Do not disable field reordering on enums with big discriminant #70748
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The field are always re-ordered to minimize padding, regardless of the alignment of the discriminant
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eddyb
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Apr 4, 2020
| @@ -104,6 +127,8 @@ pub fn main() { | |||
| assert_eq!(size_of::<e3>(), 4 as usize); | |||
| assert_eq!(size_of::<ReorderedStruct>(), 4); | |||
| assert_eq!(size_of::<ReorderedEnum>(), 6); | |||
| assert_eq!(size_of::<ReorderedEnum2>(), 8); | |||
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For the record, before this PR it was 12.
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FWIW, this has some more details #70477 (comment). My reasoning is that the discriminant "commutes" with the group of fields right next to it, which have lower alignment, without changes in padding. I believe this doesn't hold for descending sorting (by alignment), but only ascending sorting, of variant fields (it's the latter we do for I haven't fully looked into what other implications partially-sorted field orders have. @bors r+ |
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Do not disable field reordering on enums with big discriminant The field are always re-ordered to minimize padding, regardless of the alignment of the discriminant (spinoff from rust-lang#70477)
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Rollup of 6 pull requests Successful merges: - rust-lang#70635 (rustc_target: Some cleanup to `no_default_libraries`) - rust-lang#70748 (Do not disable field reordering on enums with big discriminant) - rust-lang#70752 (Add slice::fill) - rust-lang#70766 (use ManuallyDrop instead of forget inside collections) - rust-lang#70768 (macro_rules: `NtLifetime` cannot start with an identifier) - rust-lang#70783 (comment refers to removed type) Failed merges: r? @ghost
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The field are always re-ordered to minimize padding, regardless of the
alignment of the discriminant
(spinoff from #70477)