Remove rustc's notion of "preferred" alignment AKA __alignof

In PR 90877 T-lang decided not 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,
    in ways Rust would not consider even with increased C++ interop
4.  is only used by rustc to overalign some globals, not correctness
5.  can be adequately replaced by other rules for globals, as it mostly
    affects alignments for a few types under 16 bytes of alignment
6.  has only one clear benefactor: automating C -> Rust translation
    for GNU extensions like `__alignof`
7.  such code was likely intended to be `alignof` or `_Alignof`,
    because the GNU extension is a "false friend" of the C keyword,
    which makes the choice to support such a mapping very questionable
8.  makes it easy to do incorrect codegen in the compiler by its mere
    presence as usual Rust rules of alignment (e.g. `size == align * N`)
    do not hold with preferred alignment

The implementation is clearly damaging the code quality of the compiler.
Thus it is within the compiler team's purview to simply rip it out.
If T-lang wishes to have this intrinsic restored for c2rust's benefit,
it would have to use a radically different implementation that somehow
does not cause internal incorrectness.

Until then, remove the intrinsic and its supporting code, as one tool
and an ill-considered GCC extension cannot justify risking correctness.

Because we touch a fair amount of the compiler to change this at all,
and unfortunately the duplication of AbiAndPrefAlign is deep-rooted,
we keep an "AbiAlign" type which we can wean code off later.

compiler/rustc_abi/src/layout.rs

@@ -8,7 +8,7 @@ use rustc_index::bit_set::BitMatrix;
 use tracing::debug;
 
 use crate::{
-    AbiAndPrefAlign, Align, BackendRepr, FieldsShape, HasDataLayout, IndexSlice, IndexVec, Integer,
+    AbiAlign, Align, BackendRepr, FieldsShape, HasDataLayout, IndexSlice, IndexVec, Integer,
     LayoutData, Niche, NonZeroUsize, Primitive, ReprOptions, Scalar, Size, StructKind, TagEncoding,
     Variants, WrappingRange,
 };
@@ -173,13 +173,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             // Non-power-of-two vectors have padding up to the next power-of-two.
             // If we're a packed repr, remove the padding while keeping the alignment as close
             // to a vector as possible.
-            (
-                BackendRepr::Memory { sized: true },
-                AbiAndPrefAlign {
-                    abi: Align::max_aligned_factor(size),
-                    pref: dl.llvmlike_vector_align(size).pref,
-                },
-            )
+            (BackendRepr::Memory { sized: true }, AbiAlign { abi: Align::max_aligned_factor(size) })
         } else {
             (BackendRepr::SimdVector { element: e_repr, count }, dl.llvmlike_vector_align(size))
         };
@@ -435,13 +429,13 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         }
 
         if let Some(pack) = repr.pack {
-            align = align.min(AbiAndPrefAlign::new(pack));
+            align = align.min(AbiAlign::new(pack));
         }
         // The unadjusted ABI alignment does not include repr(align), but does include repr(pack).
         // See documentation on `LayoutS::unadjusted_abi_align`.
         let unadjusted_abi_align = align.abi;
         if let Some(repr_align) = repr.align {
-            align = align.max(AbiAndPrefAlign::new(repr_align));
+            align = align.max(AbiAlign::new(repr_align));
         }
         // `align` must not be modified after this, or `unadjusted_abi_align` could be inaccurate.
         let align = align;
@@ -1289,7 +1283,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         if let StructKind::Prefixed(prefix_size, prefix_align) = kind {
             let prefix_align =
                 if let Some(pack) = pack { prefix_align.min(pack) } else { prefix_align };
-            align = align.max(AbiAndPrefAlign::new(prefix_align));
+            align = align.max(AbiAlign::new(prefix_align));
             offset = prefix_size.align_to(prefix_align);
         }
         for &i in &inverse_memory_index {
@@ -1308,7 +1302,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
 
             // Invariant: offset < dl.obj_size_bound() <= 1<<61
             let field_align = if let Some(pack) = pack {
-                field.align.min(AbiAndPrefAlign::new(pack))
+                field.align.min(AbiAlign::new(pack))
             } else {
                 field.align
             };
@@ -1342,7 +1336,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         // See documentation on `LayoutS::unadjusted_abi_align`.
         let unadjusted_abi_align = align.abi;
         if let Some(repr_align) = repr.align {
-            align = align.max(AbiAndPrefAlign::new(repr_align));
+            align = align.max(AbiAlign::new(repr_align));
         }
         // `align` must not be modified after this point, or `unadjusted_abi_align` could be inaccurate.
         let align = align;

compiler/rustc_abi/src/layout/ty.rs

@@ -5,7 +5,7 @@ use rustc_data_structures::intern::Interned;
 use rustc_macros::HashStable_Generic;
 
 use crate::{
-    AbiAndPrefAlign, Align, BackendRepr, FieldsShape, Float, HasDataLayout, LayoutData, Niche,
+    AbiAlign, Align, BackendRepr, FieldsShape, Float, HasDataLayout, LayoutData, Niche,
     PointeeInfo, Primitive, Scalar, Size, TargetDataLayout, Variants,
 };
 
@@ -93,7 +93,7 @@ impl<'a> Layout<'a> {
         self.0.0.largest_niche
     }
 
-    pub fn align(self) -> AbiAndPrefAlign {
+    pub fn align(self) -> AbiAlign {
         self.0.0.align
     }
 

compiler/rustc_abi/src/lib.rs

@@ -226,22 +226,22 @@ pub const MAX_SIMD_LANES: u64 = 1 << 0xF;
 #[derive(Debug, PartialEq, Eq)]
 pub struct TargetDataLayout {
     pub endian: Endian,
-    pub i1_align: AbiAndPrefAlign,
-    pub i8_align: AbiAndPrefAlign,
-    pub i16_align: AbiAndPrefAlign,
-    pub i32_align: AbiAndPrefAlign,
-    pub i64_align: AbiAndPrefAlign,
-    pub i128_align: AbiAndPrefAlign,
-    pub f16_align: AbiAndPrefAlign,
-    pub f32_align: AbiAndPrefAlign,
-    pub f64_align: AbiAndPrefAlign,
-    pub f128_align: AbiAndPrefAlign,
+    pub i1_align: AbiAlign,
+    pub i8_align: AbiAlign,
+    pub i16_align: AbiAlign,
+    pub i32_align: AbiAlign,
+    pub i64_align: AbiAlign,
+    pub i128_align: AbiAlign,
+    pub f16_align: AbiAlign,
+    pub f32_align: AbiAlign,
+    pub f64_align: AbiAlign,
+    pub f128_align: AbiAlign,
     pub pointer_size: Size,
-    pub pointer_align: AbiAndPrefAlign,
-    pub aggregate_align: AbiAndPrefAlign,
+    pub pointer_align: AbiAlign,
+    pub aggregate_align: AbiAlign,
 
     /// Alignments for vector types.
-    pub vector_align: Vec<(Size, AbiAndPrefAlign)>,
+    pub vector_align: Vec<(Size, AbiAlign)>,
 
     pub instruction_address_space: AddressSpace,
 
@@ -257,22 +257,22 @@ impl Default for TargetDataLayout {
         let align = |bits| Align::from_bits(bits).unwrap();
         TargetDataLayout {
             endian: Endian::Big,
-            i1_align: AbiAndPrefAlign::new(align(8)),
-            i8_align: AbiAndPrefAlign::new(align(8)),
-            i16_align: AbiAndPrefAlign::new(align(16)),
-            i32_align: AbiAndPrefAlign::new(align(32)),
-            i64_align: AbiAndPrefAlign { abi: align(32), pref: align(64) },
-            i128_align: AbiAndPrefAlign { abi: align(32), pref: align(64) },
-            f16_align: AbiAndPrefAlign::new(align(16)),
-            f32_align: AbiAndPrefAlign::new(align(32)),
-            f64_align: AbiAndPrefAlign::new(align(64)),
-            f128_align: AbiAndPrefAlign::new(align(128)),
+            i1_align: AbiAlign::new(align(8)),
+            i8_align: AbiAlign::new(align(8)),
+            i16_align: AbiAlign::new(align(16)),
+            i32_align: AbiAlign::new(align(32)),
+            i64_align: AbiAlign::new(align(32)),
+            i128_align: AbiAlign::new(align(32)),
+            f16_align: AbiAlign::new(align(16)),
+            f32_align: AbiAlign::new(align(32)),
+            f64_align: AbiAlign::new(align(64)),
+            f128_align: AbiAlign::new(align(128)),
             pointer_size: Size::from_bits(64),
-            pointer_align: AbiAndPrefAlign::new(align(64)),
-            aggregate_align: AbiAndPrefAlign { abi: align(0), pref: align(64) },
+            pointer_align: AbiAlign::new(align(64)),
+            aggregate_align: AbiAlign { abi: align(8) },
             vector_align: vec![
-                (Size::from_bits(64), AbiAndPrefAlign::new(align(64))),
-                (Size::from_bits(128), AbiAndPrefAlign::new(align(128))),
+                (Size::from_bits(64), AbiAlign::new(align(64))),
+                (Size::from_bits(128), AbiAlign::new(align(128))),
             ],
             instruction_address_space: AddressSpace::DATA,
             c_enum_min_size: Integer::I32,
@@ -330,8 +330,7 @@ impl TargetDataLayout {
                     .map_err(|err| TargetDataLayoutErrors::InvalidAlignment { cause, err })
             };
             let abi = parse_bits(s[0], "alignment", cause)?;
-            let pref = s.get(1).map_or(Ok(abi), |pref| parse_bits(pref, "alignment", cause))?;
-            Ok(AbiAndPrefAlign { abi: align_from_bits(abi)?, pref: align_from_bits(pref)? })
+            Ok(AbiAlign::new(align_from_bits(abi)?))
         };
 
         let mut dl = TargetDataLayout::default();
@@ -426,7 +425,7 @@ impl TargetDataLayout {
 
     /// psABI-mandated alignment for a vector type, if any
     #[inline]
-    fn cabi_vector_align(&self, vec_size: Size) -> Option<AbiAndPrefAlign> {
+    fn cabi_vector_align(&self, vec_size: Size) -> Option<AbiAlign> {
         self.vector_align
             .iter()
             .find(|(size, _align)| *size == vec_size)
@@ -435,8 +434,8 @@ impl TargetDataLayout {
 
     /// an alignment resembling the one LLVM would pick for a vector
     #[inline]
-    pub fn llvmlike_vector_align(&self, vec_size: Size) -> AbiAndPrefAlign {
-        self.cabi_vector_align(vec_size).unwrap_or(AbiAndPrefAlign::new(
+    pub fn llvmlike_vector_align(&self, vec_size: Size) -> AbiAlign {
+        self.cabi_vector_align(vec_size).unwrap_or(AbiAlign::new(
             Align::from_bytes(vec_size.bytes().next_power_of_two()).unwrap(),
         ))
     }
@@ -864,25 +863,24 @@ impl Align {
 /// It is of effectively no consequence for layout in structs and on the stack.
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 #[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
-pub struct AbiAndPrefAlign {
+pub struct AbiAlign {
     pub abi: Align,
-    pub pref: Align,
 }
 
-impl AbiAndPrefAlign {
+impl AbiAlign {
     #[inline]
-    pub fn new(align: Align) -> AbiAndPrefAlign {
-        AbiAndPrefAlign { abi: align, pref: align }
+    pub fn new(align: Align) -> AbiAlign {
+        AbiAlign { abi: align }
     }
 
     #[inline]
-    pub fn min(self, other: AbiAndPrefAlign) -> AbiAndPrefAlign {
-        AbiAndPrefAlign { abi: self.abi.min(other.abi), pref: self.pref.min(other.pref) }
+    pub fn min(self, other: AbiAlign) -> AbiAlign {
+        AbiAlign { abi: self.abi.min(other.abi) }
     }
 
     #[inline]
-    pub fn max(self, other: AbiAndPrefAlign) -> AbiAndPrefAlign {
-        AbiAndPrefAlign { abi: self.abi.max(other.abi), pref: self.pref.max(other.pref) }
+    pub fn max(self, other: AbiAlign) -> AbiAlign {
+        AbiAlign { abi: self.abi.max(other.abi) }
     }
 }
 
@@ -945,7 +943,7 @@ impl Integer {
         }
     }
 
-    pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAndPrefAlign {
+    pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAlign {
         use Integer::*;
         let dl = cx.data_layout();
 
@@ -1058,7 +1056,7 @@ impl Float {
         }
     }
 
-    pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAndPrefAlign {
+    pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAlign {
         use Float::*;
         let dl = cx.data_layout();
 
@@ -1102,7 +1100,7 @@ impl Primitive {
         }
     }
 
-    pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAndPrefAlign {
+    pub fn align<C: HasDataLayout>(self, cx: &C) -> AbiAlign {
         use Primitive::*;
         let dl = cx.data_layout();
 
@@ -1225,7 +1223,7 @@ impl Scalar {
         }
     }
 
-    pub fn align(self, cx: &impl HasDataLayout) -> AbiAndPrefAlign {
+    pub fn align(self, cx: &impl HasDataLayout) -> AbiAlign {
         self.primitive().align(cx)
     }
 
@@ -1731,7 +1729,7 @@ pub struct LayoutData<FieldIdx: Idx, VariantIdx: Idx> {
     /// especially in the case of by-pointer struct returns, which allocate stack even when unused.
     pub uninhabited: bool,
 
-    pub align: AbiAndPrefAlign,
+    pub align: AbiAlign,
     pub size: Size,
 
     /// The largest alignment explicitly requested with `repr(align)` on this type or any field.

compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

@@ -812,11 +812,7 @@ fn codegen_regular_intrinsic_call<'tcx>(
             dest.write_cvalue(fx, val);
         }
 
-        sym::pref_align_of
-        | sym::needs_drop
-        | sym::type_id
-        | sym::type_name
-        | sym::variant_count => {
+        sym::needs_drop | sym::type_id | sym::type_name | sym::variant_count => {
             intrinsic_args!(fx, args => (); intrinsic);
 
             let const_val = fx

compiler/rustc_codegen_ssa/src/mir/intrinsic.rs

@@ -151,11 +151,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 }
                 value
             }
-            sym::pref_align_of
-            | sym::needs_drop
-            | sym::type_id
-            | sym::type_name
-            | sym::variant_count => {
+            sym::needs_drop | sym::type_id | sym::type_name | sym::variant_count => {
                 let value = bx.tcx().const_eval_instance(bx.typing_env(), instance, span).unwrap();
                 OperandRef::from_const(bx, value, result.layout.ty).immediate_or_packed_pair(bx)
             }

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -50,13 +50,6 @@ pub(crate) fn eval_nullary_intrinsic<'tcx>(
             ensure_monomorphic_enough(tcx, tp_ty)?;
             ConstValue::from_bool(tp_ty.needs_drop(tcx, typing_env))
         }
-        sym::pref_align_of => {
-            // Correctly handles non-monomorphic calls, so there is no need for ensure_monomorphic_enough.
-            let layout = tcx
-                .layout_of(typing_env.as_query_input(tp_ty))
-                .map_err(|e| err_inval!(Layout(*e)))?;
-            ConstValue::from_target_usize(layout.align.pref.bytes(), &tcx)
-        }
         sym::type_id => {
             ensure_monomorphic_enough(tcx, tp_ty)?;
             ConstValue::from_u128(tcx.type_id_hash(tp_ty).as_u128())
@@ -144,14 +137,10 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 self.write_scalar(Scalar::from_target_usize(result, self), dest)?;
             }
 
-            sym::pref_align_of
-            | sym::needs_drop
-            | sym::type_id
-            | sym::type_name
-            | sym::variant_count => {
+            sym::needs_drop | sym::type_id | sym::type_name | sym::variant_count => {
                 let gid = GlobalId { instance, promoted: None };
                 let ty = match intrinsic_name {
-                    sym::pref_align_of | sym::variant_count => self.tcx.types.usize,
+                    sym::variant_count => self.tcx.types.usize,
                     sym::needs_drop => self.tcx.types.bool,
                     sym::type_id => self.tcx.types.u128,
                     sym::type_name => Ty::new_static_str(self.tcx.tcx),

compiler/rustc_feature/src/removed.rs

@@ -207,6 +207,8 @@ declare_features! (
     /// Allows exhaustive integer pattern matching with `usize::MAX`/`isize::MIN`/`isize::MAX`.
     (removed, precise_pointer_size_matching, "1.32.0", Some(56354),
      Some("removed in favor of half-open ranges")),
+    (removed, pref_align_of, "CURRENT_RUSTC_VERSION", Some(91971),
+     Some("removed due to marginal use and inducing compiler complications")),
     (removed, proc_macro_expr, "1.27.0", Some(54727),
      Some("subsumed by `#![feature(proc_macro_hygiene)]`")),
     (removed, proc_macro_gen, "1.27.0", Some(54727),

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -235,7 +235,7 @@ pub(crate) fn check_intrinsic_type(
             sym::abort => (0, 0, vec![], tcx.types.never),
             sym::unreachable => (0, 0, vec![], tcx.types.never),
             sym::breakpoint => (0, 0, vec![], tcx.types.unit),
-            sym::size_of | sym::pref_align_of | sym::min_align_of | sym::variant_count => {
+            sym::size_of | sym::min_align_of | sym::variant_count => {
                 (1, 0, vec![], tcx.types.usize)
             }
             sym::size_of_val | sym::min_align_of_val => {

library/core/src/intrinsics/mod.rs

@@ -3485,15 +3485,6 @@ pub const fn size_of<T>() -> usize;
 #[rustc_intrinsic]
 pub const fn min_align_of<T>() -> usize;
 
-/// The preferred alignment of a type.
-///
-/// This intrinsic does not have a stable counterpart.
-/// It's "tracking issue" is [#91971](https://github.com/rust-lang/rust/issues/91971).
-#[rustc_nounwind]
-#[unstable(feature = "core_intrinsics", issue = "none")]
-#[rustc_intrinsic]
-pub const unsafe fn pref_align_of<T>() -> usize;
-
 /// Returns the number of variants of the type `T` cast to a `usize`;
 /// if `T` has no variants, returns `0`. Uninhabited variants will be counted.
 ///

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

@@ -5,9 +5,8 @@ error: fn_abi_of(pass_zst) = FnAbi {
                        ty: (),
                        layout: Layout {
                            size: Size(0 bytes),
-                           align: AbiAndPrefAlign {
+                           align: AbiAlign {
                                abi: $SOME_ALIGN,
-                               pref: $SOME_ALIGN,
                            },
                            backend_repr: Memory {
                                sized: true,
@@ -34,9 +33,8 @@ error: fn_abi_of(pass_zst) = FnAbi {
                    ty: (),
                    layout: Layout {
                        size: Size(0 bytes),
-                       align: AbiAndPrefAlign {
+                       align: AbiAlign {
                            abi: $SOME_ALIGN,
-                           pref: $SOME_ALIGN,
                        },
                        backend_repr: Memory {
                            sized: true,