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Auto merge of #73453 - erikdesjardins:tuplayout, r=eddyb

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Ignore ZST offsets when deciding whether to use Scalar/ScalarPair layout

This is important because Scalar/ScalarPair layout previously would not be used if any ZST had nonzero offset.
For example, before this change, only `((), u128)` would be laid out like `u128`, not `(u128, ())`.

Fixes #63244
This commit is contained in:
bors 2020-09-25 14:42:20 +00:00
commit 5b9e886403
7 changed files with 230 additions and 76 deletions
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116
compiler/rustc_middle/src/ty/layout.rs
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@ -390,78 +390,60 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
// Unpack newtype ABIs and find scalar pairs.
if sized && size.bytes() > 0 {
// All other fields must be ZSTs, and we need them to all start at 0.
let mut zst_offsets = offsets.iter().enumerate().filter(|&(i, _)| fields[i].is_zst());
if zst_offsets.all(|(_, o)| o.bytes() == 0) {
let mut non_zst_fields = fields.iter().enumerate().filter(|&(_, f)| !f.is_zst());
// All other fields must be ZSTs.
let mut non_zst_fields = fields.iter().enumerate().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) => {
// 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 = field.abi.clone();
}
// But scalar pairs are Rust-specific and get
// treated as aggregates by C ABIs anyway.
Abi::ScalarPair(..) => {
abi = field.abi.clone();
}
_ => {}
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) => {
// 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 = field.abi.clone();
}
// But scalar pairs are Rust-specific and get
// treated as aggregates by C ABIs anyway.
Abi::ScalarPair(..) => {
abi = field.abi.clone();
}
_ => {}
}
}
// Two non-ZST fields, and they're both scalars.
(
Some((
i,
&TyAndLayout {
layout: &Layout { abi: Abi::Scalar(ref a), .. }, ..
},
)),
Some((
j,
&TyAndLayout {
layout: &Layout { abi: Abi::Scalar(ref b), .. }, ..
},
)),
None,
) => {
// 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(a.clone(), b.clone());
let pair_offsets = match pair.fields {
FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
assert_eq!(memory_index, &[0, 1]);
offsets
}
_ => bug!(),
};
if offsets[i] == pair_offsets[0]
&& offsets[j] == pair_offsets[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;
}
}
_ => {}
}
// Two non-ZST fields, and they're both scalars.
(
Some((i, &TyAndLayout { layout: &Layout { abi: Abi::Scalar(ref a), .. }, .. })),
Some((j, &TyAndLayout { layout: &Layout { abi: Abi::Scalar(ref b), .. }, .. })),
None,
) => {
// 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(a.clone(), b.clone());
let pair_offsets = match pair.fields {
FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
assert_eq!(memory_index, &[0, 1]);
offsets
}
_ => bug!(),
};
if offsets[i] == pair_offsets[0]
&& offsets[j] == pair_offsets[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;
}
}
_ => {}
}
}