src/librustc_mir/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_target::abi::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     crate 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                 let old_val = self.storage_live(*local)?;
  99                 self.deallocate_local(old_val)?;
 100             }
 101
 102             // Mark locals as dead
 103             StorageDead(local) => {
 104                 let old_val = self.storage_dead(*local);
 105                 self.deallocate_local(old_val)?;
 106             }
 107
 108             // No dynamic semantics attached to `FakeRead`; MIR
 109             // interpreter is solely intended for borrowck'ed code.
 110             FakeRead(..) => {}
 111
 112             // Stacked Borrows.
 113             Retag(kind, place) => {
 114                 let dest = self.eval_place(**place)?;
 115                 M::retag(self, *kind, dest)?;
 116             }
 117
 118             // Statements we do not track.
 119             AscribeUserType(..) => {}
 120
 121             // Currently, Miri discards Coverage statements. Coverage statements are only injected
 122             // via an optional compile time MIR pass and have no side effects. Since Coverage
 123             // statements don't exist at the source level, it is safe for Miri to ignore them, even
 124             // for undefined behavior (UB) checks.
 125             //
 126             // A coverage counter inside a const expression (for example, a counter injected in a
 127             // const function) is discarded when the const is evaluated at compile time. Whether
 128             // this should change, and/or how to implement a const eval counter, is a subject of the
 129             // following issue:
 130             //
 131             // FIXME(#73156): Handle source code coverage in const eval
 132             Coverage(..) => {}
 133
 134             // Defined to do nothing. These are added by optimization passes, to avoid changing the
 135             // size of MIR constantly.
 136             Nop => {}
 137
 138             LlvmInlineAsm { .. } => throw_unsup_format!("inline assembly is not supported"),
 139         }
 140
 141         self.stack_mut()[frame_idx].loc.as_mut().unwrap().statement_index += 1;
 142         Ok(())
 143     }
 144
 145     /// Evaluate an assignment statement.
 146     ///
 147     /// There is no separate `eval_rvalue` function. Instead, the code for handling each rvalue
 148     /// type writes its results directly into the memory specified by the place.
 149     pub fn eval_rvalue_into_place(
 150         &mut self,
 151         rvalue: &mir::Rvalue<'tcx>,
 152         place: mir::Place<'tcx>,
 153     ) -> InterpResult<'tcx> {
 154         let dest = self.eval_place(place)?;
 155
 156         use rustc_middle::mir::Rvalue::*;
 157         match *rvalue {
 158             ThreadLocalRef(did) => {
 159                 let id = M::thread_local_static_alloc_id(self, did)?;
 160                 let val = self.global_base_pointer(id.into())?;
 161                 self.write_scalar(val, dest)?;
 162             }
 163
 164             Use(ref operand) => {
 165                 // Avoid recomputing the layout
 166                 let op = self.eval_operand(operand, Some(dest.layout))?;
 167                 self.copy_op(op, dest)?;
 168             }
 169
 170             BinaryOp(bin_op, ref left, ref right) => {
 171                 let layout = binop_left_homogeneous(bin_op).then_some(dest.layout);
 172                 let left = self.read_immediate(self.eval_operand(left, layout)?)?;
 173                 let layout = binop_right_homogeneous(bin_op).then_some(left.layout);
 174                 let right = self.read_immediate(self.eval_operand(right, layout)?)?;
 175                 self.binop_ignore_overflow(bin_op, left, right, dest)?;
 176             }
 177
 178             CheckedBinaryOp(bin_op, ref left, ref right) => {
 179                 // Due to the extra boolean in the result, we can never reuse the `dest.layout`.
 180                 let left = self.read_immediate(self.eval_operand(left, None)?)?;
 181                 let layout = binop_right_homogeneous(bin_op).then_some(left.layout);
 182                 let right = self.read_immediate(self.eval_operand(right, layout)?)?;
 183                 self.binop_with_overflow(bin_op, left, right, dest)?;
 184             }
 185
 186             UnaryOp(un_op, ref operand) => {
 187                 // The operand always has the same type as the result.
 188                 let val = self.read_immediate(self.eval_operand(operand, Some(dest.layout))?)?;
 189                 let val = self.unary_op(un_op, val)?;
 190                 assert_eq!(val.layout, dest.layout, "layout mismatch for result of {:?}", un_op);
 191                 self.write_immediate(*val, dest)?;
 192             }
 193
 194             Aggregate(ref kind, ref operands) => {
 195                 let (dest, active_field_index) = match **kind {
 196                     mir::AggregateKind::Adt(adt_def, variant_index, _, _, active_field_index) => {
 197                         self.write_discriminant(variant_index, dest)?;
 198                         if adt_def.is_enum() {
 199                             (self.place_downcast(dest, variant_index)?, active_field_index)
 200                         } else {
 201                             (dest, active_field_index)
 202                         }
 203                     }
 204                     _ => (dest, None),
 205                 };
 206
 207                 for (i, operand) in operands.iter().enumerate() {
 208                     let op = self.eval_operand(operand, None)?;
 209                     // Ignore zero-sized fields.
 210                     if !op.layout.is_zst() {
 211                         let field_index = active_field_index.unwrap_or(i);
 212                         let field_dest = self.place_field(dest, field_index)?;
 213                         self.copy_op(op, field_dest)?;
 214                     }
 215                 }
 216             }
 217
 218             Repeat(ref operand, _) => {
 219                 let op = self.eval_operand(operand, None)?;
 220                 let dest = self.force_allocation(dest)?;
 221                 let length = dest.len(self)?;
 222
 223                 if let Some(first_ptr) = self.check_mplace_access(dest, None)? {
 224                     // Write the first.
 225                     let first = self.mplace_field(dest, 0)?;
 226                     self.copy_op(op, first.into())?;
 227
 228                     if length > 1 {
 229                         let elem_size = first.layout.size;
 230                         // Copy the rest. This is performance-sensitive code
 231                         // for big static/const arrays!
 232                         let rest_ptr = first_ptr.offset(elem_size, self)?;
 233                         self.memory.copy_repeatedly(
 234                             first_ptr,
 235                             rest_ptr,
 236                             elem_size,
 237                             length - 1,
 238                             /*nonoverlapping:*/ true,
 239                         )?;
 240                     }
 241                 }
 242             }
 243
 244             Len(place) => {
 245                 // FIXME(CTFE): don't allow computing the length of arrays in const eval
 246                 let src = self.eval_place(place)?;
 247                 let mplace = self.force_allocation(src)?;
 248                 let len = mplace.len(self)?;
 249                 self.write_scalar(Scalar::from_machine_usize(len, self), dest)?;
 250             }
 251
 252             AddressOf(_, place) | Ref(_, _, place) => {
 253                 let src = self.eval_place(place)?;
 254                 let place = self.force_allocation(src)?;
 255                 if place.layout.size.bytes() > 0 {
 256                     // definitely not a ZST
 257                     assert!(place.ptr.is_ptr(), "non-ZST places should be normalized to `Pointer`");
 258                 }
 259                 self.write_immediate(place.to_ref(), dest)?;
 260             }
 261
 262             NullaryOp(mir::NullOp::Box, _) => {
 263                 M::box_alloc(self, dest)?;
 264             }
 265
 266             NullaryOp(mir::NullOp::SizeOf, ty) => {
 267                 let ty = self.subst_from_current_frame_and_normalize_erasing_regions(ty);
 268                 let layout = self.layout_of(ty)?;
 269                 assert!(
 270                     !layout.is_unsized(),
 271                     "SizeOf nullary MIR operator called for unsized type"
 272                 );
 273                 self.write_scalar(Scalar::from_machine_usize(layout.size.bytes(), self), dest)?;
 274             }
 275
 276             Cast(cast_kind, ref operand, cast_ty) => {
 277                 let src = self.eval_operand(operand, None)?;
 278                 let cast_ty = self.subst_from_current_frame_and_normalize_erasing_regions(cast_ty);
 279                 self.cast(src, cast_kind, cast_ty, dest)?;
 280             }
 281
 282             Discriminant(place) => {
 283                 let op = self.eval_place_to_op(place, None)?;
 284                 let discr_val = self.read_discriminant(op)?.0;
 285                 self.write_scalar(discr_val, dest)?;
 286             }
 287         }
 288
 289         trace!("{:?}", self.dump_place(*dest));
 290
 291         Ok(())
 292     }
 293
 294     fn terminator(&mut self, terminator: &mir::Terminator<'tcx>) -> InterpResult<'tcx> {
 295         info!("{:?}", terminator.kind);
 296
 297         self.eval_terminator(terminator)?;
 298         if !self.stack().is_empty() {
 299             if let Ok(loc) = self.frame().loc {
 300                 info!("// executing {:?}", loc.block);
 301             }
 302         }
 303         Ok(())
 304     }
 305 }