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Use an enum to differentiate between kinds of structs.
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@ahicks92
ahicks92 committed Dec 14, 2016
1 parent adae9bc commit e7c3540
Showing 1 changed file with 61 additions and 30 deletions.
91 changes: 61 additions & 30 deletions src/librustc/ty/layout.rs
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Expand Up @@ -524,9 +524,20 @@ pub struct Struct {
pub min_size: Size,
}

// Info required to optimize struct layout.
#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Debug)]
enum StructKind {
// A tuple, closure, or univariant which cannot be coerced to unsized.
AlwaysSizedUnivariant,
// A univariant, the last field of which may be coerced to unsized.
MaybeUnsizedUnivariant,
// A univariant, but part of an enum.
EnumVariant,
}

impl<'a, 'gcx, 'tcx> Struct {
pub fn new(dl: &TargetDataLayout, fields: &Vec<&'a Layout>,
repr: attr::ReprAttr, is_enum_variant: bool,
fn new(dl: &TargetDataLayout, fields: &Vec<&'a Layout>,
repr: attr::ReprAttr, kind: StructKind,
scapegoat: Ty<'gcx>) -> Result<Struct, LayoutError<'gcx>> {
let packed = repr == attr::ReprPacked;
let mut ret = Struct {
Expand All @@ -538,38 +549,48 @@ impl<'a, 'gcx, 'tcx> Struct {
min_size: Size::from_bytes(0),
};

if is_enum_variant {
assert!(fields.len() >= 1, "Enum variants must have at least a discriminant field.")
}
let (optimize, sort_ascending) = match (repr, kind) {
(attr::ReprAny, StructKind::AlwaysSizedUnivariant) => (true, false),
(attr::ReprAny, StructKind::MaybeUnsizedUnivariant) => (true, true),
(attr::ReprAny, StructKind::EnumVariant) => {
assert!(fields.len() >= 1, "Enum variants must have discriminants.");
(true, fields[0].size(dl).bytes() == 1)
}
_ => (false, false)
};

if fields.len() == 0 {return Ok(ret)};

ret.offsets = vec![Size::from_bytes(0); fields.len()];
let mut inverse_memory_index: Vec<u32> = (0..fields.len() as u32).collect();

if repr == attr::ReprAny {
let start = if is_enum_variant {1} else {0};
// FIXME(camlorn): we can't reorder the last field because
// it is possible for structs to be coerced to unsized.
// Example: struct Foo<T: ?Sized> { x: i32, y: T }
// We can coerce &Foo<u8> to &Foo<Trait>.
let end = inverse_memory_index.len()-1;
if optimize {
let start = if let StructKind::EnumVariant = kind {1} else {0};
let end = if let StructKind::MaybeUnsizedUnivariant = kind { fields.len()-1 } else { 0 };
if end > start {
let optimizing = &mut inverse_memory_index[start..end];
optimizing.sort_by_key(|&x| fields[x as usize].align(dl).abi());
}
if is_enum_variant {
assert_eq!(inverse_memory_index[0], 0,
"Enums must have field 0 as the field with lowest offset.")
if sort_ascending {
optimizing.sort_by_key(|&x| fields[x as usize].align(dl).abi());
} else {
optimizing.sort_by(| &a, &b | {
let a = fields[a as usize].align(dl).abi();
let b = fields[b as usize].align(dl).abi();
b.cmp(&a)
});
}
}
}

// At this point, inverse_memory_index holds field indices by increasing 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 use inverse_memory_index to produce memory_index.

if let StructKind::EnumVariant = kind {
assert_eq!(inverse_memory_index[0], 0,
"Enum variant discriminants must have the lowest offset.");
}

let mut offset = Size::from_bytes(0);

for i in inverse_memory_index.iter() {
Expand Down Expand Up @@ -606,10 +627,16 @@ impl<'a, 'gcx, 'tcx> Struct {
// 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:
ret.memory_index = vec![0; inverse_memory_index.len()];
// Note: if we didn't optimize, it's already right.

if optimize {
ret.memory_index = vec![0; inverse_memory_index.len()];

for i in 0..inverse_memory_index.len() {
ret.memory_index[inverse_memory_index[i] as usize] = i as u32;
for i in 0..inverse_memory_index.len() {
ret.memory_index[inverse_memory_index[i] as usize] = i as u32;
}
} else {
ret.memory_index = inverse_memory_index;
}

Ok(ret)
Expand Down Expand Up @@ -985,7 +1012,8 @@ impl<'a, 'gcx, 'tcx> Layout {

// The never type.
ty::TyNever => Univariant {
variant: Struct::new(dl, &vec![], attr::ReprAny, false, ty)?,
variant: Struct::new(dl, &vec![], attr::ReprAny,
StructKind::AlwaysSizedUnivariant, ty)?,
non_zero: false
},

Expand Down Expand Up @@ -1038,12 +1066,13 @@ impl<'a, 'gcx, 'tcx> Layout {
// Odd unit types.
ty::TyFnDef(..) => {
Univariant {
variant: Struct::new(dl, &vec![], attr::ReprAny, false, ty)?,
variant: Struct::new(dl, &vec![], attr::ReprAny, StructKind::AlwaysSizedUnivariant, ty)?,
non_zero: false
}
}
ty::TyDynamic(_) => {
let mut unit = Struct::new(dl, &vec![], attr::ReprAny, false, ty)?;
let mut unit = Struct::new(dl, &vec![], attr::ReprAny,
StructKind::AlwaysSizedUnivariant, ty)?;
unit.sized = false;
Univariant { variant: unit, non_zero: false }
}
Expand All @@ -1055,15 +1084,16 @@ impl<'a, 'gcx, 'tcx> Layout {
&tys.map(|ty| ty.layout(infcx))
.collect::<Result<Vec<_>, _>>()?,
attr::ReprAny,
false, ty)?;
StructKind::AlwaysSizedUnivariant, ty)?;
Univariant { variant: st, non_zero: false }
}

ty::TyTuple(tys) => {
// FIXME(camlorn): if we ever allow unsized tuples, this needs to be checked in the same way it is for univariant.
let st = Struct::new(dl,
&tys.iter().map(|ty| ty.layout(infcx))
.collect::<Result<Vec<_>, _>>()?,
attr::ReprAny, false, ty)?;
attr::ReprAny, StructKind::AlwaysSizedUnivariant, ty)?;
Univariant { variant: st, non_zero: false }
}

Expand Down Expand Up @@ -1096,7 +1126,7 @@ impl<'a, 'gcx, 'tcx> Layout {
assert_eq!(hint, attr::ReprAny);

return success(Univariant {
variant: Struct::new(dl, &vec![], hint, false, ty)?,
variant: Struct::new(dl, &vec![], hint, StructKind::AlwaysSizedUnivariant, ty)?,
non_zero: false
});
}
Expand Down Expand Up @@ -1134,7 +1164,8 @@ impl<'a, 'gcx, 'tcx> Layout {
un.extend(dl, fields.iter().map(|&f| Ok(f)), ty)?;
UntaggedUnion { variants: un }
} else {
let st = Struct::new(dl, &fields, hint, false, ty)?;
let st = Struct::new(dl, &fields, hint,
StructKind::MaybeUnsizedUnivariant, ty)?;
let non_zero = Some(def.did) == tcx.lang_items.non_zero();
Univariant { variant: st, non_zero: non_zero }
};
Expand Down Expand Up @@ -1188,7 +1219,7 @@ impl<'a, 'gcx, 'tcx> Layout {
let st = Struct::new(dl,
&variants[discr].iter().map(|ty| ty.layout(infcx))
.collect::<Result<Vec<_>, _>>()?,
hint, false, ty)?;
hint, StructKind::AlwaysSizedUnivariant, ty)?;

// We have to fix the last element of path here.
let mut i = *path.last().unwrap();
Expand Down Expand Up @@ -1226,7 +1257,7 @@ impl<'a, 'gcx, 'tcx> Layout {
fields.insert(0, &discr);
let st = Struct::new(dl,
&fields,
hint, false, ty)?;
hint, StructKind::EnumVariant, ty)?;
// Find the first field we can't move later
// to make room for a larger discriminant.
// It is important to skip the first field.
Expand Down

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