compiler/rustc_const_eval/src/interpret/step.rs

   1 //! This module contains the `InterpCx` methods for executing a single step of the interpreter.
   2 //!
   3 //! The main entry point is the `step` method.
   4
   5 use rustc_middle::mir;
   6 use rustc_middle::mir::interpret::{InterpResult, Scalar};
   7 use rustc_middle::ty::layout::LayoutOf;
   8
   9 use super::{InterpCx, Machine};
  10
  11 /// Classify whether an operator is "left-homogeneous", i.e., the LHS has the
  12 /// same type as the result.
  13 #[inline]
  14 fn binop_left_homogeneous(op: mir::BinOp) -> bool {
  15     use rustc_middle::mir::BinOp::*;
  16     match op {
  17         Add | Sub | Mul | Div | Rem | BitXor | BitAnd | BitOr | Offset | Shl | Shr => true,
  18         Eq | Ne | Lt | Le | Gt | Ge => false,
  19     }
  20 }
  21 /// Classify whether an operator is "right-homogeneous", i.e., the RHS has the
  22 /// same type as the LHS.
  23 #[inline]
  24 fn binop_right_homogeneous(op: mir::BinOp) -> bool {
  25     use rustc_middle::mir::BinOp::*;
  26     match op {
  27         Add | Sub | Mul | Div | Rem | BitXor | BitAnd | BitOr | Eq | Ne | Lt | Le | Gt | Ge => true,
  28         Offset | Shl | Shr => false,
  29     }
  30 }
  31
  32 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
  33     pub fn run(&mut self) -> InterpResult<'tcx> {
  34         while self.step()? {}
  35         Ok(())
  36     }
  37
  38     /// Returns `true` as long as there are more things to do.
  39     ///
  40     /// This is used by [priroda](https://github.com/oli-obk/priroda)
  41     ///
  42     /// This is marked `#inline(always)` to work around adverserial codegen when `opt-level = 3`
  43     #[inline(always)]
  44     pub fn step(&mut self) -> InterpResult<'tcx, bool> {
  45         if self.stack().is_empty() {
  46             return Ok(false);
  47         }
  48
  49         let loc = match self.frame().loc {
  50             Ok(loc) => loc,
  51             Err(_) => {
  52                 // We are unwinding and this fn has no cleanup code.
  53                 // Just go on unwinding.
  54                 trace!("unwinding: skipping frame");
  55                 self.pop_stack_frame(/* unwinding */ true)?;
  56                 return Ok(true);
  57             }
  58         };
  59         let basic_block = &self.body().basic_blocks()[loc.block];
  60
  61         let old_frames = self.frame_idx();
  62
  63         if let Some(stmt) = basic_block.statements.get(loc.statement_index) {
  64             assert_eq!(old_frames, self.frame_idx());
  65             self.statement(stmt)?;
  66             return Ok(true);
  67         }
  68
  69         M::before_terminator(self)?;
  70
  71         let terminator = basic_block.terminator();
  72         assert_eq!(old_frames, self.frame_idx());
  73         self.terminator(terminator)?;
  74         Ok(true)
  75     }
  76
  77     /// Runs the interpretation logic for the given `mir::Statement` at the current frame and
  78     /// statement counter. This also moves the statement counter forward.
  79     pub fn statement(&mut self, stmt: &mir::Statement<'tcx>) -> InterpResult<'tcx> {
  80         info!("{:?}", stmt);
  81
  82         use rustc_middle::mir::StatementKind::*;
  83
  84         // Some statements (e.g., box) push new stack frames.
  85         // We have to record the stack frame number *before* executing the statement.
  86         let frame_idx = self.frame_idx();
  87
  88         match &stmt.kind {
  89             Assign(box (place, rvalue)) => self.eval_rvalue_into_place(rvalue, *place)?,
  90
  91             SetDiscriminant { place, variant_index } => {
  92                 let dest = self.eval_place(**place)?;
  93                 self.write_discriminant(*variant_index, &dest)?;
  94             }
  95
  96             // Mark locals as alive
  97             StorageLive(local) => {
  98                 self.storage_live(*local)?;
  99             }
 100
 101             // Mark locals as dead
 102             StorageDead(local) => {
 103                 self.storage_dead(*local)?;
 104             }
 105
 106             // No dynamic semantics attached to `FakeRead`; MIR
 107             // interpreter is solely intended for borrowck'ed code.
 108             FakeRead(..) => {}
 109
 110             // Stacked Borrows.
 111             Retag(kind, place) => {
 112                 let dest = self.eval_place(**place)?;
 113                 M::retag(self, *kind, &dest)?;
 114             }
 115
 116             // Call CopyNonOverlapping
 117             CopyNonOverlapping(box rustc_middle::mir::CopyNonOverlapping { src, dst, count }) => {
 118                 let src = self.eval_operand(src, None)?;
 119                 let dst = self.eval_operand(dst, None)?;
 120                 let count = self.eval_operand(count, None)?;
 121                 self.copy_intrinsic(&src, &dst, &count, /* nonoverlapping */ true)?;
 122             }
 123
 124             // Statements we do not track.
 125             AscribeUserType(..) => {}
 126
 127             // Currently, Miri discards Coverage statements. Coverage statements are only injected
 128             // via an optional compile time MIR pass and have no side effects. Since Coverage
 129             // statements don't exist at the source level, it is safe for Miri to ignore them, even
 130             // for undefined behavior (UB) checks.
 131             //
 132             // A coverage counter inside a const expression (for example, a counter injected in a
 133             // const function) is discarded when the const is evaluated at compile time. Whether
 134             // this should change, and/or how to implement a const eval counter, is a subject of the
 135             // following issue:
 136             //
 137             // FIXME(#73156): Handle source code coverage in const eval
 138             Coverage(..) => {}
 139
 140             // Defined to do nothing. These are added by optimization passes, to avoid changing the
 141             // size of MIR constantly.
 142             Nop => {}
 143         }
 144
 145         self.stack_mut()[frame_idx].loc.as_mut().unwrap().statement_index += 1;
 146         Ok(())
 147     }
 148
 149     /// Evaluate an assignment statement.
 150     ///
 151     /// There is no separate `eval_rvalue` function. Instead, the code for handling each rvalue
 152     /// type writes its results directly into the memory specified by the place.
 153     pub fn eval_rvalue_into_place(
 154         &mut self,
 155         rvalue: &mir::Rvalue<'tcx>,
 156         place: mir::Place<'tcx>,
 157     ) -> InterpResult<'tcx> {
 158         let dest = self.eval_place(place)?;
 159
 160         use rustc_middle::mir::Rvalue::*;
 161         match *rvalue {
 162             ThreadLocalRef(did) => {
 163                 let ptr = M::thread_local_static_base_pointer(self, did)?;
 164                 self.write_pointer(ptr, &dest)?;
 165             }
 166
 167             Use(ref operand) => {
 168                 // Avoid recomputing the layout
 169                 let op = self.eval_operand(operand, Some(dest.layout))?;
 170                 self.copy_op(&op, &dest)?;
 171             }
 172
 173             BinaryOp(bin_op, box (ref left, ref right)) => {
 174                 let layout = binop_left_homogeneous(bin_op).then_some(dest.layout);
 175                 let left = self.read_immediate(&self.eval_operand(left, layout)?)?;
 176                 let layout = binop_right_homogeneous(bin_op).then_some(left.layout);
 177                 let right = self.read_immediate(&self.eval_operand(right, layout)?)?;
 178                 self.binop_ignore_overflow(bin_op, &left, &right, &dest)?;
 179             }
 180
 181             CheckedBinaryOp(bin_op, box (ref left, ref right)) => {
 182                 // Due to the extra boolean in the result, we can never reuse the `dest.layout`.
 183                 let left = self.read_immediate(&self.eval_operand(left, None)?)?;
 184                 let layout = binop_right_homogeneous(bin_op).then_some(left.layout);
 185                 let right = self.read_immediate(&self.eval_operand(right, layout)?)?;
 186                 self.binop_with_overflow(bin_op, &left, &right, &dest)?;
 187             }
 188
 189             UnaryOp(un_op, ref operand) => {
 190                 // The operand always has the same type as the result.
 191                 let val = self.read_immediate(&self.eval_operand(operand, Some(dest.layout))?)?;
 192                 let val = self.unary_op(un_op, &val)?;
 193                 assert_eq!(val.layout, dest.layout, "layout mismatch for result of {:?}", un_op);
 194                 self.write_immediate(*val, &dest)?;
 195             }
 196
 197             Aggregate(ref kind, ref operands) => {
 198                 // active_field_index is for union initialization.
 199                 let (dest, active_field_index) = match **kind {
 200                     mir::AggregateKind::Adt(adt_did, variant_index, _, _, active_field_index) => {
 201                         self.write_discriminant(variant_index, &dest)?;
 202                         if self.tcx.adt_def(adt_did).is_enum() {
 203                             assert!(active_field_index.is_none());
 204                             (self.place_downcast(&dest, variant_index)?, None)
 205                         } else {
 206                             if active_field_index.is_some() {
 207                                 assert_eq!(operands.len(), 1);
 208                             }
 209                             (dest, active_field_index)
 210                         }
 211                     }
 212                     _ => (dest, None),
 213                 };
 214
 215                 for (i, operand) in operands.iter().enumerate() {
 216                     let op = self.eval_operand(operand, None)?;
 217                     let field_index = active_field_index.unwrap_or(i);
 218                     let field_dest = self.place_field(&dest, field_index)?;
 219                     self.copy_op(&op, &field_dest)?;
 220                 }
 221             }
 222
 223             Repeat(ref operand, _) => {
 224                 let src = self.eval_operand(operand, None)?;
 225                 assert!(!src.layout.is_unsized());
 226                 let dest = self.force_allocation(&dest)?;
 227                 let length = dest.len(self)?;
 228
 229                 if length == 0 {
 230                     // Nothing to copy... but let's still make sure that `dest` as a place is valid.
 231                     self.get_alloc_mut(&dest)?;
 232                 } else {
 233                     // Write the src to the first element.
 234                     let first = self.mplace_field(&dest, 0)?;
 235                     self.copy_op(&src, &first.into())?;
 236
 237                     // This is performance-sensitive code for big static/const arrays! So we
 238                     // avoid writing each operand individually and instead just make many copies
 239                     // of the first element.
 240                     let elem_size = first.layout.size;
 241                     let first_ptr = first.ptr;
 242                     let rest_ptr = first_ptr.offset(elem_size, self)?;
 243                     // For the alignment of `rest_ptr`, we crucially do *not* use `first.align` as
 244                     // that place might be more aligned than its type mandates (a `u8` array could
 245                     // be 4-aligned if it sits at the right spot in a struct). Instead we use
 246                     // `first.layout.align`, i.e., the alignment given by the type.
 247                     self.memory.copy_repeatedly(
 248                         first_ptr,
 249                         first.align,
 250                         rest_ptr,
 251                         first.layout.align.abi,
 252                         elem_size,
 253                         length - 1,
 254                         /*nonoverlapping:*/ true,
 255                     )?;
 256                 }
 257             }
 258
 259             Len(place) => {
 260                 let src = self.eval_place(place)?;
 261                 let mplace = self.force_allocation(&src)?;
 262                 let len = mplace.len(self)?;
 263                 self.write_scalar(Scalar::from_machine_usize(len, self), &dest)?;
 264             }
 265
 266             AddressOf(_, place) | Ref(_, _, place) => {
 267                 let src = self.eval_place(place)?;
 268                 let place = self.force_allocation(&src)?;
 269                 self.write_immediate(place.to_ref(self), &dest)?;
 270             }
 271
 272             NullaryOp(null_op, ty) => {
 273                 let ty = self.subst_from_current_frame_and_normalize_erasing_regions(ty)?;
 274                 let layout = self.layout_of(ty)?;
 275                 if layout.is_unsized() {
 276                     // FIXME: This should be a span_bug (#80742)
 277                     self.tcx.sess.delay_span_bug(
 278                         self.frame().current_span(),
 279                         &format!("Nullary MIR operator called for unsized type {}", ty),
 280                     );
 281                     throw_inval!(SizeOfUnsizedType(ty));
 282                 }
 283                 let val = match null_op {
 284                     mir::NullOp::SizeOf => layout.size.bytes(),
 285                     mir::NullOp::AlignOf => layout.align.abi.bytes(),
 286                 };
 287                 self.write_scalar(Scalar::from_machine_usize(val, self), &dest)?;
 288             }
 289
 290             ShallowInitBox(ref operand, _) => {
 291                 let src = self.eval_operand(operand, None)?;
 292                 let v = self.read_immediate(&src)?;
 293                 self.write_immediate(*v, &dest)?;
 294             }
 295
 296             Cast(cast_kind, ref operand, cast_ty) => {
 297                 let src = self.eval_operand(operand, None)?;
 298                 let cast_ty =
 299                     self.subst_from_current_frame_and_normalize_erasing_regions(cast_ty)?;
 300                 self.cast(&src, cast_kind, cast_ty, &dest)?;
 301             }
 302
 303             Discriminant(place) => {
 304                 let op = self.eval_place_to_op(place, None)?;
 305                 let discr_val = self.read_discriminant(&op)?.0;
 306                 self.write_scalar(discr_val, &dest)?;
 307             }
 308         }
 309
 310         trace!("{:?}", self.dump_place(*dest));
 311
 312         Ok(())
 313     }
 314
 315     fn terminator(&mut self, terminator: &mir::Terminator<'tcx>) -> InterpResult<'tcx> {
 316         info!("{:?}", terminator.kind);
 317
 318         self.eval_terminator(terminator)?;
 319         if !self.stack().is_empty() {
 320             if let Ok(loc) = self.frame().loc {
 321                 info!("// executing {:?}", loc.block);
 322             }
 323         }
 324         Ok(())
 325     }
 326 }