that the `mid` part is aligned, so make use of that.

src/libcore/slice/memchr.rs

@@ -100,24 +100,30 @@ pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
  // - the first remaining bytes, < 2 word size
  let len = text.len();
  let ptr = text.as_ptr();
- let usize_bytes = mem::size_of::<usize>();
+ type Chunk = usize;
 
- let mut offset = {
- // We call this just to obtain the length of the suffix
- let (_, _, suffix) = unsafe { text.align_to::<usize>() };
- len - suffix.len()
+ let (min_aligned_offset, max_aligned_offset) = {
+ // We call this just to obtain the length of the prefix and suffix.
+ // In the middle we always process two chunks at once.
+ let (prefix, _, suffix) = unsafe { text.align_to::<(Chunk, Chunk)>() };
+ (prefix.len(), len - suffix.len())
  };
+
+ let mut offset = max_aligned_offset;
  if let Some(index) = text[offset..].iter().rposition(|elt| *elt == x) {
  return Some(offset + index);
  }
 
- // search the body of the text
+ // search the body of the text, make sure we don't cross min_aligned_offset.
+ // offset is always aligned, so just testing `>` is sufficient and avoids possible
+ // overflow.
  let repeated_x = repeat_byte(x);
+ let chunk_bytes = mem::size_of::<Chunk>();
 
- while offset >= 2 * usize_bytes {
+ while offset > min_aligned_offset {
  unsafe {
- let u = *(ptr.offset(offset as isize - 2 * usize_bytes as isize) as *const usize);
- let v = *(ptr.offset(offset as isize - usize_bytes as isize) as *const usize);
+ let u = *(ptr.offset(offset as isize - 2 * chunk_bytes as isize) as *const Chunk);
+ let v = *(ptr.offset(offset as isize - chunk_bytes as isize) as *const Chunk);
 
  // break if there is a matching byte
  let zu = contains_zero_byte(u ^ repeated_x);
@@ -126,7 +132,7 @@ pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
  break;
  }
  }
- offset -= 2 * usize_bytes;
+ offset -= 2 * chunk_bytes;
  }
 
  // find the byte before the point the body loop stopped