Skip to content

Commit f7ed1c2

Browse files
workingjubileeworkingjubilee
committed
Remove notion of "preferred" alignment AKA __alignof
In rust-lang#90877 (comment) T-lang decided they did not wish to remove `intrinsics::pref_align_of`. However, the intrinsic and its supporting code 1. is a nightly feature, so can be removed at compiler/libs discretion 2. requires considerable effort in the compiler to support, as it necessarily complicates every single site reasoning about alignment 3. has been justified based on relevance to codegen, but it is only a requirement for C++ (not C, not Rust) stack frame layout for AIX[^1], in ways Rust would not consider even with increased C++ interop 4. is dwarfed by unrelated concerns regarding the AIX XLC compiler's layout rules for C which make it difficult to support[^2] 5. is only used by rustc to pad global layouts[^3], not correctness 6. can be adequately replaced by other rules for globals, as it mostly affects alignments for a few types under 16 bytes of alignment 7. has only one clear benefactor: automating C -> Rust translation for GNU extensions[^4] like `__alignof` 8. was likely intended to be `alignof`, `_Alignof`, AKA `mem::align_of`, because the GNU extensions were "false friends" of Standard C forms, which makes the choice to support such a mapping very questionable 9. makes it easy to do incorrect codegen with the compiler by its mere presence, as usual Rust rules of alignment (e.g. size == align * N) do not hold with preferred alignment[^5] Thus remove the intrinsic and supporting code, as a cost-benefit analysis cannot possibly justify going to such lengths and risking correctness just to help automated translation of an ill-considered GNU extension. [^1]: cite AIX cookie layout [^2]: cite AIX "power alignment" issues [^3]: cite static alignment usages [^4]: cite c2rust commit [^5]: cite bjorn3's PR
1 parent ce36a96 commit f7ed1c2

28 files changed

+221
-424
lines changed

‎compiler/rustc_abi/src/layout.rs

Lines changed: 6 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@ use rustc_index::Idx;
77
use tracing::debug;
88

99
use crate::{
10-
AbiAndPrefAlign, Align, BackendRepr, FieldsShape, HasDataLayout, IndexSlice, IndexVec, Integer,
10+
AbiAlign, Align, BackendRepr, FieldsShape, HasDataLayout, IndexSlice, IndexVec, Integer,
1111
LayoutData, Niche, NonZeroUsize, Primitive, ReprOptions, Scalar, Size, StructKind, TagEncoding,
1212
Variants, WrappingRange,
1313
};
@@ -361,13 +361,13 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
361361
}
362362

363363
if let Some(pack) = repr.pack {
364-
align = align.min(AbiAndPrefAlign::new(pack));
364+
align = align.min(AbiAlign::new(pack));
365365
}
366366
// The unadjusted ABI alignment does not include repr(align), but does include repr(pack).
367367
// See documentation on `LayoutS::unadjusted_abi_align`.
368368
let unadjusted_abi_align = align.abi;
369369
if let Some(repr_align) = repr.align {
370-
align = align.max(AbiAndPrefAlign::new(repr_align));
370+
align = align.max(AbiAlign::new(repr_align));
371371
}
372372
// `align` must not be modified after this, or `unadjusted_abi_align` could be inaccurate.
373373
let align = align;
@@ -1203,7 +1203,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
12031203
if let StructKind::Prefixed(prefix_size, prefix_align) = kind {
12041204
let prefix_align =
12051205
if let Some(pack) = pack { prefix_align.min(pack) } else { prefix_align };
1206-
align = align.max(AbiAndPrefAlign::new(prefix_align));
1206+
align = align.max(AbiAlign::new(prefix_align));
12071207
offset = prefix_size.align_to(prefix_align);
12081208
}
12091209
for &i in &inverse_memory_index {
@@ -1222,7 +1222,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
12221222

12231223
// Invariant: offset < dl.obj_size_bound() <= 1<<61
12241224
let field_align = if let Some(pack) = pack {
1225-
field.align.min(AbiAndPrefAlign::new(pack))
1225+
field.align.min(AbiAlign::new(pack))
12261226
} else {
12271227
field.align
12281228
};
@@ -1256,7 +1256,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
12561256
// See documentation on `LayoutS::unadjusted_abi_align`.
12571257
let unadjusted_abi_align = align.abi;
12581258
if let Some(repr_align) = repr.align {
1259-
align = align.max(AbiAndPrefAlign::new(repr_align));
1259+
align = align.max(AbiAlign::new(repr_align));
12601260
}
12611261
// `align` must not be modified after this point, or `unadjusted_abi_align` could be inaccurate.
12621262
let align = align;

‎compiler/rustc_abi/src/layout/ty.rs

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -5,7 +5,7 @@ use rustc_data_structures::intern::Interned;
55
use rustc_macros::HashStable_Generic;
66

77
use crate::{
8-
AbiAndPrefAlign, Align, BackendRepr, FieldsShape, Float, HasDataLayout, LayoutData, Niche,
8+
AbiAlign, Align, BackendRepr, FieldsShape, Float, HasDataLayout, LayoutData, Niche,
99
PointeeInfo, Primitive, Scalar, Size, TargetDataLayout, Variants,
1010
};
1111

@@ -93,7 +93,7 @@ impl<'a> Layout<'a> {
9393
self.0.0.largest_niche
9494
}
9595

96-
pub fn align(self) -> AbiAndPrefAlign {
96+
pub fn align(self) -> AbiAlign {
9797
self.0.0.align
9898
}
9999

‎compiler/rustc_abi/src/lib.rs

Lines changed: 44 additions & 46 deletions
Original file line numberDiff line numberDiff line change
@@ -210,22 +210,22 @@ impl ReprOptions {
210210
#[derive(Debug, PartialEq, Eq)]
211211
pub struct TargetDataLayout {
212212
pub endian: Endian,
213-
pub i1_align: AbiAndPrefAlign,
214-
pub i8_align: AbiAndPrefAlign,
215-
pub i16_align: AbiAndPrefAlign,
216-
pub i32_align: AbiAndPrefAlign,
217-
pub i64_align: AbiAndPrefAlign,
218-
pub i128_align: AbiAndPrefAlign,
219-
pub f16_align: AbiAndPrefAlign,
220-
pub f32_align: AbiAndPrefAlign,
221-
pub f64_align: AbiAndPrefAlign,
222-
pub f128_align: AbiAndPrefAlign,
213+
pub i1_align: AbiAlign,
214+
pub i8_align: AbiAlign,
215+
pub i16_align: AbiAlign,
216+
pub i32_align: AbiAlign,
217+
pub i64_align: AbiAlign,
218+
pub i128_align: AbiAlign,
219+
pub f16_align: AbiAlign,
220+
pub f32_align: AbiAlign,
221+
pub f64_align: AbiAlign,
222+
pub f128_align: AbiAlign,
223223
pub pointer_size: Size,
224-
pub pointer_align: AbiAndPrefAlign,
225-
pub aggregate_align: AbiAndPrefAlign,
224+
pub pointer_align: AbiAlign,
225+
pub aggregate_align: AbiAlign,
226226

227227
/// Alignments for vector types.
228-
pub vector_align: Vec<(Size, AbiAndPrefAlign)>,
228+
pub vector_align: Vec<(Size, AbiAlign)>,
229229

230230
pub instruction_address_space: AddressSpace,
231231

@@ -241,22 +241,22 @@ impl Default for TargetDataLayout {
241241
let align = |bits| Align::from_bits(bits).unwrap();
242242
TargetDataLayout {
243243
endian: Endian::Big,
244-
i1_align: AbiAndPrefAlign::new(align(8)),
245-
i8_align: AbiAndPrefAlign::new(align(8)),
246-
i16_align: AbiAndPrefAlign::new(align(16)),
247-
i32_align: AbiAndPrefAlign::new(align(32)),
248-
i64_align: AbiAndPrefAlign { abi: align(32), pref: align(64) },
249-
i128_align: AbiAndPrefAlign { abi: align(32), pref: align(64) },
250-
f16_align: AbiAndPrefAlign::new(align(16)),
251-
f32_align: AbiAndPrefAlign::new(align(32)),
252-
f64_align: AbiAndPrefAlign::new(align(64)),
253-
f128_align: AbiAndPrefAlign::new(align(128)),
244+
i1_align: AbiAlign::new(align(8)),
245+
i8_align: AbiAlign::new(align(8)),
246+
i16_align: AbiAlign::new(align(16)),
247+
i32_align: AbiAlign::new(align(32)),
248+
i64_align: AbiAlign::new(align(32)),
249+
i128_align: AbiAlign::new(align(32)),
250+
f16_align: AbiAlign::new(align(16)),
251+
f32_align: AbiAlign::new(align(32)),
252+
f64_align: AbiAlign::new(align(64)),
253+
f128_align: AbiAlign::new(align(128)),
254254
pointer_size: Size::from_bits(64),
255-
pointer_align: AbiAndPrefAlign::new(align(64)),
256-
aggregate_align: AbiAndPrefAlign { abi: align(0), pref: align(64) },
255+
pointer_align: AbiAlign::new(align(64)),
256+
aggregate_align: AbiAlign { abi: align(8) },
257257
vector_align: vec![
258-
(Size::from_bits(64), AbiAndPrefAlign::new(align(64))),
259-
(Size::from_bits(128), AbiAndPrefAlign::new(align(128))),
258+
(Size::from_bits(64), AbiAlign::new(align(64))),
259+
(Size::from_bits(128), AbiAlign::new(align(128))),
260260
],
261261
instruction_address_space: AddressSpace::DATA,
262262
c_enum_min_size: Integer::I32,
@@ -314,8 +314,7 @@ impl TargetDataLayout {
314314
.map_err(|err| TargetDataLayoutErrors::InvalidAlignment { cause, err })
315315
};
316316
let abi = parse_bits(s[0], "alignment", cause)?;
317-
let pref = s.get(1).map_or(Ok(abi), |pref| parse_bits(pref, "alignment", cause))?;
318-
Ok(AbiAndPrefAlign { abi: align_from_bits(abi)?, pref: align_from_bits(pref)? })
317+
Ok(AbiAlign::new(align_from_bits(abi)?))
319318
};
320319

321320
let mut dl = TargetDataLayout::default();
@@ -409,15 +408,15 @@ impl TargetDataLayout {
409408
}
410409

411410
#[inline]
412-
pub fn vector_align(&self, vec_size: Size) -> AbiAndPrefAlign {
411+
pub fn vector_align(&self, vec_size: Size) -> AbiAlign {
413412
for &(size, align) in &self.vector_align {
414413
if size == vec_size {
415414
return align;
416415
}
417416
}
418417
// Default to natural alignment, which is what LLVM does.
419418
// That is, use the size, rounded up to a power of 2.
420-
AbiAndPrefAlign::new(Align::from_bytes(vec_size.bytes().next_power_of_two()).unwrap())
419+
AbiAlign::new(Align::from_bytes(vec_size.bytes().next_power_of_two()).unwrap())
421420
}
422421
}
423422

@@ -838,25 +837,24 @@ impl Align {
838837
/// It is of effectively no consequence for layout in structs and on the stack.
839838
#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
840839
#[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
841-
pub struct AbiAndPrefAlign {
840+
pub struct AbiAlign {
842841
pub abi: Align,
843-
pub pref: Align,
844842
}
845843

846-
impl AbiAndPrefAlign {
844+
impl AbiAlign {
847845
#[inline]
848-
pub fn new(align: Align) -> AbiAndPrefAlign {
849-
AbiAndPrefAlign { abi: align, pref: align }
846+
pub fn new(align: Align) -> AbiAlign {
847+
AbiAlign { abi: align }
850848
}
851849

852850
#[inline]
853-
pub fn min(self, other: AbiAndPrefAlign) -> AbiAndPrefAlign {
854-
AbiAndPrefAlign { abi: self.abi.min(other.abi), pref: self.pref.min(other.pref) }
851+
pub fn min(self, other: AbiAlign) -> AbiAlign {
852+
AbiAlign { abi: self.abi.min(other.abi) }
855853
}
856854

857855
#[inline]
858-
pub fn max(self, other: AbiAndPrefAlign) -> AbiAndPrefAlign {
859-
AbiAndPrefAlign { abi: self.abi.max(other.abi), pref: self.pref.max(other.pref) }
856+
pub fn max(self, other: AbiAlign) -> AbiAlign {
857+
AbiAlign { abi: self.abi.max(other.abi) }
860858
}
861859
}
862860

@@ -916,7 +914,7 @@ impl Integer {
916914
}
917915
}
918916

919-
pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAndPrefAlign {
917+
pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAlign {
920918
use Integer::*;
921919
let dl = cx.data_layout();
922920

@@ -1029,7 +1027,7 @@ impl Float {
10291027
}
10301028
}
10311029

1032-
pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAndPrefAlign {
1030+
pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAlign {
10331031
use Float::*;
10341032
let dl = cx.data_layout();
10351033

@@ -1073,7 +1071,7 @@ impl Primitive {
10731071
}
10741072
}
10751073

1076-
pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAndPrefAlign {
1074+
pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAlign {
10771075
use Primitive::*;
10781076
let dl = cx.data_layout();
10791077

@@ -1196,7 +1194,7 @@ impl Scalar {
11961194
}
11971195
}
11981196

1199-
pub fn align(self, cx: &impl HasDataLayout) -> AbiAndPrefAlign {
1197+
pub fn align(self, cx: &impl HasDataLayout) -> AbiAlign {
12001198
self.primitive().align(cx)
12011199
}
12021200

@@ -1464,7 +1462,7 @@ impl BackendRepr {
14641462
}
14651463

14661464
/// Returns the fixed alignment of this ABI, if any is mandated.
1467-
pub fn inherent_align<C: HasDataLayout>(&self, cx: &C) -> Option<AbiAndPrefAlign> {
1465+
pub fn inherent_align<C: HasDataLayout>(&self, cx: &C) -> Option<AbiAlign> {
14681466
Some(match *self {
14691467
BackendRepr::Scalar(s) => s.align(cx),
14701468
BackendRepr::ScalarPair(s1, s2) => s1.align(cx).max(s2.align(cx)),
@@ -1705,7 +1703,7 @@ pub struct LayoutData<FieldIdx: Idx, VariantIdx: Idx> {
17051703
/// (i.e. outside of its `valid_range`), if it exists.
17061704
pub largest_niche: Option<Niche>,
17071705

1708-
pub align: AbiAndPrefAlign,
1706+
pub align: AbiAlign,
17091707
pub size: Size,
17101708

17111709
/// The largest alignment explicitly requested with `repr(align)` on this type or any field.

‎compiler/rustc_const_eval/src/interpret/intrinsics.rs

Lines changed: 0 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -50,13 +50,6 @@ pub(crate) fn eval_nullary_intrinsic<'tcx>(
5050
ensure_monomorphic_enough(tcx, tp_ty)?;
5151
ConstValue::from_bool(tp_ty.needs_drop(tcx, typing_env))
5252
}
53-
sym::pref_align_of => {
54-
// Correctly handles non-monomorphic calls, so there is no need for ensure_monomorphic_enough.
55-
let layout = tcx
56-
.layout_of(typing_env.as_query_input(tp_ty))
57-
.map_err(|e| err_inval!(Layout(*e)))?;
58-
ConstValue::from_target_usize(layout.align.pref.bytes(), &tcx)
59-
}
6053
sym::type_id => {
6154
ensure_monomorphic_enough(tcx, tp_ty)?;
6255
ConstValue::from_u128(tcx.type_id_hash(tp_ty).as_u128())

‎compiler/rustc_ty_utils/src/layout.rs

Lines changed: 2 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -5,7 +5,7 @@ use hir::def_id::DefId;
55
use rustc_abi::Integer::{I8, I32};
66
use rustc_abi::Primitive::{self, Float, Int, Pointer};
77
use rustc_abi::{
8-
AbiAndPrefAlign, AddressSpace, Align, BackendRepr, FIRST_VARIANT, FieldIdx, FieldsShape,
8+
AbiAlign, AddressSpace, Align, BackendRepr, FIRST_VARIANT, FieldIdx, FieldsShape,
99
HasDataLayout, Layout, LayoutCalculatorError, LayoutData, Niche, ReprOptions, Scalar, Size,
1010
StructKind, TagEncoding, VariantIdx, Variants, WrappingRange,
1111
};
@@ -560,13 +560,7 @@ fn layout_of_uncached<'tcx>(
560560
// Non-power-of-two vectors have padding up to the next power-of-two.
561561
// If we're a packed repr, remove the padding while keeping the alignment as close
562562
// to a vector as possible.
563-
(
564-
BackendRepr::Memory { sized: true },
565-
AbiAndPrefAlign {
566-
abi: Align::max_for_offset(size),
567-
pref: dl.vector_align(size).pref,
568-
},
569-
)
563+
(BackendRepr::Memory { sized: true }, AbiAlign { abi: Align::max_for_offset(size) })
570564
} else {
571565
(BackendRepr::Vector { element: e_abi, count: e_len }, dl.vector_align(size))
572566
};

‎library/core/src/intrinsics/mod.rs

Lines changed: 0 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -4173,18 +4173,6 @@ pub const fn min_align_of<T>() -> usize {
41734173
unreachable!()
41744174
}
41754175

4176-
/// The preferred alignment of a type.
4177-
///
4178-
/// This intrinsic does not have a stable counterpart.
4179-
/// It's "tracking issue" is [#91971](https://github.com/rust-lang/rust/issues/91971).
4180-
#[rustc_nounwind]
4181-
#[unstable(feature = "core_intrinsics", issue = "none")]
4182-
#[rustc_intrinsic]
4183-
#[rustc_intrinsic_must_be_overridden]
4184-
pub const unsafe fn pref_align_of<T>() -> usize {
4185-
unreachable!()
4186-
}
4187-
41884176
/// Returns the number of variants of the type `T` cast to a `usize`;
41894177
/// if `T` has no variants, returns `0`. Uninhabited variants will be counted.
41904178
///

‎tests/ui/abi/c-zst.aarch64-darwin.stderr

Lines changed: 2 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -5,9 +5,8 @@ error: fn_abi_of(pass_zst) = FnAbi {
55
ty: (),
66
layout: Layout {
77
size: Size(0 bytes),
8-
align: AbiAndPrefAlign {
8+
align: AbiAlign {
99
abi: $SOME_ALIGN,
10-
pref: $SOME_ALIGN,
1110
},
1211
abi: Memory {
1312
sized: true,
@@ -33,9 +32,8 @@ error: fn_abi_of(pass_zst) = FnAbi {
3332
ty: (),
3433
layout: Layout {
3534
size: Size(0 bytes),
36-
align: AbiAndPrefAlign {
35+
align: AbiAlign {
3736
abi: $SOME_ALIGN,
38-
pref: $SOME_ALIGN,
3937
},
4038
abi: Memory {
4139
sized: true,

‎tests/ui/abi/c-zst.powerpc-linux.stderr

Lines changed: 2 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -5,9 +5,8 @@ error: fn_abi_of(pass_zst) = FnAbi {
55
ty: (),
66
layout: Layout {
77
size: Size(0 bytes),
8-
align: AbiAndPrefAlign {
8+
align: AbiAlign {
99
abi: $SOME_ALIGN,
10-
pref: $SOME_ALIGN,
1110
},
1211
abi: Memory {
1312
sized: true,
@@ -44,9 +43,8 @@ error: fn_abi_of(pass_zst) = FnAbi {
4443
ty: (),
4544
layout: Layout {
4645
size: Size(0 bytes),
47-
align: AbiAndPrefAlign {
46+
align: AbiAlign {
4847
abi: $SOME_ALIGN,
49-
pref: $SOME_ALIGN,
5048
},
5149
abi: Memory {
5250
sized: true,

‎tests/ui/abi/c-zst.s390x-linux.stderr

Lines changed: 2 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -5,9 +5,8 @@ error: fn_abi_of(pass_zst) = FnAbi {
55
ty: (),
66
layout: Layout {
77
size: Size(0 bytes),
8-
align: AbiAndPrefAlign {
8+
align: AbiAlign {
99
abi: $SOME_ALIGN,
10-
pref: $SOME_ALIGN,
1110
},
1211
abi: Memory {
1312
sized: true,
@@ -44,9 +43,8 @@ error: fn_abi_of(pass_zst) = FnAbi {
4443
ty: (),
4544
layout: Layout {
4645
size: Size(0 bytes),
47-
align: AbiAndPrefAlign {
46+
align: AbiAlign {
4847
abi: $SOME_ALIGN,
49-
pref: $SOME_ALIGN,
5048
},
5149
abi: Memory {
5250
sized: true,

0 commit comments

Comments
 (0)