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Fix for #62691: use the largest niche across all fields #70411

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merged 4 commits into from
Mar 26, 2020
Merged

Fix for #62691: use the largest niche across all fields #70411

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4d77d01
Fix for #62691: use the largest niche across all fields
ogoffart Mar 25, 2020
81006f6
Optimize slightly by avoiding to call Niche::reserve when not needed
ogoffart Mar 25, 2020
bb3e513
Revert previous commit and make the optimisation in a nicer way
ogoffart Mar 25, 2020
0b00c20
Reorganize a bit the code and add a comment
ogoffart Mar 26, 2020
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36 changes: 22 additions & 14 deletions src/librustc/ty/layout.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -282,8 +282,6 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {

let mut align = if pack.is_some() { dl.i8_align } else { dl.aggregate_align };

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

let mut optimize = !repr.inhibit_struct_field_reordering_opt();
Expand Down Expand Up @@ -320,6 +318,8 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
// At the bottom of this function, we invert `inverse_memory_index` to
// produce `memory_index` (see `invert_mapping`).

let mut sized = true;
let mut offsets = vec![Size::ZERO; fields.len()];
let mut offset = Size::ZERO;
let mut largest_niche = None;
let mut largest_niche_available = 0;
Expand Down Expand Up @@ -907,18 +907,26 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
let count = (niche_variants.end().as_u32()
- niche_variants.start().as_u32()
+ 1) as u128;
// FIXME(#62691) use the largest niche across all fields,
// not just the first one.
for (field_index, &field) in variants[i].iter().enumerate() {
let niche = match &field.largest_niche {
Some(niche) => niche,
_ => continue,
};
let (niche_start, niche_scalar) = match niche.reserve(self, count) {
Some(pair) => pair,
None => continue,
};

let mut niche_size = 0;
if let Some((field_index, niche, niche_start, niche_scalar)) =
variants[i].iter().enumerate().fold(None, |acc, (j, &field)| {
let niche = match &field.largest_niche {
Some(niche) => niche,
_ => return acc,
};
let ns = niche.available(dl);
if ns <= niche_size {
return acc;
}
match niche.reserve(self, count) {
Some(pair) => {
niche_size = ns;
Some((j, niche, pair.0, pair.1))
}
None => acc,
}
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You don't need this complication, just have .max_by_key(|(_, niche, _)| niche.available(dl)) before .and_then(|(_, niche)| Some((i, niche, niche.reserve(self, count)?))).

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I wanted to do that but that's not exactly the same thing. If it fails for the largest niche, but would have succeeded for a smaller niche, we would end up with None instead of using the smaller niche.
Now, maybe this cannot succeed for a smaller niche and fail for a bigger one so in that case that would be indeed best.

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Now, maybe this cannot succeed for a smaller niche and fail for a bigger one so in that case that would be indeed best.

Yeah, that's why we want bigger niches in general: they succeed strictly more often.

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This is what i've done now.

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But looking at the code of Niche::reserve, it seems it can still fail if valid_range_contains despite being the largest niche while a smaller niche would work. or is that really unlikely?

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The calculations are equivalent, I think I even double-checked them using bruteforce for small bitwidths.

niche.reserve(count) is checking niche.available() >= count but using values it has to compute anyway (i.e. the remaining validity range), instead of actually using available.

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Consider sprinkling some comments here btw, and/or extracting functions :)

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Hopefully we can keep something close to the first commit, as it's more functional and way easier to explain as "find the largest niche among the fields" + "make sure it can fit our variant count".

})
{
let mut align = dl.aggregate_align;
let st = variants
.iter_enumerated()
Expand Down
7 changes: 7 additions & 0 deletions src/test/ui/type-sizes.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -74,6 +74,11 @@ enum NicheFilledEnumWithAbsentVariant {
C,
}

enum Option2<A, B> {
Some(A, B),
None
}

pub fn main() {
assert_eq!(size_of::<u8>(), 1 as usize);
assert_eq!(size_of::<u32>(), 4 as usize);
Expand Down Expand Up @@ -113,4 +118,6 @@ pub fn main() {

assert_eq!(size_of::<Option<Option<(bool, &())>>>(), size_of::<(bool, &())>());
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, &())>());
}