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author | The 8472 <git@infinite-source.de> | 2023-04-22 19:24:39 +0200 |
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committer | The 8472 <git@infinite-source.de> | 2023-04-28 23:08:54 +0200 |
commit | 61fb5a91b794d7ab9c5f923b26c92cfc473b976b (patch) | |
tree | fab8cdc1c35362e85b38171b32ed6133e7e41d6f /compiler/rustc_abi | |
parent | afe106cdc8b5dbcbeedb292b87dc7d7ae58964f1 (diff) | |
download | rust-61fb5a91b794d7ab9c5f923b26c92cfc473b976b.tar.gz |
layout-alignment-promotion logic should depend on the niche-bias
For start-biased layout we want to avoid overpromoting so that
the niche doesn't get pushed back.
For end-biased layout we want to avoid promoting fields that
may contain one of the niches of interest.
Diffstat (limited to 'compiler/rustc_abi')
-rw-r--r-- | compiler/rustc_abi/src/layout.rs | 29 |
1 files changed, 22 insertions, 7 deletions
diff --git a/compiler/rustc_abi/src/layout.rs b/compiler/rustc_abi/src/layout.rs index a49c9e58297..b4597d5bc78 100644 --- a/compiler/rustc_abi/src/layout.rs +++ b/compiler/rustc_abi/src/layout.rs @@ -794,7 +794,7 @@ fn univariant( let mut align = if pack.is_some() { dl.i8_align } else { dl.aggregate_align }; let mut inverse_memory_index: IndexVec<u32, FieldIdx> = fields.indices().collect(); let optimize = !repr.inhibit_struct_field_reordering_opt(); - if optimize { + if optimize && fields.len() > 1 { let end = if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() }; let optimizing = &mut inverse_memory_index.raw[..end]; @@ -814,7 +814,12 @@ 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 largest_niche_size = fields + .iter() + .filter_map(|f| f.largest_niche()) + .map(|n| n.available(dl)) + .max() + .unwrap_or(0); // Calculates a sort key to group fields by their alignment or possibly some size-derived // pseudo-alignment. @@ -829,13 +834,23 @@ fn univariant( // let align = layout.align().abi.bytes(); let size = layout.size().bytes(); + let niche_size = layout.largest_niche().map(|n| n.available(dl)).unwrap_or(0); // group [u8; 4] with align-4 or [u8; 6] with align-2 fields let size_as_align = align.max(size).trailing_zeros(); - // Given `A(u8, [u8; 16])` and `B(bool, [u8; 16])` we want to bump the array - // to the front in the first case (for aligned loads) but keep the bool in front - // in the second case for its niches. - let size_as_align = if any_niche { - max_field_align.trailing_zeros().min(size_as_align) + let size_as_align = if largest_niche_size > 0 { + match niche_bias { + // Given `A(u8, [u8; 16])` and `B(bool, [u8; 16])` we want to bump the array + // to the front in the first case (for aligned loads) but keep the bool in front + // in the second case for its niches. + NicheBias::Start => max_field_align.trailing_zeros().min(size_as_align), + // When moving niches towards the end of the struct then for + // A((u8, u8, u8, bool), (u8, bool, u8)) we want to keep the first tuple + // in the align-1 group because its bool can be moved closer to the end. + NicheBias::End if niche_size == largest_niche_size => { + align.trailing_zeros() + } + NicheBias::End => size_as_align, + } } else { size_as_align }; |