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Auto merge of #112062 - lukas-code:unsized-layout, r=wesleywiser

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Make struct layout not depend on unsizeable tail

fixes (after backport) https://github.com/rust-lang/rust/issues/112048

Since unsizing `Ptr<Foo<T>>` -> `Ptr<Foo<U>` just copies the pointer and adds the metadata, the layout of `Foo` must not depend on niches in and alignment of the tail `T`.

Nominating for beta 1.71, because it will have this issue: `@rustbot` label beta-nominated
This commit is contained in:
bors 2023-06-13 22:34:59 +00:00
commit 6330daade9
3 changed files with 104 additions and 39 deletions
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88
compiler/rustc_abi/src/layout.rs
View File

@ -57,48 +57,54 @@ fn univariant(
// run and bias niches to the right and then check which one is closer to one of the struct's
// edges.
if let Some(layout) = &layout {
if let Some(niche) = layout.largest_niche {
let head_space = niche.offset.bytes();
let niche_length = niche.value.size(dl).bytes();
let tail_space = layout.size.bytes() - head_space - niche_length;
// 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_length = niche.value.size(dl).bytes();
let tail_space = layout.size.bytes() - head_space - niche_length;
// 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 = univariant(self, dl, fields, repr, kind, NicheBias::End)
.expect("alt layout should always work");
let niche = alt_layout
.largest_niche
.expect("alt layout should have a niche like the regular one");
let alt_head_space = niche.offset.bytes();
let alt_niche_len = niche.value.size(dl).bytes();
let alt_tail_space = alt_layout.size.bytes() - alt_head_space - alt_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 = univariant(self, dl, fields, repr, kind, NicheBias::End)
.expect("alt layout should always work");
let niche = alt_layout
.largest_niche
.expect("alt layout should have a niche like the regular one");
let alt_head_space = niche.offset.bytes();
let alt_niche_len = 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());
debug_assert_eq!(layout.size.bytes(), alt_layout.size.bytes());
let prefer_alt_layout =
alt_head_space > head_space && alt_head_space > tail_space;
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_length,
tail_space,
alt_head_space,
alt_niche_len,
alt_tail_space,
layout.fields.count(),
prefer_alt_layout,
format_field_niches(&layout, &fields, &dl),
format_field_niches(&alt_layout, &fields, &dl),
);
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_length,
tail_space,
alt_head_space,
alt_niche_len,
alt_tail_space,
layout.fields.count(),
prefer_alt_layout,
format_field_niches(&layout, &fields, &dl),
format_field_niches(&alt_layout, &fields, &dl),
);
if prefer_alt_layout {
return Some(alt_layout);
if prefer_alt_layout {
return Some(alt_layout);
}
}
}
}
@ -828,6 +834,7 @@ fn univariant(
if optimize && 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
@ -844,8 +851,11 @@ fn univariant(
}
// Otherwise we just leave things alone and actually optimize the type's fields
} else {
let max_field_align = fields.iter().map(|f| f.align().abi.bytes()).max().unwrap_or(1);
let largest_niche_size = fields
// 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))

25
tests/ui/layout/issue-112048-unsizing-field-order.rs Normal file
View File

@ -0,0 +1,25 @@
// run-pass
// Check that unsizing doesn't reorder fields.
#![allow(dead_code)]
use std::fmt::Debug;
#[derive(Debug)]
struct GcNode<T: ?Sized> {
gets_swapped_with_next: usize,
next: Option<&'static GcNode<dyn Debug>>,
tail: T,
}
fn main() {
let node: Box<GcNode<dyn Debug>> = Box::new(GcNode {
gets_swapped_with_next: 42,
next: None,
tail: Box::new(1),
});
assert_eq!(node.gets_swapped_with_next, 42);
assert!(node.next.is_none());
}

30
tests/ui/layout/issue-112048-unsizing-niche.rs Normal file
View File

@ -0,0 +1,30 @@
// run-pass
// Check that unsizing does not change which field is considered for niche layout.
#![feature(offset_of)]
#![allow(dead_code)]
#[derive(Clone)]
struct WideptrField<T: ?Sized> {
first: usize,
second: usize,
niche: NicheAtEnd,
tail: T,
}
#[derive(Clone)]
#[repr(C)]
struct NicheAtEnd {
arr: [u8; 7],
b: bool,
}
type Tail = [bool; 8];
fn main() {
assert_eq!(
core::mem::offset_of!(WideptrField<Tail>, niche),
core::mem::offset_of!(WideptrField<dyn Send>, niche)
);
}