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optimize field ordering by grouping power-of-two arrays with larger types

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The 8472 2022-10-06 21:20:02 +02:00
parent 66ccf36f16
commit 9f0cb566ea
1 changed files with 13 additions and 4 deletions
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17
compiler/rustc_ty_utils/src/layout.rs
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@ -138,8 +138,17 @@ fn univariant_uninterned<'tcx>(
if optimize {
let end = if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
let optimizing = &mut inverse_memory_index[..end];
let field_align = |f: &TyAndLayout<'_>| {
if let Some(pack) = pack { f.align.abi.min(pack) } else { f.align.abi }
let effective_field_align = |f: &TyAndLayout<'_>| {
if let Some(pack) = pack {
f.align.abi.min(pack)
} else if f.size.bytes().is_power_of_two() && f.size.bytes() >= f.align.abi.bytes() {
// Try to put fields which have a 2^n size and smaller alignment together with
// fields that have an alignment matching that size.
// E.g. group [u8; 4] with u32 fields
Align::from_bytes(f.align.abi.bytes()).unwrap_or(f.align.abi)
} else {
f.align.abi
}
};
// If `-Z randomize-layout` was enabled for the type definition we can shuffle
@ -161,14 +170,14 @@ fn univariant_uninterned<'tcx>(
// Place ZSTs first to avoid "interesting offsets",
// especially with only one or two non-ZST fields.
let f = &fields[x as usize];
(!f.is_zst(), cmp::Reverse(field_align(f)))
(!f.is_zst(), cmp::Reverse(effective_field_align(f)))
});
}
StructKind::Prefixed(..) => {
// Sort in ascending alignment so that the layout stays optimal
// regardless of the prefix
optimizing.sort_by_key(|&x| field_align(&fields[x as usize]));
optimizing.sort_by_key(|&x| effective_field_align(&fields[x as usize]));
}
}