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Rollup merge of #46809 - eddyb:issue-46769-optimal, r=arielb1

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rustc: do not raise the alignment of optimized enums to the niche's alignment.

This is the improved fix for #46769 that does not increase the size of any types (see also #46808).
This commit is contained in:
kennytm 2017-12-22 02:50:49 +08:00 committed by GitHub
commit d4981e9742
2 changed files with 16 additions and 9 deletions
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12
src/librustc/ty/layout.rs
View File

@ -1484,27 +1484,25 @@ impl<'a, 'tcx> LayoutDetails {
Some(niche) => niche,
None => continue
};
let mut align = dl.aggregate_align;
let st = variants.iter().enumerate().map(|(j, v)| {
let mut st = univariant_uninterned(v,
&def.repr, StructKind::AlwaysSized)?;
st.variants = Variants::Single { index: j };
align = align.max(st.align);
Ok(st)
}).collect::<Result<Vec<_>, _>>()?;
let offset = st[i].fields.offset(field_index) + offset;
let LayoutDetails { mut size, mut align, .. } = st[i];
let size = st[i].size;
let mut niche_align = niche.value.align(dl);
let abi = if offset.bytes() == 0 && niche.value.size(dl) == size {
Abi::Scalar(niche.clone())
} else {
if offset.abi_align(niche_align) != offset {
niche_align = dl.i8_align;
}
Abi::Aggregate { sized: true }
};
align = align.max(niche_align);
size = size.abi_align(align);
return Ok(tcx.intern_layout(LayoutDetails {
variants: Variants::NicheFilling {

13
src/test/run-pass/packed-struct-optimized-enum.rs
View File

@ -19,7 +19,9 @@ impl<T: Copy> Clone for Packed<T> {
fn sanity_check_size<T: Copy>(one: T) {
let two = [one, one];
let stride = (&two[1] as *const _ as usize) - (&two[0] as *const _ as usize);
assert_eq!(stride, std::mem::size_of_val(&one));
let (size, align) = (std::mem::size_of::<T>(), std::mem::align_of::<T>());
assert_eq!(stride, size);
assert_eq!(size % align, 0);
}
fn main() {
@ -32,5 +34,12 @@ fn main() {
// In #46769, `Option<(Packed<&()>, bool)>` was found to have
// pointer alignment, without actually being aligned in size.
// E.g. on 64-bit platforms, it had alignment `8` but size `9`.
sanity_check_size(Some((Packed(&()), true)));
type PackedRefAndBool<'a> = (Packed<&'a ()>, bool);
sanity_check_size::<Option<PackedRefAndBool>>(Some((Packed(&()), true)));
// Make sure we don't pay for the enum optimization in size,
// e.g. we shouldn't need extra padding after the packed data.
assert_eq!(std::mem::align_of::<Option<PackedRefAndBool>>(), 1);
assert_eq!(std::mem::size_of::<Option<PackedRefAndBool>>(),
std::mem::size_of::<PackedRefAndBool>());
}