rustc_abi/layout.rs
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use std::fmt::{self, Write};
use std::ops::{Bound, Deref};
use std::{cmp, iter};
use rustc_index::Idx;
use tracing::debug;
use crate::{
Abi, AbiAndPrefAlign, Align, FieldsShape, HasDataLayout, IndexSlice, IndexVec, Integer,
LayoutS, Niche, NonZeroUsize, Primitive, ReprOptions, Scalar, Size, StructKind, TagEncoding,
Variants, WrappingRange,
};
#[cfg(feature = "nightly")]
mod ty;
#[cfg(feature = "nightly")]
pub use ty::{FIRST_VARIANT, FieldIdx, Layout, TyAbiInterface, TyAndLayout, VariantIdx};
// A variant is absent if it's uninhabited and only has ZST fields.
// Present uninhabited variants only require space for their fields,
// but *not* an encoding of the discriminant (e.g., a tag value).
// See issue #49298 for more details on the need to leave space
// for non-ZST uninhabited data (mostly partial initialization).
fn absent<'a, FieldIdx, VariantIdx, F>(fields: &IndexSlice<FieldIdx, F>) -> bool
where
FieldIdx: Idx,
VariantIdx: Idx,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
{
let uninhabited = fields.iter().any(|f| f.abi.is_uninhabited());
// 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.is_1zst());
uninhabited && is_1zst
}
/// Determines towards which end of a struct layout optimizations will try to place the best niches.
enum NicheBias {
Start,
End,
}
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum LayoutCalculatorError<F> {
/// An unsized type was found in a location where a sized type was expected.
///
/// This is not always a compile error, for example if there is a `[T]: Sized`
/// bound in a where clause.
///
/// Contains the field that was unexpectedly unsized.
UnexpectedUnsized(F),
/// A type was too large for the target platform.
SizeOverflow,
/// A union had no fields.
EmptyUnion,
/// The fields or variants have irreconcilable reprs
ReprConflict,
}
impl<F> LayoutCalculatorError<F> {
pub fn without_payload(&self) -> LayoutCalculatorError<()> {
match self {
LayoutCalculatorError::UnexpectedUnsized(_) => {
LayoutCalculatorError::UnexpectedUnsized(())
}
LayoutCalculatorError::SizeOverflow => LayoutCalculatorError::SizeOverflow,
LayoutCalculatorError::EmptyUnion => LayoutCalculatorError::EmptyUnion,
LayoutCalculatorError::ReprConflict => LayoutCalculatorError::ReprConflict,
}
}
/// Format an untranslated diagnostic for this type
///
/// Intended for use by rust-analyzer, as neither it nor `rustc_abi` depend on fluent infra.
pub fn fallback_fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str(match self {
LayoutCalculatorError::UnexpectedUnsized(_) => {
"an unsized type was found where a sized type was expected"
}
LayoutCalculatorError::SizeOverflow => "size overflow",
LayoutCalculatorError::EmptyUnion => "type is a union with no fields",
LayoutCalculatorError::ReprConflict => "type has an invalid repr",
})
}
}
type LayoutCalculatorResult<FieldIdx, VariantIdx, F> =
Result<LayoutS<FieldIdx, VariantIdx>, LayoutCalculatorError<F>>;
#[derive(Clone, Copy, Debug)]
pub struct LayoutCalculator<Cx> {
pub cx: Cx,
}
impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
pub fn new(cx: Cx) -> Self {
Self { cx }
}
pub fn scalar_pair<FieldIdx: Idx, VariantIdx: Idx>(
&self,
a: Scalar,
b: Scalar,
) -> LayoutS<FieldIdx, VariantIdx> {
let dl = self.cx.data_layout();
let b_align = b.align(dl);
let align = a.align(dl).max(b_align).max(dl.aggregate_align);
let b_offset = a.size(dl).align_to(b_align.abi);
let size = (b_offset + b.size(dl)).align_to(align.abi);
// HACK(nox): We iter on `b` and then `a` because `max_by_key`
// returns the last maximum.
let largest_niche = Niche::from_scalar(dl, b_offset, b)
.into_iter()
.chain(Niche::from_scalar(dl, Size::ZERO, a))
.max_by_key(|niche| niche.available(dl));
LayoutS {
variants: Variants::Single { index: VariantIdx::new(0) },
fields: FieldsShape::Arbitrary {
offsets: [Size::ZERO, b_offset].into(),
memory_index: [0, 1].into(),
},
abi: Abi::ScalarPair(a, b),
largest_niche,
align,
size,
max_repr_align: None,
unadjusted_abi_align: align.abi,
}
}
pub fn univariant<
'a,
FieldIdx: Idx,
VariantIdx: Idx,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
>(
&self,
fields: &IndexSlice<FieldIdx, F>,
repr: &ReprOptions,
kind: StructKind,
) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
let dl = self.cx.data_layout();
let layout = self.univariant_biased(fields, repr, kind, NicheBias::Start);
// Enums prefer niches close to the beginning or the end of the variants so that other
// (smaller) data-carrying variants can be packed into the space after/before the niche.
// If the default field ordering does not give us a niche at the front then we do a second
// run and bias niches to the right and then check which one is closer to one of the
// struct's edges.
if let Ok(layout) = &layout {
// Don't try to calculate an end-biased layout for unsizable structs,
// otherwise we could end up with different layouts for
// Foo<Type> and Foo<dyn Trait> which would break unsizing.
if !matches!(kind, StructKind::MaybeUnsized) {
if let Some(niche) = layout.largest_niche {
let head_space = niche.offset.bytes();
let niche_len = niche.value.size(dl).bytes();
let tail_space = layout.size.bytes() - head_space - niche_len;
// This may end up doing redundant work if the niche is already in the last
// field (e.g. a trailing bool) and there is tail padding. But it's non-trivial
// to get the unpadded size so we try anyway.
if fields.len() > 1 && head_space != 0 && tail_space > 0 {
let alt_layout = self
.univariant_biased(fields, repr, kind, NicheBias::End)
.expect("alt layout should always work");
let alt_niche = alt_layout
.largest_niche
.expect("alt layout should have a niche like the regular one");
let alt_head_space = alt_niche.offset.bytes();
let alt_niche_len = alt_niche.value.size(dl).bytes();
let alt_tail_space =
alt_layout.size.bytes() - alt_head_space - alt_niche_len;
debug_assert_eq!(layout.size.bytes(), alt_layout.size.bytes());
let prefer_alt_layout =
alt_head_space > head_space && alt_head_space > tail_space;
debug!(
"sz: {}, default_niche_at: {}+{}, default_tail_space: {}, alt_niche_at/head_space: {}+{}, alt_tail: {}, num_fields: {}, better: {}\n\
layout: {}\n\
alt_layout: {}\n",
layout.size.bytes(),
head_space,
niche_len,
tail_space,
alt_head_space,
alt_niche_len,
alt_tail_space,
layout.fields.count(),
prefer_alt_layout,
self.format_field_niches(layout, fields),
self.format_field_niches(&alt_layout, fields),
);
if prefer_alt_layout {
return Ok(alt_layout);
}
}
}
}
}
layout
}
pub fn layout_of_never_type<FieldIdx: Idx, VariantIdx: Idx>(
&self,
) -> LayoutS<FieldIdx, VariantIdx> {
let dl = self.cx.data_layout();
LayoutS {
variants: Variants::Single { index: VariantIdx::new(0) },
fields: FieldsShape::Primitive,
abi: Abi::Uninhabited,
largest_niche: None,
align: dl.i8_align,
size: Size::ZERO,
max_repr_align: None,
unadjusted_abi_align: dl.i8_align.abi,
}
}
pub fn layout_of_struct_or_enum<
'a,
FieldIdx: Idx,
VariantIdx: Idx,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
>(
&self,
repr: &ReprOptions,
variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
is_enum: bool,
is_unsafe_cell: bool,
scalar_valid_range: (Bound<u128>, Bound<u128>),
discr_range_of_repr: impl Fn(i128, i128) -> (Integer, bool),
discriminants: impl Iterator<Item = (VariantIdx, i128)>,
dont_niche_optimize_enum: bool,
always_sized: bool,
) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
let (present_first, present_second) = {
let mut present_variants = variants
.iter_enumerated()
.filter_map(|(i, v)| if !repr.c() && absent(v) { None } else { Some(i) });
(present_variants.next(), present_variants.next())
};
let present_first = match present_first {
Some(present_first) => present_first,
// Uninhabited because it has no variants, or only absent ones.
None if is_enum => {
return Ok(self.layout_of_never_type());
}
// If it's a struct, still compute a layout so that we can still compute the
// field offsets.
None => VariantIdx::new(0),
};
// take the struct path if it is an actual struct
if !is_enum ||
// or for optimizing univariant enums
(present_second.is_none() && !repr.inhibit_enum_layout_opt())
{
self.layout_of_struct(
repr,
variants,
is_enum,
is_unsafe_cell,
scalar_valid_range,
always_sized,
present_first,
)
} else {
// At this point, we have handled all unions and
// structs. (We have also handled univariant enums
// that allow representation optimization.)
assert!(is_enum);
self.layout_of_enum(
repr,
variants,
discr_range_of_repr,
discriminants,
dont_niche_optimize_enum,
)
}
}
pub fn layout_of_union<
'a,
FieldIdx: Idx,
VariantIdx: Idx,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
>(
&self,
repr: &ReprOptions,
variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
let dl = self.cx.data_layout();
let mut align = if repr.pack.is_some() { dl.i8_align } else { dl.aggregate_align };
let mut max_repr_align = repr.align;
// If all the non-ZST fields have the same ABI and union ABI optimizations aren't
// disabled, we can use that common ABI for the union as a whole.
struct AbiMismatch;
let mut common_non_zst_abi_and_align = if repr.inhibits_union_abi_opt() {
// Can't optimize
Err(AbiMismatch)
} else {
Ok(None)
};
let mut size = Size::ZERO;
let only_variant_idx = VariantIdx::new(0);
let only_variant = &variants[only_variant_idx];
for field in only_variant {
if field.is_unsized() {
return Err(LayoutCalculatorError::UnexpectedUnsized(*field));
}
align = align.max(field.align);
max_repr_align = max_repr_align.max(field.max_repr_align);
size = cmp::max(size, field.size);
if field.is_zst() {
// Nothing more to do for ZST fields
continue;
}
if let Ok(common) = common_non_zst_abi_and_align {
// Discard valid range information and allow undef
let field_abi = field.abi.to_union();
if let Some((common_abi, common_align)) = common {
if common_abi != field_abi {
// Different fields have different ABI: disable opt
common_non_zst_abi_and_align = Err(AbiMismatch);
} else {
// Fields with the same non-Aggregate ABI should also
// have the same alignment
if !matches!(common_abi, Abi::Aggregate { .. }) {
assert_eq!(
common_align, field.align.abi,
"non-Aggregate field with matching ABI but differing alignment"
);
}
}
} else {
// First non-ZST field: record its ABI and alignment
common_non_zst_abi_and_align = Ok(Some((field_abi, field.align.abi)));
}
}
}
if let Some(pack) = repr.pack {
align = align.min(AbiAndPrefAlign::new(pack));
}
// The unadjusted ABI alignment does not include repr(align), but does include repr(pack).
// See documentation on `LayoutS::unadjusted_abi_align`.
let unadjusted_abi_align = align.abi;
if let Some(repr_align) = repr.align {
align = align.max(AbiAndPrefAlign::new(repr_align));
}
// `align` must not be modified after this, or `unadjusted_abi_align` could be inaccurate.
let align = align;
// If all non-ZST fields have the same ABI, we may forward that ABI
// for the union as a whole, unless otherwise inhibited.
let abi = match common_non_zst_abi_and_align {
Err(AbiMismatch) | Ok(None) => Abi::Aggregate { sized: true },
Ok(Some((abi, _))) => {
if abi.inherent_align(dl).map(|a| a.abi) != Some(align.abi) {
// Mismatched alignment (e.g. union is #[repr(packed)]): disable opt
Abi::Aggregate { sized: true }
} else {
abi
}
}
};
let Some(union_field_count) = NonZeroUsize::new(only_variant.len()) else {
return Err(LayoutCalculatorError::EmptyUnion);
};
Ok(LayoutS {
variants: Variants::Single { index: only_variant_idx },
fields: FieldsShape::Union(union_field_count),
abi,
largest_niche: None,
align,
size: size.align_to(align.abi),
max_repr_align,
unadjusted_abi_align,
})
}
/// single-variant enums are just structs, if you think about it
fn layout_of_struct<
'a,
FieldIdx: Idx,
VariantIdx: Idx,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
>(
&self,
repr: &ReprOptions,
variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
is_enum: bool,
is_unsafe_cell: bool,
scalar_valid_range: (Bound<u128>, Bound<u128>),
always_sized: bool,
present_first: VariantIdx,
) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
// Struct, or univariant enum equivalent to a struct.
// (Typechecking will reject discriminant-sizing attrs.)
let dl = self.cx.data_layout();
let v = present_first;
let kind = if is_enum || variants[v].is_empty() || always_sized {
StructKind::AlwaysSized
} else {
StructKind::MaybeUnsized
};
let mut st = self.univariant(&variants[v], repr, kind)?;
st.variants = Variants::Single { index: v };
if is_unsafe_cell {
let hide_niches = |scalar: &mut _| match scalar {
Scalar::Initialized { value, valid_range } => {
*valid_range = WrappingRange::full(value.size(dl))
}
// Already doesn't have any niches
Scalar::Union { .. } => {}
};
match &mut st.abi {
Abi::Uninhabited => {}
Abi::Scalar(scalar) => hide_niches(scalar),
Abi::ScalarPair(a, b) => {
hide_niches(a);
hide_niches(b);
}
Abi::Vector { element, count: _ } => hide_niches(element),
Abi::Aggregate { sized: _ } => {}
}
st.largest_niche = None;
return Ok(st);
}
let (start, end) = scalar_valid_range;
match st.abi {
Abi::Scalar(ref mut scalar) | Abi::ScalarPair(ref mut scalar, _) => {
// Enlarging validity ranges would result in missed
// optimizations, *not* wrongly assuming the inner
// 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.
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.
assert!(end <= max_value, "{end} > {max_value}");
scalar.valid_range_mut().end = end;
}
// Update `largest_niche` if we have introduced a larger niche.
let niche = Niche::from_scalar(dl, Size::ZERO, *scalar);
if let Some(niche) = niche {
match st.largest_niche {
Some(largest_niche) => {
// Replace the existing niche even if they're equal,
// because this one is at a lower offset.
if largest_niche.available(dl) <= niche.available(dl) {
st.largest_niche = Some(niche);
}
}
None => st.largest_niche = Some(niche),
}
}
}
_ => assert!(
start == Bound::Unbounded && end == Bound::Unbounded,
"nonscalar layout for layout_scalar_valid_range type: {st:#?}",
),
}
Ok(st)
}
fn layout_of_enum<
'a,
FieldIdx: Idx,
VariantIdx: Idx,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
>(
&self,
repr: &ReprOptions,
variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
discr_range_of_repr: impl Fn(i128, i128) -> (Integer, bool),
discriminants: impl Iterator<Item = (VariantIdx, i128)>,
dont_niche_optimize_enum: bool,
) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
// Until we've decided whether to use the tagged or
// niche filling LayoutS, we don't want to intern the
// variant layouts, so we can't store them in the
// overall LayoutS. Store the overall LayoutS
// and the variant LayoutSs here until then.
struct TmpLayout<FieldIdx: Idx, VariantIdx: Idx> {
layout: LayoutS<FieldIdx, VariantIdx>,
variants: IndexVec<VariantIdx, LayoutS<FieldIdx, VariantIdx>>,
}
let dl = self.cx.data_layout();
// bail if the enum has an incoherent repr that cannot be computed
if repr.packed() {
return Err(LayoutCalculatorError::ReprConflict);
}
let calculate_niche_filling_layout = || -> Option<TmpLayout<FieldIdx, VariantIdx>> {
if dont_niche_optimize_enum {
return None;
}
if variants.len() < 2 {
return None;
}
let mut align = dl.aggregate_align;
let mut max_repr_align = repr.align;
let mut unadjusted_abi_align = align.abi;
let mut variant_layouts = variants
.iter_enumerated()
.map(|(j, v)| {
let mut st = self.univariant(v, repr, StructKind::AlwaysSized).ok()?;
st.variants = Variants::Single { index: j };
align = align.max(st.align);
max_repr_align = max_repr_align.max(st.max_repr_align);
unadjusted_abi_align = unadjusted_abi_align.max(st.unadjusted_abi_align);
Some(st)
})
.collect::<Option<IndexVec<VariantIdx, _>>>()?;
let largest_variant_index = variant_layouts
.iter_enumerated()
.max_by_key(|(_i, layout)| layout.size.bytes())
.map(|(i, _layout)| i)?;
let all_indices = variants.indices();
let needs_disc =
|index: VariantIdx| index != largest_variant_index && !absent(&variants[index]);
let niche_variants = all_indices.clone().find(|v| needs_disc(*v)).unwrap()
..=all_indices.rev().find(|v| needs_disc(*v)).unwrap();
let count =
(niche_variants.end().index() as u128 - niche_variants.start().index() as u128) + 1;
// Use the largest niche in the largest variant.
let niche = variant_layouts[largest_variant_index].largest_niche?;
let (niche_start, niche_scalar) = niche.reserve(dl, count)?;
let niche_offset = niche.offset;
let niche_size = niche.value.size(dl);
let size = variant_layouts[largest_variant_index].size.align_to(align.abi);
let all_variants_fit = variant_layouts.iter_enumerated_mut().all(|(i, layout)| {
if i == largest_variant_index {
return true;
}
layout.largest_niche = None;
if layout.size <= niche_offset {
// This variant will fit before the niche.
return true;
}
// Determine if it'll fit after the niche.
let this_align = layout.align.abi;
let this_offset = (niche_offset + niche_size).align_to(this_align);
if this_offset + layout.size > size {
return false;
}
// It'll fit, but we need to make some adjustments.
match layout.fields {
FieldsShape::Arbitrary { ref mut offsets, .. } => {
for offset in offsets.iter_mut() {
*offset += this_offset;
}
}
FieldsShape::Primitive | FieldsShape::Array { .. } | FieldsShape::Union(..) => {
panic!("Layout of fields should be Arbitrary for variants")
}
}
// It can't be a Scalar or ScalarPair because the offset isn't 0.
if !layout.abi.is_uninhabited() {
layout.abi = Abi::Aggregate { sized: true };
}
layout.size += this_offset;
true
});
if !all_variants_fit {
return None;
}
let largest_niche = Niche::from_scalar(dl, niche_offset, niche_scalar);
let others_zst = variant_layouts
.iter_enumerated()
.all(|(i, layout)| i == largest_variant_index || layout.size == Size::ZERO);
let same_size = size == variant_layouts[largest_variant_index].size;
let same_align = align == variant_layouts[largest_variant_index].align;
let abi = if variant_layouts.iter().all(|v| v.abi.is_uninhabited()) {
Abi::Uninhabited
} else if same_size && same_align && others_zst {
match variant_layouts[largest_variant_index].abi {
// When the total alignment and size match, we can use the
// same ABI as the scalar variant with the reserved niche.
Abi::Scalar(_) => Abi::Scalar(niche_scalar),
Abi::ScalarPair(first, second) => {
// Only the niche is guaranteed to be initialised,
// so use union layouts for the other primitive.
if niche_offset == Size::ZERO {
Abi::ScalarPair(niche_scalar, second.to_union())
} else {
Abi::ScalarPair(first.to_union(), niche_scalar)
}
}
_ => Abi::Aggregate { sized: true },
}
} else {
Abi::Aggregate { sized: true }
};
let layout = LayoutS {
variants: Variants::Multiple {
tag: niche_scalar,
tag_encoding: TagEncoding::Niche {
untagged_variant: largest_variant_index,
niche_variants,
niche_start,
},
tag_field: 0,
variants: IndexVec::new(),
},
fields: FieldsShape::Arbitrary {
offsets: [niche_offset].into(),
memory_index: [0].into(),
},
abi,
largest_niche,
size,
align,
max_repr_align,
unadjusted_abi_align,
};
Some(TmpLayout { layout, variants: variant_layouts })
};
let niche_filling_layout = calculate_niche_filling_layout();
let (mut min, mut max) = (i128::MAX, i128::MIN);
let discr_type = repr.discr_type();
let bits = Integer::from_attr(dl, discr_type).size().bits();
for (i, mut val) in discriminants {
if !repr.c() && variants[i].iter().any(|f| f.abi.is_uninhabited()) {
continue;
}
if discr_type.is_signed() {
// sign extend the raw representation to be an i128
val = (val << (128 - bits)) >> (128 - bits);
}
if val < min {
min = val;
}
if val > max {
max = val;
}
}
// We might have no inhabited variants, so pretend there's at least one.
if (min, max) == (i128::MAX, i128::MIN) {
min = 0;
max = 0;
}
assert!(min <= max, "discriminant range is {min}...{max}");
let (min_ity, signed) = discr_range_of_repr(min, max); //Integer::repr_discr(tcx, ty, &repr, min, max);
let mut align = dl.aggregate_align;
let mut max_repr_align = repr.align;
let mut unadjusted_abi_align = align.abi;
let mut size = Size::ZERO;
// We're interested in the smallest alignment, so start large.
let mut start_align = Align::from_bytes(256).unwrap();
assert_eq!(Integer::for_align(dl, start_align), None);
// repr(C) on an enum tells us to make a (tag, union) layout,
// so we need to grow the prefix alignment to be at least
// the alignment of the union. (This value is used both for
// determining the alignment of the overall enum, and the
// determining the alignment of the payload after the tag.)
let mut prefix_align = min_ity.align(dl).abi;
if repr.c() {
for fields in variants {
for field in fields {
prefix_align = prefix_align.max(field.align.abi);
}
}
}
// Create the set of structs that represent each variant.
let mut layout_variants = variants
.iter_enumerated()
.map(|(i, field_layouts)| {
let mut st = self.univariant(
field_layouts,
repr,
StructKind::Prefixed(min_ity.size(), prefix_align),
)?;
st.variants = Variants::Single { index: i };
// Find the first field we can't move later
// to make room for a larger discriminant.
for field_idx in st.fields.index_by_increasing_offset() {
let field = &field_layouts[FieldIdx::new(field_idx)];
if !field.is_1zst() {
start_align = start_align.min(field.align.abi);
break;
}
}
size = cmp::max(size, st.size);
align = align.max(st.align);
max_repr_align = max_repr_align.max(st.max_repr_align);
unadjusted_abi_align = unadjusted_abi_align.max(st.unadjusted_abi_align);
Ok(st)
})
.collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
// Align the maximum variant size to the largest alignment.
size = size.align_to(align.abi);
// FIXME(oli-obk): deduplicate and harden these checks
if size.bytes() >= dl.obj_size_bound() {
return Err(LayoutCalculatorError::SizeOverflow);
}
let typeck_ity = Integer::from_attr(dl, repr.discr_type());
if typeck_ity < min_ity {
// It is a bug if Layout decided on a greater discriminant size than typeck for
// some reason at this point (based on values discriminant can take on). Mostly
// because this discriminant will be loaded, and then stored into variable of
// type calculated by typeck. Consider such case (a bug): typeck decided on
// byte-sized discriminant, but layout thinks we need a 16-bit to store all
// discriminant values. That would be a bug, because then, in codegen, in order
// to store this 16-bit discriminant into 8-bit sized temporary some of the
// space necessary to represent would have to be discarded (or layout is wrong
// on thinking it needs 16 bits)
panic!(
"layout decided on a larger discriminant type ({min_ity:?}) than typeck ({typeck_ity:?})"
);
// However, it is fine to make discr type however large (as an optimisation)
// after this point – we’ll just truncate the value we load in codegen.
}
// Check to see if we should use a different type for the
// discriminant. We can safely use a type with the same size
// as the alignment of the first field of each variant.
// We increase the size of the discriminant to avoid LLVM copying
// padding when it doesn't need to. This normally causes unaligned
// load/stores and excessive memcpy/memset operations. By using a
// bigger integer size, LLVM can be sure about its contents and
// won't be so conservative.
// Use the initial field alignment
let mut ity = if repr.c() || repr.int.is_some() {
min_ity
} else {
Integer::for_align(dl, start_align).unwrap_or(min_ity)
};
// If the alignment is not larger than the chosen discriminant size,
// don't use the alignment as the final size.
if ity <= min_ity {
ity = min_ity;
} else {
// Patch up the variants' first few fields.
let old_ity_size = min_ity.size();
let new_ity_size = ity.size();
for variant in &mut layout_variants {
match variant.fields {
FieldsShape::Arbitrary { ref mut offsets, .. } => {
for i in offsets {
if *i <= old_ity_size {
assert_eq!(*i, old_ity_size);
*i = new_ity_size;
}
}
// We might be making the struct larger.
if variant.size <= old_ity_size {
variant.size = new_ity_size;
}
}
FieldsShape::Primitive | FieldsShape::Array { .. } | FieldsShape::Union(..) => {
panic!("encountered a non-arbitrary layout during enum layout")
}
}
}
}
let tag_mask = ity.size().unsigned_int_max();
let tag = Scalar::Initialized {
value: Primitive::Int(ity, signed),
valid_range: WrappingRange {
start: (min as u128 & tag_mask),
end: (max as u128 & tag_mask),
},
};
let mut abi = Abi::Aggregate { sized: true };
if layout_variants.iter().all(|v| v.abi.is_uninhabited()) {
abi = Abi::Uninhabited;
} else if tag.size(dl) == size {
// Make sure we only use scalar layout when the enum is entirely its
// own tag (i.e. it has no padding nor any non-ZST variant fields).
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!("encountered a non-arbitrary layout during enum layout");
};
// 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.is_zst());
let (field, offset) = match (fields.next(), fields.next()) {
(None, None) => {
common_prim_initialized_in_all_variants = false;
continue;
}
(Some(pair), None) => pair,
_ => {
common_prim = None;
break;
}
};
let prim = match field.abi {
Abi::Scalar(scalar) => {
common_prim_initialized_in_all_variants &=
matches!(scalar, Scalar::Initialized { .. });
scalar.primitive()
}
_ => {
common_prim = None;
break;
}
};
if let Some((old_prim, common_offset)) = common_prim {
// All variants must be at the same offset
if offset != common_offset {
common_prim = None;
break;
}
// This is pretty conservative. We could go fancier
// by realising that (u8, u8) could just cohabit with
// u16 or even u32.
let new_prim = match (old_prim, prim) {
// Allow all identical primitives.
(x, y) if x == y => x,
// Allow integers of the same size with differing signedness.
// We arbitrarily choose the signedness of the first variant.
(p @ Primitive::Int(x, _), Primitive::Int(y, _)) if x == y => p,
// Allow integers mixed with pointers of the same layout.
// We must represent this using a pointer, to avoid
// roundtripping pointers through ptrtoint/inttoptr.
(p @ Primitive::Pointer(_), i @ Primitive::Int(..))
| (i @ Primitive::Int(..), p @ Primitive::Pointer(_))
if p.size(dl) == i.size(dl) && p.align(dl) == i.align(dl) =>
{
p
}
_ => {
common_prim = None;
break;
}
};
// We may be updating the primitive here, for example from int->ptr.
common_prim = Some((new_prim, common_offset));
} else {
common_prim = Some((prim, offset));
}
}
if let Some((prim, offset)) = common_prim {
let prim_scalar = if common_prim_initialized_in_all_variants {
let size = prim.size(dl);
assert!(size.bits() <= 128);
Scalar::Initialized { value: prim, valid_range: WrappingRange::full(size) }
} else {
// Common prim might be uninit.
Scalar::Union { value: prim }
};
let pair = self.scalar_pair::<FieldIdx, VariantIdx>(tag, prim_scalar);
let pair_offsets = match pair.fields {
FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
assert_eq!(memory_index.raw, [0, 1]);
offsets
}
_ => panic!("encountered a non-arbitrary layout during enum layout"),
};
if pair_offsets[FieldIdx::new(0)] == Size::ZERO
&& pair_offsets[FieldIdx::new(1)] == *offset
&& align == pair.align
&& size == pair.size
{
// We can use `ScalarPair` only when it matches our
// already computed layout (including `#[repr(C)]`).
abi = pair.abi;
}
}
}
// If we pick a "clever" (by-value) ABI, we might have to adjust the ABI of the
// variants to ensure they are consistent. This is because a downcast is
// semantically a NOP, and thus should not affect layout.
if matches!(abi, Abi::Scalar(..) | Abi::ScalarPair(..)) {
for variant in &mut layout_variants {
// We only do this for variants with fields; the others are not accessed anyway.
// Also do not overwrite any already existing "clever" ABIs.
if variant.fields.count() > 0 && matches!(variant.abi, Abi::Aggregate { .. }) {
variant.abi = abi;
// Also need to bump up the size and alignment, so that the entire value fits
// in here.
variant.size = cmp::max(variant.size, size);
variant.align.abi = cmp::max(variant.align.abi, align.abi);
}
}
}
let largest_niche = Niche::from_scalar(dl, Size::ZERO, tag);
let tagged_layout = LayoutS {
variants: Variants::Multiple {
tag,
tag_encoding: TagEncoding::Direct,
tag_field: 0,
variants: IndexVec::new(),
},
fields: FieldsShape::Arbitrary {
offsets: [Size::ZERO].into(),
memory_index: [0].into(),
},
largest_niche,
abi,
align,
size,
max_repr_align,
unadjusted_abi_align,
};
let tagged_layout = TmpLayout { layout: tagged_layout, variants: layout_variants };
let mut best_layout = match (tagged_layout, niche_filling_layout) {
(tl, Some(nl)) => {
// Pick the smaller layout; otherwise,
// pick the layout with the larger niche; otherwise,
// pick tagged as it has simpler codegen.
use cmp::Ordering::*;
let niche_size = |tmp_l: &TmpLayout<FieldIdx, VariantIdx>| {
tmp_l.layout.largest_niche.map_or(0, |n| n.available(dl))
};
match (tl.layout.size.cmp(&nl.layout.size), niche_size(&tl).cmp(&niche_size(&nl))) {
(Greater, _) => nl,
(Equal, Less) => nl,
_ => tl,
}
}
(tl, None) => tl,
};
// Now we can intern the variant layouts and store them in the enum layout.
best_layout.layout.variants = match best_layout.layout.variants {
Variants::Multiple { tag, tag_encoding, tag_field, .. } => {
Variants::Multiple { tag, tag_encoding, tag_field, variants: best_layout.variants }
}
Variants::Single { .. } => {
panic!("encountered a single-variant enum during multi-variant layout")
}
};
Ok(best_layout.layout)
}
fn univariant_biased<
'a,
FieldIdx: Idx,
VariantIdx: Idx,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug + Copy,
>(
&self,
fields: &IndexSlice<FieldIdx, F>,
repr: &ReprOptions,
kind: StructKind,
niche_bias: NicheBias,
) -> LayoutCalculatorResult<FieldIdx, VariantIdx, F> {
let dl = self.cx.data_layout();
let pack = repr.pack;
let mut align = if pack.is_some() { dl.i8_align } else { dl.aggregate_align };
let mut max_repr_align = repr.align;
let mut inverse_memory_index: IndexVec<u32, FieldIdx> = fields.indices().collect();
let optimize_field_order = !repr.inhibit_struct_field_reordering();
if optimize_field_order && fields.len() > 1 {
let end =
if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
let optimizing = &mut inverse_memory_index.raw[..end];
let fields_excluding_tail = &fields.raw[..end];
// If `-Z randomize-layout` was enabled for the type definition we can shuffle
// the field ordering to try and catch some code making assumptions about layouts
// we don't guarantee.
if repr.can_randomize_type_layout() && cfg!(feature = "randomize") {
#[cfg(feature = "randomize")]
{
use rand::SeedableRng;
use rand::seq::SliceRandom;
// `ReprOptions.field_shuffle_seed` is a deterministic seed we can use to randomize field
// ordering.
let mut rng =
rand_xoshiro::Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed);
// Shuffle the ordering of the fields.
optimizing.shuffle(&mut rng);
}
// Otherwise we just leave things alone and actually optimize the type's fields
} else {
// To allow unsizing `&Foo<Type>` -> `&Foo<dyn Trait>`, the layout of the struct must
// not depend on the layout of the tail.
let max_field_align =
fields_excluding_tail.iter().map(|f| f.align.abi.bytes()).max().unwrap_or(1);
let largest_niche_size = fields_excluding_tail
.iter()
.filter_map(|f| f.largest_niche)
.map(|n| n.available(dl))
.max()
.unwrap_or(0);
// Calculates a sort key to group fields by their alignment or possibly some
// size-derived pseudo-alignment.
let alignment_group_key = |layout: &F| {
// The two branches here return values that cannot be meaningfully compared with
// each other. However, we know that consistently for all executions of
// `alignment_group_key`, one or the other branch will be taken, so this is okay.
if let Some(pack) = pack {
// Return the packed alignment in bytes.
layout.align.abi.min(pack).bytes()
} else {
// Returns `log2(effective-align)`. The calculation assumes that size is an
// integer multiple of align, except for ZSTs.
let align = layout.align.abi.bytes();
let size = layout.size.bytes();
let niche_size = layout.largest_niche.map(|n| n.available(dl)).unwrap_or(0);
// Group [u8; 4] with align-4 or [u8; 6] with align-2 fields.
let size_as_align = align.max(size).trailing_zeros();
let size_as_align = if largest_niche_size > 0 {
match niche_bias {
// Given `A(u8, [u8; 16])` and `B(bool, [u8; 16])` we want to bump the
// array to the front in the first case (for aligned loads) but keep
// the bool in front in the second case for its niches.
NicheBias::Start => {
max_field_align.trailing_zeros().min(size_as_align)
}
// When moving niches towards the end of the struct then for
// A((u8, u8, u8, bool), (u8, bool, u8)) we want to keep the first tuple
// in the align-1 group because its bool can be moved closer to the end.
NicheBias::End if niche_size == largest_niche_size => {
align.trailing_zeros()
}
NicheBias::End => size_as_align,
}
} else {
size_as_align
};
size_as_align as u64
}
};
match kind {
StructKind::AlwaysSized | StructKind::MaybeUnsized => {
// Currently `LayoutS` only exposes a single niche so sorting is usually
// sufficient to get one niche into the preferred position. If it ever
// supported multiple niches then a more advanced pick-and-pack approach could
// provide better results. But even for the single-niche cache it's not
// optimal. E.g. for A(u32, (bool, u8), u16) it would be possible to move the
// bool to the front but it would require packing the tuple together with the
// u16 to build a 4-byte group so that the u32 can be placed after it without
// padding. This kind of packing can't be achieved by sorting.
optimizing.sort_by_key(|&x| {
let f = &fields[x];
let field_size = f.size.bytes();
let niche_size = f.largest_niche.map_or(0, |n| n.available(dl));
let niche_size_key = match niche_bias {
// large niche first
NicheBias::Start => !niche_size,
// large niche last
NicheBias::End => niche_size,
};
let inner_niche_offset_key = match niche_bias {
NicheBias::Start => f.largest_niche.map_or(0, |n| n.offset.bytes()),
NicheBias::End => f.largest_niche.map_or(0, |n| {
!(field_size - n.value.size(dl).bytes() - n.offset.bytes())
}),
};
(
// Then place largest alignments first.
cmp::Reverse(alignment_group_key(f)),
// Then prioritize niche placement within alignment group according to
// `niche_bias_start`.
niche_size_key,
// Then among fields with equally-sized niches prefer the ones
// closer to the start/end of the field.
inner_niche_offset_key,
)
});
}
StructKind::Prefixed(..) => {
// Sort in ascending alignment so that the layout stays optimal
// regardless of the prefix.
// And put the largest niche in an alignment group at the end
// so it can be used as discriminant in jagged enums
optimizing.sort_by_key(|&x| {
let f = &fields[x];
let niche_size = f.largest_niche.map_or(0, |n| n.available(dl));
(alignment_group_key(f), niche_size)
});
}
}
// FIXME(Kixiron): We can always shuffle fields within a given alignment class
// regardless of the status of `-Z randomize-layout`
}
}
// inverse_memory_index holds field indices by increasing memory offset.
// That is, if field 5 has offset 0, the first element of inverse_memory_index is 5.
// We now write field offsets to the corresponding offset slot;
// field 5 with offset 0 puts 0 in offsets[5].
// At the bottom of this function, we invert `inverse_memory_index` to
// produce `memory_index` (see `invert_mapping`).
let mut unsized_field = None::<&F>;
let mut offsets = IndexVec::from_elem(Size::ZERO, fields);
let mut offset = Size::ZERO;
let mut largest_niche = None;
let mut largest_niche_available = 0;
if let StructKind::Prefixed(prefix_size, prefix_align) = kind {
let prefix_align =
if let Some(pack) = pack { prefix_align.min(pack) } else { prefix_align };
align = align.max(AbiAndPrefAlign::new(prefix_align));
offset = prefix_size.align_to(prefix_align);
}
for &i in &inverse_memory_index {
let field = &fields[i];
if let Some(unsized_field) = unsized_field {
return Err(LayoutCalculatorError::UnexpectedUnsized(*unsized_field));
}
if field.is_unsized() {
if let StructKind::MaybeUnsized = kind {
unsized_field = Some(field);
} else {
return Err(LayoutCalculatorError::UnexpectedUnsized(*field));
}
}
// Invariant: offset < dl.obj_size_bound() <= 1<<61
let field_align = if let Some(pack) = pack {
field.align.min(AbiAndPrefAlign::new(pack))
} else {
field.align
};
offset = offset.align_to(field_align.abi);
align = align.max(field_align);
max_repr_align = max_repr_align.max(field.max_repr_align);
debug!("univariant offset: {:?} field: {:#?}", offset, field);
offsets[i] = offset;
if let Some(mut niche) = field.largest_niche {
let available = niche.available(dl);
// Pick up larger niches.
let prefer_new_niche = match niche_bias {
NicheBias::Start => available > largest_niche_available,
// if there are several niches of the same size then pick the last one
NicheBias::End => available >= largest_niche_available,
};
if prefer_new_niche {
largest_niche_available = available;
niche.offset += offset;
largest_niche = Some(niche);
}
}
offset =
offset.checked_add(field.size, dl).ok_or(LayoutCalculatorError::SizeOverflow)?;
}
// The unadjusted ABI alignment does not include repr(align), but does include repr(pack).
// See documentation on `LayoutS::unadjusted_abi_align`.
let unadjusted_abi_align = align.abi;
if let Some(repr_align) = repr.align {
align = align.max(AbiAndPrefAlign::new(repr_align));
}
// `align` must not be modified after this point, or `unadjusted_abi_align` could be inaccurate.
let align = align;
debug!("univariant min_size: {:?}", offset);
let min_size = offset;
// As stated above, inverse_memory_index holds field indices by increasing offset.
// This makes it an already-sorted view of the offsets vec.
// To invert it, consider:
// If field 5 has offset 0, offsets[0] is 5, and memory_index[5] should be 0.
// Field 5 would be the first element, so memory_index is i:
// Note: if we didn't optimize, it's already right.
let memory_index = if optimize_field_order {
inverse_memory_index.invert_bijective_mapping()
} else {
debug_assert!(inverse_memory_index.iter().copied().eq(fields.indices()));
inverse_memory_index.into_iter().map(|it| it.index() as u32).collect()
};
let size = min_size.align_to(align.abi);
// FIXME(oli-obk): deduplicate and harden these checks
if size.bytes() >= dl.obj_size_bound() {
return Err(LayoutCalculatorError::SizeOverflow);
}
let mut layout_of_single_non_zst_field = None;
let sized = unsized_field.is_none();
let mut abi = Abi::Aggregate { sized };
let optimize_abi = !repr.inhibit_newtype_abi_optimization();
// Try to make this a Scalar/ScalarPair.
if sized && size.bytes() > 0 {
// 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.is_zst());
match (non_zst_fields.next(), non_zst_fields.next(), non_zst_fields.next()) {
// We have exactly one non-ZST field.
(Some((i, field)), None, None) => {
layout_of_single_non_zst_field = Some(field);
// Field fills the struct and it has a scalar or scalar pair ABI.
if offsets[i].bytes() == 0 && align.abi == field.align.abi && size == field.size
{
match field.abi {
// For plain scalars, or vectors of them, we can't unpack
// newtypes for `#[repr(C)]`, as that affects C ABIs.
Abi::Scalar(_) | Abi::Vector { .. } if optimize_abi => {
abi = field.abi;
}
// But scalar pairs are Rust-specific and get
// treated as aggregates by C ABIs anyway.
Abi::ScalarPair(..) => {
abi = field.abi;
}
_ => {}
}
}
}
// Two non-ZST fields, and they're both scalars.
(Some((i, a)), Some((j, b)), None) => {
match (a.abi, b.abi) {
(Abi::Scalar(a), Abi::Scalar(b)) => {
// Order by the memory placement, not source order.
let ((i, a), (j, b)) = if offsets[i] < offsets[j] {
((i, a), (j, b))
} else {
((j, b), (i, a))
};
let pair = self.scalar_pair::<FieldIdx, VariantIdx>(a, b);
let pair_offsets = match pair.fields {
FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
assert_eq!(memory_index.raw, [0, 1]);
offsets
}
FieldsShape::Primitive
| FieldsShape::Array { .. }
| FieldsShape::Union(..) => {
panic!("encountered a non-arbitrary layout during enum layout")
}
};
if offsets[i] == pair_offsets[FieldIdx::new(0)]
&& offsets[j] == pair_offsets[FieldIdx::new(1)]
&& align == pair.align
&& size == pair.size
{
// We can use `ScalarPair` only when it matches our
// already computed layout (including `#[repr(C)]`).
abi = pair.abi;
}
}
_ => {}
}
}
_ => {}
}
}
if fields.iter().any(|f| f.abi.is_uninhabited()) {
abi = Abi::Uninhabited;
}
let unadjusted_abi_align = if repr.transparent() {
match layout_of_single_non_zst_field {
Some(l) => l.unadjusted_abi_align,
None => {
// `repr(transparent)` with all ZST fields.
align.abi
}
}
} else {
unadjusted_abi_align
};
Ok(LayoutS {
variants: Variants::Single { index: VariantIdx::new(0) },
fields: FieldsShape::Arbitrary { offsets, memory_index },
abi,
largest_niche,
align,
size,
max_repr_align,
unadjusted_abi_align,
})
}
fn format_field_niches<
'a,
FieldIdx: Idx,
VariantIdx: Idx,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
>(
&self,
layout: &LayoutS<FieldIdx, VariantIdx>,
fields: &IndexSlice<FieldIdx, F>,
) -> String {
let dl = self.cx.data_layout();
let mut s = String::new();
for i in layout.fields.index_by_increasing_offset() {
let offset = layout.fields.offset(i);
let f = &fields[FieldIdx::new(i)];
write!(s, "[o{}a{}s{}", offset.bytes(), f.align.abi.bytes(), f.size.bytes()).unwrap();
if let Some(n) = f.largest_niche {
write!(
s,
" n{}b{}s{}",
n.offset.bytes(),
n.available(dl).ilog2(),
n.value.size(dl).bytes()
)
.unwrap();
}
write!(s, "] ").unwrap();
}
s
}
}