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Extract fn layout_of_enum

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This commit is contained in:
Jubilee Young 2023-12-16 01:44:13 -08:00
parent f116bc6e27
commit b525f76bb5
1 changed files with 496 additions and 476 deletions
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972
compiler/rustc_abi/src/layout.rs
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@ -291,482 +291,15 @@ pub trait LayoutCalculator {
// structs. (We have also handled univariant enums
// that allow representation optimization.)
assert!(is_enum);
// 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 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(dl, v, repr, StructKind::AlwaysSized)?;
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;
// Find the field with the largest niche
let (field_index, niche, (niche_start, niche_scalar)) = variants[largest_variant_index]
.iter()
.enumerate()
.filter_map(|(j, field)| Some((j, field.largest_niche?)))
.max_by_key(|(_, niche)| niche.available(dl))
.and_then(|(j, niche)| Some((j, niche, niche.reserve(dl, count)?)))?;
let niche_offset =
niche.offset + variant_layouts[largest_variant_index].fields.offset(field_index);
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 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(
dl,
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);
Some(st)
})
.collect::<Option<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 None;
}
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(pair) = common_prim {
// This is pretty conservative. We could go fancier
// by conflating things like i32 and u32, or even
// realising that (u8, u8) could just cohabit with
// u16 or even u32.
if pair != (prim, offset) {
common_prim = None;
break;
}
} 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")
}
};
Some(best_layout.layout)
layout_of_enum(
self,
repr,
variants,
discr_range_of_repr,
discriminants,
dont_niche_optimize_enum,
dl,
)
}
fn layout_of_union<
@ -874,6 +407,493 @@ pub trait LayoutCalculator {
}
}
fn layout_of_enum<'a, LC, FieldIdx: Idx, VariantIdx: Idx, F>(
layout_calc: &LC,
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,
dl: &TargetDataLayout,
) -> Option<LayoutS<FieldIdx, VariantIdx>>
where
LC: LayoutCalculator + ?Sized,
F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
{
// 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 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 = layout_calc.univariant(dl, v, repr, StructKind::AlwaysSized)?;
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;
// Find the field with the largest niche
let (field_index, niche, (niche_start, niche_scalar)) = variants[largest_variant_index]
.iter()
.enumerate()
.filter_map(|(j, field)| Some((j, field.largest_niche?)))
.max_by_key(|(_, niche)| niche.available(dl))
.and_then(|(j, niche)| Some((j, niche, niche.reserve(dl, count)?)))?;
let niche_offset =
niche.offset + variant_layouts[largest_variant_index].fields.offset(field_index);
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 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 = layout_calc.univariant(
dl,
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);
Some(st)
})
.collect::<Option<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 None;
}
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(pair) = common_prim {
// This is pretty conservative. We could go fancier
// by conflating things like i32 and u32, or even
// realising that (u8, u8) could just cohabit with
// u16 or even u32.
if pair != (prim, offset) {
common_prim = None;
break;
}
} 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 = layout_calc.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")
}
};
Some(best_layout.layout)
}
/// Determines towards which end of a struct layout optimizations will try to place the best niches.
enum NicheBias {
Start,