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Revert "Compare tagged/niche-filling layout and pick the best one"

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Mark Rousskov 2020-07-21 20:02:30 -04:00 committed by GitHub
parent 8ad7bc3f42
commit 1d6860082f
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4 changed files with 7 additions and 51 deletions
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1
src/librustc_middle/lib.rs
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@ -27,7 +27,6 @@
#![feature(bool_to_option)]
#![feature(box_patterns)]
#![feature(box_syntax)]
#![feature(cmp_min_max_by)]
#![feature(const_fn)]
#![feature(const_panic)]
#![feature(const_fn_transmute)]

26
src/librustc_middle/ty/layout.rs
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@ -876,8 +876,6 @@ fn layout_raw_uncached(&self, ty: Ty<'tcx>) -> Result<&'tcx Layout, LayoutError<
.iter_enumerated()
.all(|(i, v)| v.discr == ty::VariantDiscr::Relative(i.as_u32()));
let mut niche_filling_layout = None;
// Niche-filling enum optimization.
if !def.repr.inhibit_enum_layout_opt() && no_explicit_discriminants {
let mut dataful_variant = None;
@ -974,7 +972,7 @@ fn layout_raw_uncached(&self, ty: Ty<'tcx>) -> Result<&'tcx Layout, LayoutError<
let largest_niche =
Niche::from_scalar(dl, offset, niche_scalar.clone());
niche_filling_layout = Some(Layout {
return Ok(tcx.intern_layout(Layout {
variants: Variants::Multiple {
tag: niche_scalar,
tag_encoding: TagEncoding::Niche {
@ -993,7 +991,7 @@ fn layout_raw_uncached(&self, ty: Ty<'tcx>) -> Result<&'tcx Layout, LayoutError<
largest_niche,
size,
align,
});
}));
}
}
}
@ -1216,7 +1214,7 @@ fn layout_raw_uncached(&self, ty: Ty<'tcx>) -> Result<&'tcx Layout, LayoutError<
let largest_niche = Niche::from_scalar(dl, Size::ZERO, tag.clone());
let tagged_layout = Layout {
tcx.intern_layout(Layout {
variants: Variants::Multiple {
tag,
tag_encoding: TagEncoding::Direct,
@ -1231,23 +1229,7 @@ fn layout_raw_uncached(&self, ty: Ty<'tcx>) -> Result<&'tcx Layout, LayoutError<
abi,
align,
size,
};
let best_layout = match (tagged_layout, niche_filling_layout) {
(tagged_layout, Some(niche_filling_layout)) => {
// Pick the smaller layout; otherwise,
// pick the layout with the larger niche; otherwise,
// pick tagged as it has simpler codegen.
cmp::min_by_key(tagged_layout, niche_filling_layout, |layout| {
let niche_size =
layout.largest_niche.as_ref().map_or(0, |n| n.available(dl));
(layout.size, cmp::Reverse(niche_size))
})
}
(tagged_layout, None) => tagged_layout,
};
tcx.intern_layout(best_layout)
})
}
// Types with no meaningful known layout.

6
src/test/ui/print_type_sizes/niche-filling.stdout
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@ -8,12 +8,12 @@ print-type-size variant `Some`: 12 bytes
print-type-size field `.0`: 12 bytes
print-type-size variant `None`: 0 bytes
print-type-size type: `EmbeddedDiscr`: 8 bytes, alignment: 4 bytes
print-type-size discriminant: 1 bytes
print-type-size variant `Record`: 7 bytes
print-type-size field `.pre`: 1 bytes
print-type-size field `.post`: 2 bytes
print-type-size field `.val`: 4 bytes
print-type-size field `.post`: 2 bytes
print-type-size field `.pre`: 1 bytes
print-type-size variant `None`: 0 bytes
print-type-size end padding: 1 bytes
print-type-size type: `MyOption<Union1<std::num::NonZeroU32>>`: 8 bytes, alignment: 4 bytes
print-type-size discriminant: 4 bytes
print-type-size variant `Some`: 4 bytes

25
src/test/ui/type-sizes.rs
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@ -5,7 +5,6 @@
#![feature(never_type)]
use std::mem::size_of;
use std::num::NonZeroU8;
struct t {a: u8, b: i8}
struct u {a: u8, b: i8, c: u8}
@ -103,23 +102,6 @@ enum Option2<A, B> {
None
}
// Two layouts are considered for `CanBeNicheFilledButShouldnt`:
// Niche-filling:
// { u32 (4 bytes), NonZeroU8 + tag in niche (1 byte), padding (3 bytes) }
// Tagged:
// { tag (1 byte), NonZeroU8 (1 byte), padding (2 bytes), u32 (4 bytes) }
// Both are the same size (due to padding),
// but the tagged layout is better as the tag creates a niche with 254 invalid values,
// allowing types like `Option<Option<CanBeNicheFilledButShouldnt>>` to fit into 8 bytes.
pub enum CanBeNicheFilledButShouldnt {
A(NonZeroU8, u32),
B
}
pub enum AlwaysTaggedBecauseItHasNoNiche {
A(u8, u32),
B
}
pub fn main() {
assert_eq!(size_of::<u8>(), 1 as usize);
assert_eq!(size_of::<u32>(), 4 as usize);
@ -163,11 +145,4 @@ pub fn main() {
assert_eq!(size_of::<Option<Option<(&(), bool)>>>(), size_of::<(bool, &())>());
assert_eq!(size_of::<Option<Option2<bool, &()>>>(), size_of::<(bool, &())>());
assert_eq!(size_of::<Option<Option2<&(), bool>>>(), size_of::<(bool, &())>());
assert_eq!(size_of::<CanBeNicheFilledButShouldnt>(), 8);
assert_eq!(size_of::<Option<CanBeNicheFilledButShouldnt>>(), 8);
assert_eq!(size_of::<Option<Option<CanBeNicheFilledButShouldnt>>>(), 8);
assert_eq!(size_of::<AlwaysTaggedBecauseItHasNoNiche>(), 8);
assert_eq!(size_of::<Option<AlwaysTaggedBecauseItHasNoNiche>>(), 8);
assert_eq!(size_of::<Option<Option<AlwaysTaggedBecauseItHasNoNiche>>>(), 8);
}