src/librustc_codegen_ssa/mir/place.rs

   1 use super::operand::OperandValue;
   2 use super::{FunctionCx, LocalRef};
   3
   4 use crate::common::IntPredicate;
   5 use crate::glue;
   6 use crate::traits::*;
   7 use crate::MemFlags;
   8
   9 use rustc::mir;
  10 use rustc::mir::tcx::PlaceTy;
  11 use rustc::ty::layout::{self, Align, HasTyCtxt, LayoutOf, TyLayout, VariantIdx};
  12 use rustc::ty::{self, Ty};
  13
  14 #[derive(Copy, Clone, Debug)]
  15 pub struct PlaceRef<'tcx, V> {
  16     /// A pointer to the contents of the place.
  17     pub llval: V,
  18
  19     /// This place's extra data if it is unsized, or `None` if null.
  20     pub llextra: Option<V>,
  21
  22     /// The monomorphized type of this place, including variant information.
  23     pub layout: TyLayout<'tcx>,
  24
  25     /// The alignment we know for this place.
  26     pub align: Align,
  27 }
  28
  29 impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
  30     pub fn new_sized(llval: V, layout: TyLayout<'tcx>) -> PlaceRef<'tcx, V> {
  31         assert!(!layout.is_unsized());
  32         PlaceRef { llval, llextra: None, layout, align: layout.align.abi }
  33     }
  34
  35     pub fn new_sized_aligned(llval: V, layout: TyLayout<'tcx>, align: Align) -> PlaceRef<'tcx, V> {
  36         assert!(!layout.is_unsized());
  37         PlaceRef { llval, llextra: None, layout, align }
  38     }
  39
  40     // FIXME(eddyb) pass something else for the name so no work is done
  41     // unless LLVM IR names are turned on (e.g. for `--emit=llvm-ir`).
  42     pub fn alloca<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
  43         bx: &mut Bx,
  44         layout: TyLayout<'tcx>,
  45     ) -> Self {
  46         assert!(!layout.is_unsized(), "tried to statically allocate unsized place");
  47         let tmp = bx.alloca(bx.cx().backend_type(layout), layout.align.abi);
  48         Self::new_sized(tmp, layout)
  49     }
  50
  51     /// Returns a place for an indirect reference to an unsized place.
  52     // FIXME(eddyb) pass something else for the name so no work is done
  53     // unless LLVM IR names are turned on (e.g. for `--emit=llvm-ir`).
  54     pub fn alloca_unsized_indirect<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
  55         bx: &mut Bx,
  56         layout: TyLayout<'tcx>,
  57     ) -> Self {
  58         assert!(layout.is_unsized(), "tried to allocate indirect place for sized values");
  59         let ptr_ty = bx.cx().tcx().mk_mut_ptr(layout.ty);
  60         let ptr_layout = bx.cx().layout_of(ptr_ty);
  61         Self::alloca(bx, ptr_layout)
  62     }
  63
  64     pub fn len<Cx: ConstMethods<'tcx, Value = V>>(&self, cx: &Cx) -> V {
  65         if let layout::FieldPlacement::Array { count, .. } = self.layout.fields {
  66             if self.layout.is_unsized() {
  67                 assert_eq!(count, 0);
  68                 self.llextra.unwrap()
  69             } else {
  70                 cx.const_usize(count)
  71             }
  72         } else {
  73             bug!("unexpected layout `{:#?}` in PlaceRef::len", self.layout)
  74         }
  75     }
  76 }
  77
  78 impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
  79     /// Access a field, at a point when the value's case is known.
  80     pub fn project_field<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
  81         self,
  82         bx: &mut Bx,
  83         ix: usize,
  84     ) -> Self {
  85         let field = self.layout.field(bx.cx(), ix);
  86         let offset = self.layout.fields.offset(ix);
  87         let effective_field_align = self.align.restrict_for_offset(offset);
  88
  89         let mut simple = || {
  90             // Unions and newtypes only use an offset of 0.
  91             let llval = if offset.bytes() == 0 {
  92                 self.llval
  93             } else if let layout::Abi::ScalarPair(ref a, ref b) = self.layout.abi {
  94                 // Offsets have to match either first or second field.
  95                 assert_eq!(offset, a.value.size(bx.cx()).align_to(b.value.align(bx.cx()).abi));
  96                 bx.struct_gep(self.llval, 1)
  97             } else {
  98                 bx.struct_gep(self.llval, bx.cx().backend_field_index(self.layout, ix))
  99             };
 100             PlaceRef {
 101                 // HACK(eddyb): have to bitcast pointers until LLVM removes pointee types.
 102                 llval: bx.pointercast(llval, bx.cx().type_ptr_to(bx.cx().backend_type(field))),
 103                 llextra: if bx.cx().type_has_metadata(field.ty) { self.llextra } else { None },
 104                 layout: field,
 105                 align: effective_field_align,
 106             }
 107         };
 108
 109         // Simple cases, which don't need DST adjustment:
 110         //   * no metadata available - just log the case
 111         //   * known alignment - sized types, `[T]`, `str` or a foreign type
 112         //   * packed struct - there is no alignment padding
 113         match field.ty.kind {
 114             _ if self.llextra.is_none() => {
 115                 debug!(
 116                     "unsized field `{}`, of `{:?}` has no metadata for adjustment",
 117                     ix, self.llval
 118                 );
 119                 return simple();
 120             }
 121             _ if !field.is_unsized() => return simple(),
 122             ty::Slice(..) | ty::Str | ty::Foreign(..) => return simple(),
 123             ty::Adt(def, _) => {
 124                 if def.repr.packed() {
 125                     // FIXME(eddyb) generalize the adjustment when we
 126                     // start supporting packing to larger alignments.
 127                     assert_eq!(self.layout.align.abi.bytes(), 1);
 128                     return simple();
 129                 }
 130             }
 131             _ => {}
 132         }
 133
 134         // We need to get the pointer manually now.
 135         // We do this by casting to a `*i8`, then offsetting it by the appropriate amount.
 136         // We do this instead of, say, simply adjusting the pointer from the result of a GEP
 137         // because the field may have an arbitrary alignment in the LLVM representation
 138         // anyway.
 139         //
 140         // To demonstrate:
 141         //
 142         //     struct Foo<T: ?Sized> {
 143         //         x: u16,
 144         //         y: T
 145         //     }
 146         //
 147         // The type `Foo<Foo<Trait>>` is represented in LLVM as `{ u16, { u16, u8 }}`, meaning that
 148         // the `y` field has 16-bit alignment.
 149
 150         let meta = self.llextra;
 151
 152         let unaligned_offset = bx.cx().const_usize(offset.bytes());
 153
 154         // Get the alignment of the field
 155         let (_, unsized_align) = glue::size_and_align_of_dst(bx, field.ty, meta);
 156
 157         // Bump the unaligned offset up to the appropriate alignment using the
 158         // following expression:
 159         //
 160         //     (unaligned offset + (align - 1)) & -align
 161
 162         // Calculate offset.
 163         let align_sub_1 = bx.sub(unsized_align, bx.cx().const_usize(1u64));
 164         let and_lhs = bx.add(unaligned_offset, align_sub_1);
 165         let and_rhs = bx.neg(unsized_align);
 166         let offset = bx.and(and_lhs, and_rhs);
 167
 168         debug!("struct_field_ptr: DST field offset: {:?}", offset);
 169
 170         // Cast and adjust pointer.
 171         let byte_ptr = bx.pointercast(self.llval, bx.cx().type_i8p());
 172         let byte_ptr = bx.gep(byte_ptr, &[offset]);
 173
 174         // Finally, cast back to the type expected.
 175         let ll_fty = bx.cx().backend_type(field);
 176         debug!("struct_field_ptr: Field type is {:?}", ll_fty);
 177
 178         PlaceRef {
 179             llval: bx.pointercast(byte_ptr, bx.cx().type_ptr_to(ll_fty)),
 180             llextra: self.llextra,
 181             layout: field,
 182             align: effective_field_align,
 183         }
 184     }
 185
 186     /// Obtain the actual discriminant of a value.
 187     pub fn codegen_get_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
 188         self,
 189         bx: &mut Bx,
 190         cast_to: Ty<'tcx>,
 191     ) -> V {
 192         let cast_to = bx.cx().immediate_backend_type(bx.cx().layout_of(cast_to));
 193         if self.layout.abi.is_uninhabited() {
 194             return bx.cx().const_undef(cast_to);
 195         }
 196         let (discr_scalar, discr_kind, discr_index) = match self.layout.variants {
 197             layout::Variants::Single { index } => {
 198                 let discr_val = self
 199                     .layout
 200                     .ty
 201                     .discriminant_for_variant(bx.cx().tcx(), index)
 202                     .map_or(index.as_u32() as u128, |discr| discr.val);
 203                 return bx.cx().const_uint_big(cast_to, discr_val);
 204             }
 205             layout::Variants::Multiple { ref discr, ref discr_kind, discr_index, .. } => {
 206                 (discr, discr_kind, discr_index)
 207             }
 208         };
 209
 210         // Read the tag/niche-encoded discriminant from memory.
 211         let encoded_discr = self.project_field(bx, discr_index);
 212         let encoded_discr = bx.load_operand(encoded_discr);
 213
 214         // Decode the discriminant (specifically if it's niche-encoded).
 215         match *discr_kind {
 216             layout::DiscriminantKind::Tag => {
 217                 let signed = match discr_scalar.value {
 218                     // We use `i1` for bytes that are always `0` or `1`,
 219                     // e.g., `#[repr(i8)] enum E { A, B }`, but we can't
 220                     // let LLVM interpret the `i1` as signed, because
 221                     // then `i1 1` (i.e., `E::B`) is effectively `i8 -1`.
 222                     layout::Int(_, signed) => !discr_scalar.is_bool() && signed,
 223                     _ => false,
 224                 };
 225                 bx.intcast(encoded_discr.immediate(), cast_to, signed)
 226             }
 227             layout::DiscriminantKind::Niche {
 228                 dataful_variant,
 229                 ref niche_variants,
 230                 niche_start,
 231             } => {
 232                 // Rebase from niche values to discriminants, and check
 233                 // whether the result is in range for the niche variants.
 234                 let niche_llty = bx.cx().immediate_backend_type(encoded_discr.layout);
 235                 let encoded_discr = encoded_discr.immediate();
 236
 237                 // We first compute the "relative discriminant" (wrt `niche_variants`),
 238                 // that is, if `n = niche_variants.end() - niche_variants.start()`,
 239                 // we remap `niche_start..=niche_start + n` (which may wrap around)
 240                 // to (non-wrap-around) `0..=n`, to be able to check whether the
 241                 // discriminant corresponds to a niche variant with one comparison.
 242                 // We also can't go directly to the (variant index) discriminant
 243                 // and check that it is in the range `niche_variants`, because
 244                 // that might not fit in the same type, on top of needing an extra
 245                 // comparison (see also the comment on `let niche_discr`).
 246                 let relative_discr = if niche_start == 0 {
 247                     // Avoid subtracting `0`, which wouldn't work for pointers.
 248                     // FIXME(eddyb) check the actual primitive type here.
 249                     encoded_discr
 250                 } else {
 251                     bx.sub(encoded_discr, bx.cx().const_uint_big(niche_llty, niche_start))
 252                 };
 253                 let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32();
 254                 let is_niche = {
 255                     let relative_max = if relative_max == 0 {
 256                         // Avoid calling `const_uint`, which wouldn't work for pointers.
 257                         // FIXME(eddyb) check the actual primitive type here.
 258                         bx.cx().const_null(niche_llty)
 259                     } else {
 260                         bx.cx().const_uint(niche_llty, relative_max as u64)
 261                     };
 262                     bx.icmp(IntPredicate::IntULE, relative_discr, relative_max)
 263                 };
 264
 265                 // NOTE(eddyb) this addition needs to be performed on the final
 266                 // type, in case the niche itself can't represent all variant
 267                 // indices (e.g. `u8` niche with more than `256` variants,
 268                 // but enough uninhabited variants so that the remaining variants
 269                 // fit in the niche).
 270                 // In other words, `niche_variants.end - niche_variants.start`
 271                 // is representable in the niche, but `niche_variants.end`
 272                 // might not be, in extreme cases.
 273                 let niche_discr = {
 274                     let relative_discr = if relative_max == 0 {
 275                         // HACK(eddyb) since we have only one niche, we know which
 276                         // one it is, and we can avoid having a dynamic value here.
 277                         bx.cx().const_uint(cast_to, 0)
 278                     } else {
 279                         bx.intcast(relative_discr, cast_to, false)
 280                     };
 281                     bx.add(
 282                         relative_discr,
 283                         bx.cx().const_uint(cast_to, niche_variants.start().as_u32() as u64),
 284                     )
 285                 };
 286
 287                 bx.select(
 288                     is_niche,
 289                     niche_discr,
 290                     bx.cx().const_uint(cast_to, dataful_variant.as_u32() as u64),
 291                 )
 292             }
 293         }
 294     }
 295
 296     /// Sets the discriminant for a new value of the given case of the given
 297     /// representation.
 298     pub fn codegen_set_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
 299         &self,
 300         bx: &mut Bx,
 301         variant_index: VariantIdx,
 302     ) {
 303         if self.layout.for_variant(bx.cx(), variant_index).abi.is_uninhabited() {
 304             // We play it safe by using a well-defined `abort`, but we could go for immediate UB
 305             // if that turns out to be helpful.
 306             bx.abort();
 307             return;
 308         }
 309         match self.layout.variants {
 310             layout::Variants::Single { index } => {
 311                 assert_eq!(index, variant_index);
 312             }
 313             layout::Variants::Multiple {
 314                 discr_kind: layout::DiscriminantKind::Tag,
 315                 discr_index,
 316                 ..
 317             } => {
 318                 let ptr = self.project_field(bx, discr_index);
 319                 let to =
 320                     self.layout.ty.discriminant_for_variant(bx.tcx(), variant_index).unwrap().val;
 321                 bx.store(
 322                     bx.cx().const_uint_big(bx.cx().backend_type(ptr.layout), to),
 323                     ptr.llval,
 324                     ptr.align,
 325                 );
 326             }
 327             layout::Variants::Multiple {
 328                 discr_kind:
 329                     layout::DiscriminantKind::Niche { dataful_variant, ref niche_variants, niche_start },
 330                 discr_index,
 331                 ..
 332             } => {
 333                 if variant_index != dataful_variant {
 334                     if bx.cx().sess().target.target.arch == "arm"
 335                         || bx.cx().sess().target.target.arch == "aarch64"
 336                     {
 337                         // FIXME(#34427): as workaround for LLVM bug on ARM,
 338                         // use memset of 0 before assigning niche value.
 339                         let fill_byte = bx.cx().const_u8(0);
 340                         let size = bx.cx().const_usize(self.layout.size.bytes());
 341                         bx.memset(self.llval, fill_byte, size, self.align, MemFlags::empty());
 342                     }
 343
 344                     let niche = self.project_field(bx, discr_index);
 345                     let niche_llty = bx.cx().immediate_backend_type(niche.layout);
 346                     let niche_value = variant_index.as_u32() - niche_variants.start().as_u32();
 347                     let niche_value = (niche_value as u128).wrapping_add(niche_start);
 348                     // FIXME(eddyb): check the actual primitive type here.
 349                     let niche_llval = if niche_value == 0 {
 350                         // HACK(eddyb): using `c_null` as it works on all types.
 351                         bx.cx().const_null(niche_llty)
 352                     } else {
 353                         bx.cx().const_uint_big(niche_llty, niche_value)
 354                     };
 355                     OperandValue::Immediate(niche_llval).store(bx, niche);
 356                 }
 357             }
 358         }
 359     }
 360
 361     pub fn project_index<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
 362         &self,
 363         bx: &mut Bx,
 364         llindex: V,
 365     ) -> Self {
 366         // Statically compute the offset if we can, otherwise just use the element size,
 367         // as this will yield the lowest alignment.
 368         let layout = self.layout.field(bx, 0);
 369         let offset = if let Some(llindex) = bx.const_to_opt_uint(llindex) {
 370             layout.size.checked_mul(llindex, bx).unwrap_or(layout.size)
 371         } else {
 372             layout.size
 373         };
 374
 375         PlaceRef {
 376             llval: bx.inbounds_gep(self.llval, &[bx.cx().const_usize(0), llindex]),
 377             llextra: None,
 378             layout,
 379             align: self.align.restrict_for_offset(offset),
 380         }
 381     }
 382
 383     pub fn project_downcast<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
 384         &self,
 385         bx: &mut Bx,
 386         variant_index: VariantIdx,
 387     ) -> Self {
 388         let mut downcast = *self;
 389         downcast.layout = self.layout.for_variant(bx.cx(), variant_index);
 390
 391         // Cast to the appropriate variant struct type.
 392         let variant_ty = bx.cx().backend_type(downcast.layout);
 393         downcast.llval = bx.pointercast(downcast.llval, bx.cx().type_ptr_to(variant_ty));
 394
 395         downcast
 396     }
 397
 398     pub fn storage_live<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) {
 399         bx.lifetime_start(self.llval, self.layout.size);
 400     }
 401
 402     pub fn storage_dead<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) {
 403         bx.lifetime_end(self.llval, self.layout.size);
 404     }
 405 }
 406
 407 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 408     pub fn codegen_place(
 409         &mut self,
 410         bx: &mut Bx,
 411         place_ref: &mir::PlaceRef<'_, 'tcx>,
 412     ) -> PlaceRef<'tcx, Bx::Value> {
 413         debug!("codegen_place(place_ref={:?})", place_ref);
 414         let cx = self.cx;
 415         let tcx = self.cx.tcx();
 416
 417         let result = match place_ref {
 418             mir::PlaceRef { local, projection: [] } => match self.locals[**local] {
 419                 LocalRef::Place(place) => {
 420                     return place;
 421                 }
 422                 LocalRef::UnsizedPlace(place) => {
 423                     return bx.load_operand(place).deref(cx);
 424                 }
 425                 LocalRef::Operand(..) => {
 426                     bug!("using operand local {:?} as place", place_ref);
 427                 }
 428             },
 429             mir::PlaceRef { local, projection: [proj_base @ .., mir::ProjectionElem::Deref] } => {
 430                 // Load the pointer from its location.
 431                 self.codegen_consume(bx, &mir::PlaceRef { local, projection: proj_base })
 432                     .deref(bx.cx())
 433             }
 434             mir::PlaceRef { local, projection: [proj_base @ .., elem] } => {
 435                 // FIXME turn this recursion into iteration
 436                 let cg_base =
 437                     self.codegen_place(bx, &mir::PlaceRef { local, projection: proj_base });
 438
 439                 match elem {
 440                     mir::ProjectionElem::Deref => bug!(),
 441                     mir::ProjectionElem::Field(ref field, _) => {
 442                         cg_base.project_field(bx, field.index())
 443                     }
 444                     mir::ProjectionElem::Index(index) => {
 445                         let index = &mir::Operand::Copy(mir::Place::from(*index));
 446                         let index = self.codegen_operand(bx, index);
 447                         let llindex = index.immediate();
 448                         cg_base.project_index(bx, llindex)
 449                     }
 450                     mir::ProjectionElem::ConstantIndex {
 451                         offset,
 452                         from_end: false,
 453                         min_length: _,
 454                     } => {
 455                         let lloffset = bx.cx().const_usize(*offset as u64);
 456                         cg_base.project_index(bx, lloffset)
 457                     }
 458                     mir::ProjectionElem::ConstantIndex {
 459                         offset,
 460                         from_end: true,
 461                         min_length: _,
 462                     } => {
 463                         let lloffset = bx.cx().const_usize(*offset as u64);
 464                         let lllen = cg_base.len(bx.cx());
 465                         let llindex = bx.sub(lllen, lloffset);
 466                         cg_base.project_index(bx, llindex)
 467                     }
 468                     mir::ProjectionElem::Subslice { from, to, from_end } => {
 469                         let mut subslice =
 470                             cg_base.project_index(bx, bx.cx().const_usize(*from as u64));
 471                         let projected_ty =
 472                             PlaceTy::from_ty(cg_base.layout.ty).projection_ty(tcx, elem).ty;
 473                         subslice.layout = bx.cx().layout_of(self.monomorphize(&projected_ty));
 474
 475                         if subslice.layout.is_unsized() {
 476                             assert!(from_end, "slice subslices should be `from_end`");
 477                             subslice.llextra = Some(bx.sub(
 478                                 cg_base.llextra.unwrap(),
 479                                 bx.cx().const_usize((*from as u64) + (*to as u64)),
 480                             ));
 481                         }
 482
 483                         // Cast the place pointer type to the new
 484                         // array or slice type (`*[%_; new_len]`).
 485                         subslice.llval = bx.pointercast(
 486                             subslice.llval,
 487                             bx.cx().type_ptr_to(bx.cx().backend_type(subslice.layout)),
 488                         );
 489
 490                         subslice
 491                     }
 492                     mir::ProjectionElem::Downcast(_, v) => cg_base.project_downcast(bx, *v),
 493                 }
 494             }
 495         };
 496         debug!("codegen_place(place={:?}) => {:?}", place_ref, result);
 497         result
 498     }
 499
 500     pub fn monomorphized_place_ty(&self, place_ref: &mir::PlaceRef<'_, 'tcx>) -> Ty<'tcx> {
 501         let tcx = self.cx.tcx();
 502         let place_ty = mir::Place::ty_from(place_ref.local, place_ref.projection, *self.mir, tcx);
 503         self.monomorphize(&place_ty.ty)
 504     }
 505 }