compiler/rustc_ty_utils/src/layout_sanity_check.rs

   1 use rustc_middle::ty::{
   2     layout::{LayoutCx, TyAndLayout},
   3     TyCtxt,
   4 };
   5 use rustc_target::abi::*;
   6
   7 use std::cmp;
   8
   9 /// Enforce some basic invariants on layouts.
  10 pub(super) fn sanity_check_layout<'tcx>(
  11     cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
  12     layout: &TyAndLayout<'tcx>,
  13 ) {
  14     // Type-level uninhabitedness should always imply ABI uninhabitedness.
  15     if layout.ty.is_privately_uninhabited(cx.tcx, cx.param_env) {
  16         assert!(layout.abi.is_uninhabited());
  17     }
  18
  19     if layout.size.bytes() % layout.align.abi.bytes() != 0 {
  20         bug!("size is not a multiple of align, in the following layout:\n{layout:#?}");
  21     }
  22
  23     if !cfg!(debug_assertions) {
  24         // Stop here, the rest is kind of expensive.
  25         return;
  26     }
  27
  28     /// Yields non-ZST fields of the type
  29     fn non_zst_fields<'tcx, 'a>(
  30         cx: &'a LayoutCx<'tcx, TyCtxt<'tcx>>,
  31         layout: &'a TyAndLayout<'tcx>,
  32     ) -> impl Iterator<Item = (Size, TyAndLayout<'tcx>)> + 'a {
  33         (0..layout.layout.fields().count()).filter_map(|i| {
  34             let field = layout.field(cx, i);
  35             // Also checking `align == 1` here leads to test failures in
  36             // `layout/zero-sized-array-union.rs`, where a type has a zero-size field with
  37             // alignment 4 that still gets ignored during layout computation (which is okay
  38             // since other fields already force alignment 4).
  39             let zst = field.is_zst();
  40             (!zst).then(|| (layout.fields.offset(i), field))
  41         })
  42     }
  43
  44     fn skip_newtypes<'tcx>(
  45         cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
  46         layout: &TyAndLayout<'tcx>,
  47     ) -> TyAndLayout<'tcx> {
  48         if matches!(layout.layout.variants(), Variants::Multiple { .. }) {
  49             // Definitely not a newtype of anything.
  50             return *layout;
  51         }
  52         let mut fields = non_zst_fields(cx, layout);
  53         let Some(first) = fields.next() else {
  54             // No fields here, so this could be a primitive or enum -- either way it's not a newtype around a thing
  55             return *layout
  56         };
  57         if fields.next().is_none() {
  58             let (offset, first) = first;
  59             if offset == Size::ZERO && first.layout.size() == layout.size {
  60                 // This is a newtype, so keep recursing.
  61                 // FIXME(RalfJung): I don't think it would be correct to do any checks for
  62                 // alignment here, so we don't. Is that correct?
  63                 return skip_newtypes(cx, &first);
  64             }
  65         }
  66         // No more newtypes here.
  67         *layout
  68     }
  69
  70     fn check_layout_abi<'tcx>(cx: &LayoutCx<'tcx, TyCtxt<'tcx>>, layout: &TyAndLayout<'tcx>) {
  71         match layout.layout.abi() {
  72             Abi::Scalar(scalar) => {
  73                 // No padding in scalars.
  74                 let size = scalar.size(cx);
  75                 let align = scalar.align(cx).abi;
  76                 assert_eq!(
  77                     layout.layout.size(),
  78                     size,
  79                     "size mismatch between ABI and layout in {layout:#?}"
  80                 );
  81                 assert_eq!(
  82                     layout.layout.align().abi,
  83                     align,
  84                     "alignment mismatch between ABI and layout in {layout:#?}"
  85                 );
  86                 // Check that this matches the underlying field.
  87                 let inner = skip_newtypes(cx, layout);
  88                 assert!(
  89                     matches!(inner.layout.abi(), Abi::Scalar(_)),
  90                     "`Scalar` type {} is newtype around non-`Scalar` type {}",
  91                     layout.ty,
  92                     inner.ty
  93                 );
  94                 match inner.layout.fields() {
  95                     FieldsShape::Primitive => {
  96                         // Fine.
  97                     }
  98                     FieldsShape::Union(..) => {
  99                         // FIXME: I guess we could also check something here? Like, look at all fields?
 100                         return;
 101                     }
 102                     FieldsShape::Arbitrary { .. } => {
 103                         // Should be an enum, the only field is the discriminant.
 104                         assert!(
 105                             inner.ty.is_enum(),
 106                             "`Scalar` layout for non-primitive non-enum type {}",
 107                             inner.ty
 108                         );
 109                         assert_eq!(
 110                             inner.layout.fields().count(),
 111                             1,
 112                             "`Scalar` layout for multiple-field type in {inner:#?}",
 113                         );
 114                         let offset = inner.layout.fields().offset(0);
 115                         let field = inner.field(cx, 0);
 116                         // The field should be at the right offset, and match the `scalar` layout.
 117                         assert_eq!(
 118                             offset,
 119                             Size::ZERO,
 120                             "`Scalar` field at non-0 offset in {inner:#?}",
 121                         );
 122                         assert_eq!(field.size, size, "`Scalar` field with bad size in {inner:#?}",);
 123                         assert_eq!(
 124                             field.align.abi, align,
 125                             "`Scalar` field with bad align in {inner:#?}",
 126                         );
 127                         assert!(
 128                             matches!(field.abi, Abi::Scalar(_)),
 129                             "`Scalar` field with bad ABI in {inner:#?}",
 130                         );
 131                     }
 132                     _ => {
 133                         panic!("`Scalar` layout for non-primitive non-enum type {}", inner.ty);
 134                     }
 135                 }
 136             }
 137             Abi::ScalarPair(scalar1, scalar2) => {
 138                 // Sanity-check scalar pairs. These are a bit more flexible and support
 139                 // padding, but we can at least ensure both fields actually fit into the layout
 140                 // and the alignment requirement has not been weakened.
 141                 let size1 = scalar1.size(cx);
 142                 let align1 = scalar1.align(cx).abi;
 143                 let size2 = scalar2.size(cx);
 144                 let align2 = scalar2.align(cx).abi;
 145                 assert!(
 146                     layout.layout.align().abi >= cmp::max(align1, align2),
 147                     "alignment mismatch between ABI and layout in {layout:#?}",
 148                 );
 149                 let field2_offset = size1.align_to(align2);
 150                 assert!(
 151                     layout.layout.size() >= field2_offset + size2,
 152                     "size mismatch between ABI and layout in {layout:#?}"
 153                 );
 154                 // Check that the underlying pair of fields matches.
 155                 let inner = skip_newtypes(cx, layout);
 156                 assert!(
 157                     matches!(inner.layout.abi(), Abi::ScalarPair(..)),
 158                     "`ScalarPair` type {} is newtype around non-`ScalarPair` type {}",
 159                     layout.ty,
 160                     inner.ty
 161                 );
 162                 if matches!(inner.layout.variants(), Variants::Multiple { .. }) {
 163                     // FIXME: ScalarPair for enums is enormously complicated and it is very hard
 164                     // to check anything about them.
 165                     return;
 166                 }
 167                 match inner.layout.fields() {
 168                     FieldsShape::Arbitrary { .. } => {
 169                         // Checked below.
 170                     }
 171                     FieldsShape::Union(..) => {
 172                         // FIXME: I guess we could also check something here? Like, look at all fields?
 173                         return;
 174                     }
 175                     _ => {
 176                         panic!("`ScalarPair` layout with unexpected field shape in {inner:#?}");
 177                     }
 178                 }
 179                 let mut fields = non_zst_fields(cx, &inner);
 180                 let (offset1, field1) = fields.next().unwrap_or_else(|| {
 181                     panic!(
 182                         "`ScalarPair` layout for type with not even one non-ZST field: {inner:#?}"
 183                     )
 184                 });
 185                 let (offset2, field2) = fields.next().unwrap_or_else(|| {
 186                     panic!(
 187                         "`ScalarPair` layout for type with less than two non-ZST fields: {inner:#?}"
 188                     )
 189                 });
 190                 assert!(
 191                     fields.next().is_none(),
 192                     "`ScalarPair` layout for type with at least three non-ZST fields: {inner:#?}"
 193                 );
 194                 // The fields might be in opposite order.
 195                 let (offset1, field1, offset2, field2) = if offset1 <= offset2 {
 196                     (offset1, field1, offset2, field2)
 197                 } else {
 198                     (offset2, field2, offset1, field1)
 199                 };
 200                 // The fields should be at the right offset, and match the `scalar` layout.
 201                 assert_eq!(
 202                     offset1,
 203                     Size::ZERO,
 204                     "`ScalarPair` first field at non-0 offset in {inner:#?}",
 205                 );
 206                 assert_eq!(
 207                     field1.size, size1,
 208                     "`ScalarPair` first field with bad size in {inner:#?}",
 209                 );
 210                 assert_eq!(
 211                     field1.align.abi, align1,
 212                     "`ScalarPair` first field with bad align in {inner:#?}",
 213                 );
 214                 assert!(
 215                     matches!(field1.abi, Abi::Scalar(_)),
 216                     "`ScalarPair` first field with bad ABI in {inner:#?}",
 217                 );
 218                 assert_eq!(
 219                     offset2, field2_offset,
 220                     "`ScalarPair` second field at bad offset in {inner:#?}",
 221                 );
 222                 assert_eq!(
 223                     field2.size, size2,
 224                     "`ScalarPair` second field with bad size in {inner:#?}",
 225                 );
 226                 assert_eq!(
 227                     field2.align.abi, align2,
 228                     "`ScalarPair` second field with bad align in {inner:#?}",
 229                 );
 230                 assert!(
 231                     matches!(field2.abi, Abi::Scalar(_)),
 232                     "`ScalarPair` second field with bad ABI in {inner:#?}",
 233                 );
 234             }
 235             Abi::Vector { count, element } => {
 236                 // No padding in vectors. Alignment can be strengthened, though.
 237                 assert!(
 238                     layout.layout.align().abi >= element.align(cx).abi,
 239                     "alignment mismatch between ABI and layout in {layout:#?}"
 240                 );
 241                 let size = element.size(cx) * count;
 242                 assert_eq!(
 243                     layout.layout.size(),
 244                     size.align_to(cx.data_layout().vector_align(size).abi),
 245                     "size mismatch between ABI and layout in {layout:#?}"
 246                 );
 247             }
 248             Abi::Uninhabited | Abi::Aggregate { .. } => {} // Nothing to check.
 249         }
 250     }
 251
 252     check_layout_abi(cx, layout);
 253
 254     if let Variants::Multiple { variants, .. } = &layout.variants {
 255         for variant in variants.iter() {
 256             // No nested "multiple".
 257             assert!(matches!(variant.variants, Variants::Single { .. }));
 258             // Variants should have the same or a smaller size as the full thing,
 259             // and same for alignment.
 260             if variant.size > layout.size {
 261                 bug!(
 262                     "Type with size {} bytes has variant with size {} bytes: {layout:#?}",
 263                     layout.size.bytes(),
 264                     variant.size.bytes(),
 265                 )
 266             }
 267             if variant.align.abi > layout.align.abi {
 268                 bug!(
 269                     "Type with alignment {} bytes has variant with alignment {} bytes: {layout:#?}",
 270                     layout.align.abi.bytes(),
 271                     variant.align.abi.bytes(),
 272                 )
 273             }
 274             // Skip empty variants.
 275             if variant.size == Size::ZERO
 276                 || variant.fields.count() == 0
 277                 || variant.abi.is_uninhabited()
 278             {
 279                 // These are never actually accessed anyway, so we can skip the coherence check
 280                 // for them. They also fail that check, since they have
 281                 // `Aggregate`/`Uninhbaited` ABI even when the main type is
 282                 // `Scalar`/`ScalarPair`. (Note that sometimes, variants with fields have size
 283                 // 0, and sometimes, variants without fields have non-0 size.)
 284                 continue;
 285             }
 286             // The top-level ABI and the ABI of the variants should be coherent.
 287             let scalar_coherent =
 288                 |s1: Scalar, s2: Scalar| s1.size(cx) == s2.size(cx) && s1.align(cx) == s2.align(cx);
 289             let abi_coherent = match (layout.abi, variant.abi) {
 290                 (Abi::Scalar(s1), Abi::Scalar(s2)) => scalar_coherent(s1, s2),
 291                 (Abi::ScalarPair(a1, b1), Abi::ScalarPair(a2, b2)) => {
 292                     scalar_coherent(a1, a2) && scalar_coherent(b1, b2)
 293                 }
 294                 (Abi::Uninhabited, _) => true,
 295                 (Abi::Aggregate { .. }, _) => true,
 296                 _ => false,
 297             };
 298             if !abi_coherent {
 299                 bug!(
 300                     "Variant ABI is incompatible with top-level ABI:\nvariant={:#?}\nTop-level: {layout:#?}",
 301                     variant
 302                 );
 303             }
 304         }
 305     }
 306 }