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don't promote large fields to higher alignments if that would affect niche placement

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The 8472 2023-03-05 16:15:16 +01:00
parent faf2da3e2f
commit 4907dac54c
2 changed files with 42 additions and 13 deletions
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37
compiler/rustc_abi/src/layout.rs
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@ -772,19 +772,6 @@ fn univariant(
if optimize {
let end = if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
let optimizing = &mut inverse_memory_index.raw[..end];
let effective_field_align = |layout: Layout<'_>| {
if let Some(pack) = pack {
// return the packed alignment in bytes
layout.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
layout.align().abi.bytes().max(layout.size().bytes()).trailing_zeros() as u64
}
};
// If `-Z randomize-layout` was enabled for the type definition we can shuffle
// the field ordering to try and catch some code making assumptions about layouts
@ -801,6 +788,30 @@ fn univariant(
}
// Otherwise we just leave things alone and actually optimize the type's fields
} else {
let max_field_align = fields.iter().map(|f| f.align().abi.bytes()).max().unwrap_or(1);
let any_niche = fields.iter().any(|f| f.largest_niche().is_some());
let effective_field_align = |layout: Layout<'_>| {
if let Some(pack) = pack {
// return the packed alignment in bytes
layout.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
let align = layout.align().abi.bytes();
let size = layout.size().bytes();
let size_as_align = align.max(size).trailing_zeros();
let size_as_align = if any_niche {
max_field_align.trailing_zeros().min(size_as_align)
} else {
size_as_align
};
size_as_align as u64
}
};
match kind {
StructKind::AlwaysSized | StructKind::MaybeUnsized => {
optimizing.sort_by_key(|&x| {