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also sort fields by niche sizes to retain optimizations

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The 8472 2022-10-07 01:09:54 +02:00
parent a3450d060d
commit 97d8a9bdd3
1 changed files with 16 additions and 3 deletions
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19
compiler/rustc_ty_utils/src/layout.rs
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@ -140,8 +140,13 @@ fn univariant_uninterned<'tcx>(
let optimizing = &mut inverse_memory_index[..end];
let effective_field_align = |f: &TyAndLayout<'_>| {
if let Some(pack) = pack {
// return the packed alignment in bytes
f.align.abi.min(pack).bytes()
} else {
// returns log2(effective-align).
// This is ok since `pack` applies to all fields equally.
// The calculation assumes that size is an integer multiple of align, except for ZSTs.
//
// group [u8; 4] with align-4 or [u8; 6] with align-2 fields
f.align.abi.bytes().max(f.size.bytes()).trailing_zeros() as u64
}
@ -165,15 +170,23 @@ fn univariant_uninterned<'tcx>(
optimizing.sort_by_key(|&x| {
// Place ZSTs first to avoid "interesting offsets",
// especially with only one or two non-ZST fields.
// Then place largest alignments first, largest niches within an alignment group last
let f = &fields[x as usize];
(!f.is_zst(), cmp::Reverse(effective_field_align(f)))
let niche_size = f.largest_niche.map_or(0, |n| n.available(cx));
(!f.is_zst(), cmp::Reverse(effective_field_align(f)), niche_size)
});
}
StructKind::Prefixed(..) => {
// Sort in ascending alignment so that the layout stays optimal
// regardless of the prefix
optimizing.sort_by_key(|&x| effective_field_align(&fields[x as usize]));
// regardless of the prefix.
// And put the largest niche in an alignment group at the end
// so it can be used as discriminant in jagged enums
optimizing.sort_by_key(|&x| {
let f = &fields[x as usize];
let niche_size = f.largest_niche.map_or(0, |n| n.available(cx));
(effective_field_align(f), niche_size)
});
}
}