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explain why prepare_relocation_copy works the way it does

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This commit is contained in:
Ralf Jung 2022-04-17 19:43:02 -04:00
parent 3ec1febbf5
commit 29cc8ec2d1
1 changed files with 11 additions and 4 deletions
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15
compiler/rustc_middle/src/mir/interpret/allocation.rs
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@ -567,8 +567,10 @@ impl<Tag> Deref for Relocations<Tag> {
}
/// A partial, owned list of relocations to transfer into another allocation.
///
/// Offsets are already adjusted to the destination allocation.
pub struct AllocationRelocations<Tag> {
relative_relocations: Vec<(Size, Tag)>,
dest_relocations: Vec<(Size, Tag)>,
}
impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
@ -581,12 +583,17 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
) -> AllocationRelocations<Tag> {
let relocations = self.get_relocations(cx, src);
if relocations.is_empty() {
return AllocationRelocations { relative_relocations: Vec::new() };
return AllocationRelocations { dest_relocations: Vec::new() };
}
let size = src.size;
let mut new_relocations = Vec::with_capacity(relocations.len() * (count as usize));
// If `count` is large, this is rather wasteful -- we are allocating a big array here, which
// is mostly filled with redundant information since it's just N copies of the same `Tag`s
// at slightly adjusted offsets. The reason we do this is so that in `mark_relocation_range`
// we can use `insert_presorted`. That wouldn't work with an `Iterator` that just produces
// the right sequence of relocations for all N copies.
for i in 0..count {
new_relocations.extend(relocations.iter().map(|&(offset, reloc)| {
// compute offset for current repetition
@ -599,7 +606,7 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
}));
}
AllocationRelocations { relative_relocations: new_relocations }
AllocationRelocations { dest_relocations: new_relocations }
}
/// Applies a relocation copy.
@ -609,7 +616,7 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
/// This is dangerous to use as it can violate internal `Allocation` invariants!
/// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
pub fn mark_relocation_range(&mut self, relocations: AllocationRelocations<Tag>) {
self.relocations.0.insert_presorted(relocations.relative_relocations);
self.relocations.0.insert_presorted(relocations.dest_relocations);
}
}