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Simplify and document range layout computation

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This commit is contained in:
Oli Scherer 2022-11-07 10:26:14 +00:00
parent fcb1f1874f
commit d7f5d784d7
1 changed files with 11 additions and 13 deletions
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24
compiler/rustc_abi/src/layout.rs
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@ -382,28 +382,26 @@ fn layout_of_struct_or_enum<'a, V: Idx, F: Deref<Target = &'a LayoutS<V>> + Debu
let (start, end) = scalar_valid_range;
match st.abi {
Abi::Scalar(ref mut scalar) | Abi::ScalarPair(ref mut scalar, _) => {
// the asserts ensure that we are not using the
// `#[rustc_layout_scalar_valid_range(n)]`
// attribute to widen the range of anything as that would probably
// result in UB somewhere
// FIXME(eddyb) the asserts are probably not needed,
// as larger validity ranges would result in missed
// Enlarging validity ranges would result in missed
// optimizations, *not* wrongly assuming the inner
// value is valid. e.g. unions enlarge validity ranges,
// value is valid. e.g. unions already enlarge validity ranges,
// because the values may be uninitialized.
//
// Because of that we only check that the start and end
// of the range is representable with this scalar type.
let max_value = scalar.size(dl).unsigned_int_max();
if let Bound::Included(start) = start {
// FIXME(eddyb) this might be incorrect - it doesn't
// account for wrap-around (end < start) ranges.
let valid_range = scalar.valid_range_mut();
assert!(valid_range.start <= start);
valid_range.start = start;
assert!(start <= max_value, "{start} > {max_value}");
scalar.valid_range_mut().start = start;
}
if let Bound::Included(end) = end {
// FIXME(eddyb) this might be incorrect - it doesn't
// account for wrap-around (end < start) ranges.
let valid_range = scalar.valid_range_mut();
assert!(valid_range.end >= end);
valid_range.end = end;
assert!(end <= max_value, "{end} > {max_value}");
scalar.valid_range_mut().end = end;
}
// Update `largest_niche` if we have introduced a larger niche.