bjoernager/rust
bjoernager/rust
1
Fork 0
Code Issues Pull requests Activity

rustc_abi: audit uses of is_zst; fix a case of giving an enum insufficient alignment

Browse source
This commit is contained in:
Ralf Jung 2023-08-27 18:01:05 +02:00
parent 0360b6740b
commit 0da9409e08
5 changed files with 76 additions and 7 deletions
Download patch file Download diff file Expand all files Collapse all files

25
compiler/rustc_abi/src/layout.rs
View file

@ -157,8 +157,10 @@ pub trait LayoutCalculator {
// for non-ZST uninhabited data (mostly partial initialization).
let absent = |fields: &IndexSlice<FieldIdx, Layout<'_>>| {
let uninhabited = fields.iter().any(|f| f.abi().is_uninhabited());
let is_zst = fields.iter().all(|f| f.0.is_zst());
uninhabited && is_zst
// We cannot ignore alignment; that might lead us to entirely discard a variant and
// produce an enum that is less aligned than it should be!
let is_1zst = fields.iter().all(|f| f.0.is_1zst());
uninhabited && is_1zst
};
let (present_first, present_second) = {
let mut present_variants = variants
@ -358,8 +360,11 @@ pub trait LayoutCalculator {
match layout.fields {
FieldsShape::Arbitrary { ref mut offsets, .. } => {
for (j, offset) in offsets.iter_enumerated_mut() {
// keep ZST at offset 0 to simplify Scalar/ScalarPair layout
if !variants[i][j].0.is_zst() {
*offset += this_offset;
} else {
debug_assert_eq!(offset.bytes(), 0);
}
}
}
@ -504,7 +509,7 @@ pub trait LayoutCalculator {
// to make room for a larger discriminant.
for field_idx in st.fields.index_by_increasing_offset() {
let field = &field_layouts[FieldIdx::from_usize(field_idx)];
if !field.0.is_zst() || field.align().abi.bytes() != 1 {
if !field.0.is_1zst() {
start_align = start_align.min(field.align().abi);
break;
}
@ -603,12 +608,15 @@ pub trait LayoutCalculator {
abi = Abi::Scalar(tag);
} else {
// Try to use a ScalarPair for all tagged enums.
// That's possible only if we can find a common primitive type for all variants.
let mut common_prim = None;
let mut common_prim_initialized_in_all_variants = true;
for (field_layouts, layout_variant) in iter::zip(variants, &layout_variants) {
let FieldsShape::Arbitrary { ref offsets, .. } = layout_variant.fields else {
panic!();
};
// We skip *all* ZST here and later check if we are good in terms of alignment.
// This lets us handle some cases involving aligned ZST.
let mut fields = iter::zip(field_layouts, offsets).filter(|p| !p.0.0.is_zst());
let (field, offset) = match (fields.next(), fields.next()) {
(None, None) => {
@ -954,8 +962,10 @@ fn univariant(
};
(
// Place ZSTs first to avoid "interesting offsets", especially with only one
// or two non-ZST fields. This helps Scalar/ScalarPair layouts.
// Place ZSTs first to avoid "interesting offsets", especially with only
// one or two non-ZST fields. This helps Scalar/ScalarPair layouts. Note
// that these can ignore even some aligned ZST as long as the alignment
// is less than that of the scalar, hence we treat *all* ZST like that.
!f.0.is_zst(),
// Then place largest alignments first.
cmp::Reverse(alignment_group_key(f)),
@ -1073,9 +1083,10 @@ fn univariant(
let size = min_size.align_to(align.abi);
let mut layout_of_single_non_zst_field = None;
let mut abi = Abi::Aggregate { sized };
// Unpack newtype ABIs and find scalar pairs.
// Try to make this a Scalar/ScalarPair.
if sized && size.bytes() > 0 {
// All other fields must be ZSTs.
// We skip *all* ZST here and later check if we are good in terms of alignment.
// This lets us handle some cases involving aligned ZST.
let mut non_zst_fields = fields.iter_enumerated().filter(|&(_, f)| !f.0.is_zst());
match (non_zst_fields.next(), non_zst_fields.next(), non_zst_fields.next()) {