| //! Propagates constants for early reporting of statically known |
| //! assertion failures |
| |
| use std::cell::Cell; |
| |
| use rustc_ast::ast::Mutability; |
| use rustc_data_structures::fx::FxHashSet; |
| use rustc_hir::def::DefKind; |
| use rustc_hir::HirId; |
| use rustc_index::bit_set::BitSet; |
| use rustc_index::vec::IndexVec; |
| use rustc_middle::mir::interpret::{InterpResult, Scalar}; |
| use rustc_middle::mir::visit::{ |
| MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor, |
| }; |
| use rustc_middle::mir::{ |
| AggregateKind, AssertKind, BasicBlock, BinOp, Body, ClearCrossCrate, Constant, Local, |
| LocalDecl, LocalKind, Location, Operand, Place, Rvalue, SourceInfo, SourceScope, |
| SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE, |
| }; |
| use rustc_middle::ty::layout::{HasTyCtxt, LayoutError, TyAndLayout}; |
| use rustc_middle::ty::subst::{InternalSubsts, Subst}; |
| use rustc_middle::ty::{self, ConstInt, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeFoldable}; |
| use rustc_session::lint; |
| use rustc_span::{def_id::DefId, Span}; |
| use rustc_target::abi::{HasDataLayout, LayoutOf, Size, TargetDataLayout}; |
| use rustc_trait_selection::traits; |
| |
| use crate::const_eval::error_to_const_error; |
| use crate::interpret::{ |
| self, compile_time_machine, AllocId, Allocation, Frame, ImmTy, Immediate, InterpCx, LocalState, |
| LocalValue, MemPlace, Memory, MemoryKind, OpTy, Operand as InterpOperand, PlaceTy, Pointer, |
| ScalarMaybeUninit, StackPopCleanup, |
| }; |
| use crate::transform::{MirPass, MirSource}; |
| |
| /// The maximum number of bytes that we'll allocate space for a return value. |
| const MAX_ALLOC_LIMIT: u64 = 1024; |
| |
| /// Macro for machine-specific `InterpError` without allocation. |
| /// (These will never be shown to the user, but they help diagnose ICEs.) |
| macro_rules! throw_machine_stop_str { |
| ($($tt:tt)*) => {{ |
| // We make a new local type for it. The type itself does not carry any information, |
| // but its vtable (for the `MachineStopType` trait) does. |
| struct Zst; |
| // Printing this type shows the desired string. |
| impl std::fmt::Display for Zst { |
| fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { |
| write!(f, $($tt)*) |
| } |
| } |
| impl rustc_middle::mir::interpret::MachineStopType for Zst {} |
| throw_machine_stop!(Zst) |
| }}; |
| } |
| |
| pub struct ConstProp; |
| |
| impl<'tcx> MirPass<'tcx> for ConstProp { |
| fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) { |
| // will be evaluated by miri and produce its errors there |
| if source.promoted.is_some() { |
| return; |
| } |
| |
| use rustc_middle::hir::map::blocks::FnLikeNode; |
| let hir_id = tcx.hir().as_local_hir_id(source.def_id().expect_local()); |
| |
| let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some(); |
| let is_assoc_const = tcx.def_kind(source.def_id()) == DefKind::AssocConst; |
| |
| // Only run const prop on functions, methods, closures and associated constants |
| if !is_fn_like && !is_assoc_const { |
| // skip anon_const/statics/consts because they'll be evaluated by miri anyway |
| trace!("ConstProp skipped for {:?}", source.def_id()); |
| return; |
| } |
| |
| let is_generator = tcx.type_of(source.def_id()).is_generator(); |
| // FIXME(welseywiser) const prop doesn't work on generators because of query cycles |
| // computing their layout. |
| if is_generator { |
| trace!("ConstProp skipped for generator {:?}", source.def_id()); |
| return; |
| } |
| |
| // Check if it's even possible to satisfy the 'where' clauses |
| // for this item. |
| // This branch will never be taken for any normal function. |
| // However, it's possible to `#!feature(trivial_bounds)]` to write |
| // a function with impossible to satisfy clauses, e.g.: |
| // `fn foo() where String: Copy {}` |
| // |
| // We don't usually need to worry about this kind of case, |
| // since we would get a compilation error if the user tried |
| // to call it. However, since we can do const propagation |
| // even without any calls to the function, we need to make |
| // sure that it even makes sense to try to evaluate the body. |
| // If there are unsatisfiable where clauses, then all bets are |
| // off, and we just give up. |
| // |
| // We manually filter the predicates, skipping anything that's not |
| // "global". We are in a potentially generic context |
| // (e.g. we are evaluating a function without substituting generic |
| // parameters, so this filtering serves two purposes: |
| // |
| // 1. We skip evaluating any predicates that we would |
| // never be able prove are unsatisfiable (e.g. `<T as Foo>` |
| // 2. We avoid trying to normalize predicates involving generic |
| // parameters (e.g. `<T as Foo>::MyItem`). This can confuse |
| // the normalization code (leading to cycle errors), since |
| // it's usually never invoked in this way. |
| let predicates = tcx |
| .predicates_of(source.def_id()) |
| .predicates |
| .iter() |
| .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None }); |
| if !traits::normalize_and_test_predicates( |
| tcx, |
| traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(), |
| ) { |
| trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", source.def_id()); |
| return; |
| } |
| |
| trace!("ConstProp starting for {:?}", source.def_id()); |
| |
| let dummy_body = &Body::new( |
| body.basic_blocks().clone(), |
| body.source_scopes.clone(), |
| body.local_decls.clone(), |
| Default::default(), |
| body.arg_count, |
| Default::default(), |
| tcx.def_span(source.def_id()), |
| body.generator_kind, |
| ); |
| |
| // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold |
| // constants, instead of just checking for const-folding succeeding. |
| // That would require an uniform one-def no-mutation analysis |
| // and RPO (or recursing when needing the value of a local). |
| let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx, source); |
| optimization_finder.visit_body(body); |
| |
| trace!("ConstProp done for {:?}", source.def_id()); |
| } |
| } |
| |
| struct ConstPropMachine<'mir, 'tcx> { |
| /// The virtual call stack. |
| stack: Vec<Frame<'mir, 'tcx, (), ()>>, |
| /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end. |
| written_only_inside_own_block_locals: FxHashSet<Local>, |
| /// Locals that need to be cleared after every block terminates. |
| only_propagate_inside_block_locals: BitSet<Local>, |
| } |
| |
| impl<'mir, 'tcx> ConstPropMachine<'mir, 'tcx> { |
| fn new(only_propagate_inside_block_locals: BitSet<Local>) -> Self { |
| Self { |
| stack: Vec::new(), |
| written_only_inside_own_block_locals: Default::default(), |
| only_propagate_inside_block_locals, |
| } |
| } |
| } |
| |
| impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> { |
| compile_time_machine!(<'mir, 'tcx>); |
| |
| type MemoryExtra = (); |
| |
| fn find_mir_or_eval_fn( |
| _ecx: &mut InterpCx<'mir, 'tcx, Self>, |
| _instance: ty::Instance<'tcx>, |
| _args: &[OpTy<'tcx>], |
| _ret: Option<(PlaceTy<'tcx>, BasicBlock)>, |
| _unwind: Option<BasicBlock>, |
| ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> { |
| Ok(None) |
| } |
| |
| fn call_intrinsic( |
| _ecx: &mut InterpCx<'mir, 'tcx, Self>, |
| _instance: ty::Instance<'tcx>, |
| _args: &[OpTy<'tcx>], |
| _ret: Option<(PlaceTy<'tcx>, BasicBlock)>, |
| _unwind: Option<BasicBlock>, |
| ) -> InterpResult<'tcx> { |
| throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp") |
| } |
| |
| fn assert_panic( |
| _ecx: &mut InterpCx<'mir, 'tcx, Self>, |
| _msg: &rustc_middle::mir::AssertMessage<'tcx>, |
| _unwind: Option<rustc_middle::mir::BasicBlock>, |
| ) -> InterpResult<'tcx> { |
| bug!("panics terminators are not evaluated in ConstProp") |
| } |
| |
| fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> { |
| throw_unsup!(ReadPointerAsBytes) |
| } |
| |
| fn binary_ptr_op( |
| _ecx: &InterpCx<'mir, 'tcx, Self>, |
| _bin_op: BinOp, |
| _left: ImmTy<'tcx>, |
| _right: ImmTy<'tcx>, |
| ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> { |
| // We can't do this because aliasing of memory can differ between const eval and llvm |
| throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp") |
| } |
| |
| fn box_alloc( |
| _ecx: &mut InterpCx<'mir, 'tcx, Self>, |
| _dest: PlaceTy<'tcx>, |
| ) -> InterpResult<'tcx> { |
| throw_machine_stop_str!("can't const prop heap allocations") |
| } |
| |
| fn access_local( |
| _ecx: &InterpCx<'mir, 'tcx, Self>, |
| frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>, |
| local: Local, |
| ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> { |
| let l = &frame.locals[local]; |
| |
| if l.value == LocalValue::Uninitialized { |
| throw_machine_stop_str!("tried to access an uninitialized local") |
| } |
| |
| l.access() |
| } |
| |
| fn access_local_mut<'a>( |
| ecx: &'a mut InterpCx<'mir, 'tcx, Self>, |
| frame: usize, |
| local: Local, |
| ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>> |
| { |
| if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) { |
| ecx.machine.written_only_inside_own_block_locals.insert(local); |
| } |
| ecx.machine.stack[frame].locals[local].access_mut() |
| } |
| |
| fn before_access_global( |
| _memory_extra: &(), |
| _alloc_id: AllocId, |
| allocation: &Allocation<Self::PointerTag, Self::AllocExtra>, |
| _static_def_id: Option<DefId>, |
| is_write: bool, |
| ) -> InterpResult<'tcx> { |
| if is_write { |
| throw_machine_stop_str!("can't write to global"); |
| } |
| // If the static allocation is mutable, then we can't const prop it as its content |
| // might be different at runtime. |
| if allocation.mutability == Mutability::Mut { |
| throw_machine_stop_str!("can't access mutable globals in ConstProp"); |
| } |
| |
| Ok(()) |
| } |
| |
| #[inline(always)] |
| fn stack( |
| ecx: &'a InterpCx<'mir, 'tcx, Self>, |
| ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] { |
| &ecx.machine.stack |
| } |
| |
| #[inline(always)] |
| fn stack_mut( |
| ecx: &'a mut InterpCx<'mir, 'tcx, Self>, |
| ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> { |
| &mut ecx.machine.stack |
| } |
| } |
| |
| /// Finds optimization opportunities on the MIR. |
| struct ConstPropagator<'mir, 'tcx> { |
| ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, |
| tcx: TyCtxt<'tcx>, |
| can_const_prop: IndexVec<Local, ConstPropMode>, |
| param_env: ParamEnv<'tcx>, |
| // FIXME(eddyb) avoid cloning these two fields more than once, |
| // by accessing them through `ecx` instead. |
| source_scopes: IndexVec<SourceScope, SourceScopeData>, |
| local_decls: IndexVec<Local, LocalDecl<'tcx>>, |
| // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store |
| // the last known `SourceInfo` here and just keep revisiting it. |
| source_info: Option<SourceInfo>, |
| } |
| |
| impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> { |
| type Ty = Ty<'tcx>; |
| type TyAndLayout = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>; |
| |
| fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyAndLayout { |
| self.tcx.layout_of(self.param_env.and(ty)) |
| } |
| } |
| |
| impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> { |
| #[inline] |
| fn data_layout(&self) -> &TargetDataLayout { |
| &self.tcx.data_layout |
| } |
| } |
| |
| impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> { |
| #[inline] |
| fn tcx(&self) -> TyCtxt<'tcx> { |
| self.tcx |
| } |
| } |
| |
| impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> { |
| fn new( |
| body: &Body<'tcx>, |
| dummy_body: &'mir Body<'tcx>, |
| tcx: TyCtxt<'tcx>, |
| source: MirSource<'tcx>, |
| ) -> ConstPropagator<'mir, 'tcx> { |
| let def_id = source.def_id(); |
| let substs = &InternalSubsts::identity_for_item(tcx, def_id); |
| let param_env = tcx.param_env(def_id).with_reveal_all(); |
| |
| let span = tcx.def_span(def_id); |
| let can_const_prop = CanConstProp::check(body); |
| let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len()); |
| for (l, mode) in can_const_prop.iter_enumerated() { |
| if *mode == ConstPropMode::OnlyInsideOwnBlock { |
| only_propagate_inside_block_locals.insert(l); |
| } |
| } |
| let mut ecx = InterpCx::new( |
| tcx, |
| span, |
| param_env, |
| ConstPropMachine::new(only_propagate_inside_block_locals), |
| (), |
| ); |
| |
| let ret = ecx |
| .layout_of(body.return_ty().subst(tcx, substs)) |
| .ok() |
| // Don't bother allocating memory for ZST types which have no values |
| // or for large values. |
| .filter(|ret_layout| { |
| !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) |
| }) |
| .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack)); |
| |
| ecx.push_stack_frame( |
| Instance::new(def_id, substs), |
| dummy_body, |
| ret.map(Into::into), |
| StackPopCleanup::None { cleanup: false }, |
| ) |
| .expect("failed to push initial stack frame"); |
| |
| ConstPropagator { |
| ecx, |
| tcx, |
| param_env, |
| can_const_prop, |
| // FIXME(eddyb) avoid cloning these two fields more than once, |
| // by accessing them through `ecx` instead. |
| source_scopes: body.source_scopes.clone(), |
| //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it |
| local_decls: body.local_decls.clone(), |
| source_info: None, |
| } |
| } |
| |
| fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> { |
| let op = match self.ecx.eval_place_to_op(place, None) { |
| Ok(op) => op, |
| Err(e) => { |
| trace!("get_const failed: {}", e); |
| return None; |
| } |
| }; |
| |
| // Try to read the local as an immediate so that if it is representable as a scalar, we can |
| // handle it as such, but otherwise, just return the value as is. |
| Some(match self.ecx.try_read_immediate(op) { |
| Ok(Ok(imm)) => imm.into(), |
| _ => op, |
| }) |
| } |
| |
| /// Remove `local` from the pool of `Locals`. Allows writing to them, |
| /// but not reading from them anymore. |
| fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) { |
| ecx.frame_mut().locals[local] = |
| LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) }; |
| } |
| |
| fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> { |
| match &self.source_scopes[source_info.scope].local_data { |
| ClearCrossCrate::Set(data) => Some(data.lint_root), |
| ClearCrossCrate::Clear => None, |
| } |
| } |
| |
| fn use_ecx<F, T>(&mut self, f: F) -> Option<T> |
| where |
| F: FnOnce(&mut Self) -> InterpResult<'tcx, T>, |
| { |
| match f(self) { |
| Ok(val) => Some(val), |
| Err(error) => { |
| // Some errors shouldn't come up because creating them causes |
| // an allocation, which we should avoid. When that happens, |
| // dedicated error variants should be introduced instead. |
| assert!( |
| !error.kind.allocates(), |
| "const-prop encountered allocating error: {}", |
| error |
| ); |
| None |
| } |
| } |
| } |
| |
| /// Returns the value, if any, of evaluating `c`. |
| fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> { |
| // FIXME we need to revisit this for #67176 |
| if c.needs_subst() { |
| return None; |
| } |
| |
| match self.ecx.eval_const_to_op(c.literal, None) { |
| Ok(op) => Some(op), |
| Err(error) => { |
| let tcx = self.ecx.tcx.at(c.span); |
| let err = error_to_const_error(&self.ecx, error, Some(c.span)); |
| if let Some(lint_root) = self.lint_root(source_info) { |
| let lint_only = match c.literal.val { |
| // Promoteds must lint and not error as the user didn't ask for them |
| ConstKind::Unevaluated(_, _, Some(_)) => true, |
| // Out of backwards compatibility we cannot report hard errors in unused |
| // generic functions using associated constants of the generic parameters. |
| _ => c.literal.needs_subst(), |
| }; |
| if lint_only { |
| // Out of backwards compatibility we cannot report hard errors in unused |
| // generic functions using associated constants of the generic parameters. |
| err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span)); |
| } else { |
| err.report_as_error(tcx, "erroneous constant used"); |
| } |
| } else { |
| err.report_as_error(tcx, "erroneous constant used"); |
| } |
| None |
| } |
| } |
| } |
| |
| /// Returns the value, if any, of evaluating `place`. |
| fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> { |
| trace!("eval_place(place={:?})", place); |
| self.use_ecx(|this| this.ecx.eval_place_to_op(place, None)) |
| } |
| |
| /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant` |
| /// or `eval_place`, depending on the variant of `Operand` used. |
| fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> { |
| match *op { |
| Operand::Constant(ref c) => self.eval_constant(c, source_info), |
| Operand::Move(place) | Operand::Copy(place) => self.eval_place(place), |
| } |
| } |
| |
| fn report_assert_as_lint( |
| &self, |
| lint: &'static lint::Lint, |
| source_info: SourceInfo, |
| message: &'static str, |
| panic: AssertKind<ConstInt>, |
| ) -> Option<()> { |
| let lint_root = self.lint_root(source_info)?; |
| self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| { |
| let mut err = lint.build(message); |
| err.span_label(source_info.span, format!("{:?}", panic)); |
| err.emit() |
| }); |
| None |
| } |
| |
| fn check_unary_op( |
| &mut self, |
| op: UnOp, |
| arg: &Operand<'tcx>, |
| source_info: SourceInfo, |
| ) -> Option<()> { |
| if let (val, true) = self.use_ecx(|this| { |
| let val = this.ecx.read_immediate(this.ecx.eval_operand(arg, None)?)?; |
| let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, val)?; |
| Ok((val, overflow)) |
| })? { |
| // `AssertKind` only has an `OverflowNeg` variant, so make sure that is |
| // appropriate to use. |
| assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow"); |
| self.report_assert_as_lint( |
| lint::builtin::ARITHMETIC_OVERFLOW, |
| source_info, |
| "this arithmetic operation will overflow", |
| AssertKind::OverflowNeg(val.to_const_int()), |
| )?; |
| } |
| |
| Some(()) |
| } |
| |
| fn check_binary_op( |
| &mut self, |
| op: BinOp, |
| left: &Operand<'tcx>, |
| right: &Operand<'tcx>, |
| source_info: SourceInfo, |
| ) -> Option<()> { |
| let r = |
| self.use_ecx(|this| this.ecx.read_immediate(this.ecx.eval_operand(right, None)?))?; |
| let l = self.use_ecx(|this| this.ecx.read_immediate(this.ecx.eval_operand(left, None)?)); |
| // Check for exceeding shifts *even if* we cannot evaluate the LHS. |
| if op == BinOp::Shr || op == BinOp::Shl { |
| // We need the type of the LHS. We cannot use `place_layout` as that is the type |
| // of the result, which for checked binops is not the same! |
| let left_ty = left.ty(&self.local_decls, self.tcx); |
| let left_size = self.ecx.layout_of(left_ty).ok()?.size; |
| let right_size = r.layout.size; |
| let r_bits = r.to_scalar().ok(); |
| // This is basically `force_bits`. |
| let r_bits = r_bits.and_then(|r| r.to_bits_or_ptr(right_size, &self.tcx).ok()); |
| if r_bits.map_or(false, |b| b >= left_size.bits() as u128) { |
| debug!("check_binary_op: reporting assert for {:?}", source_info); |
| self.report_assert_as_lint( |
| lint::builtin::ARITHMETIC_OVERFLOW, |
| source_info, |
| "this arithmetic operation will overflow", |
| AssertKind::Overflow( |
| op, |
| match l { |
| Some(l) => l.to_const_int(), |
| // Invent a dummy value, the diagnostic ignores it anyway |
| None => ConstInt::new( |
| 1, |
| left_size, |
| left_ty.is_signed(), |
| left_ty.is_ptr_sized_integral(), |
| ), |
| }, |
| r.to_const_int(), |
| ), |
| )?; |
| } |
| } |
| |
| let l = l?; |
| |
| // The remaining operators are handled through `overflowing_binary_op`. |
| if self.use_ecx(|this| { |
| let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?; |
| Ok(overflow) |
| })? { |
| self.report_assert_as_lint( |
| lint::builtin::ARITHMETIC_OVERFLOW, |
| source_info, |
| "this arithmetic operation will overflow", |
| AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()), |
| )?; |
| } |
| |
| Some(()) |
| } |
| |
| fn const_prop( |
| &mut self, |
| rvalue: &Rvalue<'tcx>, |
| place_layout: TyAndLayout<'tcx>, |
| source_info: SourceInfo, |
| place: Place<'tcx>, |
| ) -> Option<()> { |
| // #66397: Don't try to eval into large places as that can cause an OOM |
| if place_layout.size >= Size::from_bytes(MAX_ALLOC_LIMIT) { |
| return None; |
| } |
| |
| // Perform any special handling for specific Rvalue types. |
| // Generally, checks here fall into one of two categories: |
| // 1. Additional checking to provide useful lints to the user |
| // - In this case, we will do some validation and then fall through to the |
| // end of the function which evals the assignment. |
| // 2. Working around bugs in other parts of the compiler |
| // - In this case, we'll return `None` from this function to stop evaluation. |
| match rvalue { |
| // Additional checking: give lints to the user if an overflow would occur. |
| // We do this here and not in the `Assert` terminator as that terminator is |
| // only sometimes emitted (overflow checks can be disabled), but we want to always |
| // lint. |
| Rvalue::UnaryOp(op, arg) => { |
| trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg); |
| self.check_unary_op(*op, arg, source_info)?; |
| } |
| Rvalue::BinaryOp(op, left, right) => { |
| trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right); |
| self.check_binary_op(*op, left, right, source_info)?; |
| } |
| Rvalue::CheckedBinaryOp(op, left, right) => { |
| trace!( |
| "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})", |
| op, |
| left, |
| right |
| ); |
| self.check_binary_op(*op, left, right, source_info)?; |
| } |
| |
| // Do not try creating references (#67862) |
| Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => { |
| trace!("skipping AddressOf | Ref for {:?}", place); |
| |
| // This may be creating mutable references or immutable references to cells. |
| // If that happens, the pointed to value could be mutated via that reference. |
| // Since we aren't tracking references, the const propagator loses track of what |
| // value the local has right now. |
| // Thus, all locals that have their reference taken |
| // must not take part in propagation. |
| Self::remove_const(&mut self.ecx, place.local); |
| |
| return None; |
| } |
| Rvalue::ThreadLocalRef(def_id) => { |
| trace!("skipping ThreadLocalRef({:?})", def_id); |
| |
| return None; |
| } |
| |
| // There's no other checking to do at this time. |
| Rvalue::Aggregate(..) |
| | Rvalue::Use(..) |
| | Rvalue::Repeat(..) |
| | Rvalue::Len(..) |
| | Rvalue::Cast(..) |
| | Rvalue::Discriminant(..) |
| | Rvalue::NullaryOp(..) => {} |
| } |
| |
| // FIXME we need to revisit this for #67176 |
| if rvalue.needs_subst() { |
| return None; |
| } |
| |
| self.use_ecx(|this| { |
| trace!("calling eval_rvalue_into_place(rvalue = {:?}, place = {:?})", rvalue, place); |
| this.ecx.eval_rvalue_into_place(rvalue, place)?; |
| Ok(()) |
| }) |
| } |
| |
| /// Creates a new `Operand::Constant` from a `Scalar` value |
| fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> { |
| Operand::Constant(Box::new(Constant { |
| span, |
| user_ty: None, |
| literal: ty::Const::from_scalar(self.tcx, scalar, ty), |
| })) |
| } |
| |
| fn replace_with_const( |
| &mut self, |
| rval: &mut Rvalue<'tcx>, |
| value: OpTy<'tcx>, |
| source_info: SourceInfo, |
| ) { |
| if let Rvalue::Use(Operand::Constant(c)) = rval { |
| if !matches!(c.literal.val, ConstKind::Unevaluated(..)) { |
| trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c); |
| return; |
| } |
| } |
| |
| trace!("attepting to replace {:?} with {:?}", rval, value); |
| if let Err(e) = self.ecx.const_validate_operand( |
| value, |
| vec![], |
| // FIXME: is ref tracking too expensive? |
| &mut interpret::RefTracking::empty(), |
| /*may_ref_to_static*/ true, |
| ) { |
| trace!("validation error, attempt failed: {:?}", e); |
| return; |
| } |
| |
| // FIXME> figure out what to do when try_read_immediate fails |
| let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value)); |
| |
| if let Some(Ok(imm)) = imm { |
| match *imm { |
| interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => { |
| *rval = Rvalue::Use(self.operand_from_scalar( |
| scalar, |
| value.layout.ty, |
| source_info.span, |
| )); |
| } |
| Immediate::ScalarPair( |
| ScalarMaybeUninit::Scalar(one), |
| ScalarMaybeUninit::Scalar(two), |
| ) => { |
| // Found a value represented as a pair. For now only do cont-prop if type of |
| // Rvalue is also a pair with two scalars. The more general case is more |
| // complicated to implement so we'll do it later. |
| // FIXME: implement the general case stated above ^. |
| let ty = &value.layout.ty.kind; |
| // Only do it for tuples |
| if let ty::Tuple(substs) = ty { |
| // Only do it if tuple is also a pair with two scalars |
| if substs.len() == 2 { |
| let opt_ty1_ty2 = self.use_ecx(|this| { |
| let ty1 = substs[0].expect_ty(); |
| let ty2 = substs[1].expect_ty(); |
| let ty_is_scalar = |ty| { |
| this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar()) |
| == Some(true) |
| }; |
| if ty_is_scalar(ty1) && ty_is_scalar(ty2) { |
| Ok(Some((ty1, ty2))) |
| } else { |
| Ok(None) |
| } |
| }); |
| |
| if let Some(Some((ty1, ty2))) = opt_ty1_ty2 { |
| *rval = Rvalue::Aggregate( |
| Box::new(AggregateKind::Tuple), |
| vec![ |
| self.operand_from_scalar(one, ty1, source_info.span), |
| self.operand_from_scalar(two, ty2, source_info.span), |
| ], |
| ); |
| } |
| } |
| } |
| } |
| _ => {} |
| } |
| } |
| } |
| |
| /// Returns `true` if and only if this `op` should be const-propagated into. |
| fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool { |
| let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level; |
| |
| if mir_opt_level == 0 { |
| return false; |
| } |
| |
| match *op { |
| interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => { |
| s.is_bits() |
| } |
| interpret::Operand::Immediate(Immediate::ScalarPair( |
| ScalarMaybeUninit::Scalar(l), |
| ScalarMaybeUninit::Scalar(r), |
| )) => l.is_bits() && r.is_bits(), |
| _ => false, |
| } |
| } |
| } |
| |
| /// The mode that `ConstProp` is allowed to run in for a given `Local`. |
| #[derive(Clone, Copy, Debug, PartialEq)] |
| enum ConstPropMode { |
| /// The `Local` can be propagated into and reads of this `Local` can also be propagated. |
| FullConstProp, |
| /// The `Local` can only be propagated into and from its own block. |
| OnlyInsideOwnBlock, |
| /// The `Local` can be propagated into but reads cannot be propagated. |
| OnlyPropagateInto, |
| /// The `Local` cannot be part of propagation at all. Any statement |
| /// referencing it either for reading or writing will not get propagated. |
| NoPropagation, |
| } |
| |
| struct CanConstProp { |
| can_const_prop: IndexVec<Local, ConstPropMode>, |
| // False at the beginning. Once set, no more assignments are allowed to that local. |
| found_assignment: BitSet<Local>, |
| // Cache of locals' information |
| local_kinds: IndexVec<Local, LocalKind>, |
| } |
| |
| impl CanConstProp { |
| /// Returns true if `local` can be propagated |
| fn check(body: &Body<'_>) -> IndexVec<Local, ConstPropMode> { |
| let mut cpv = CanConstProp { |
| can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls), |
| found_assignment: BitSet::new_empty(body.local_decls.len()), |
| local_kinds: IndexVec::from_fn_n( |
| |local| body.local_kind(local), |
| body.local_decls.len(), |
| ), |
| }; |
| for (local, val) in cpv.can_const_prop.iter_enumerated_mut() { |
| // Cannot use args at all |
| // Cannot use locals because if x < y { y - x } else { x - y } would |
| // lint for x != y |
| // FIXME(oli-obk): lint variables until they are used in a condition |
| // FIXME(oli-obk): lint if return value is constant |
| if cpv.local_kinds[local] == LocalKind::Arg { |
| *val = ConstPropMode::OnlyPropagateInto; |
| trace!( |
| "local {:?} can't be const propagated because it's a function argument", |
| local |
| ); |
| } else if cpv.local_kinds[local] == LocalKind::Var { |
| *val = ConstPropMode::OnlyInsideOwnBlock; |
| trace!( |
| "local {:?} will only be propagated inside its block, because it's a user variable", |
| local |
| ); |
| } |
| } |
| cpv.visit_body(&body); |
| cpv.can_const_prop |
| } |
| } |
| |
| impl<'tcx> Visitor<'tcx> for CanConstProp { |
| fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) { |
| use rustc_middle::mir::visit::PlaceContext::*; |
| match context { |
| // Projections are fine, because `&mut foo.x` will be caught by |
| // `MutatingUseContext::Borrow` elsewhere. |
| MutatingUse(MutatingUseContext::Projection) |
| // These are just stores, where the storing is not propagatable, but there may be later |
| // mutations of the same local via `Store` |
| | MutatingUse(MutatingUseContext::Call) |
| // Actual store that can possibly even propagate a value |
| | MutatingUse(MutatingUseContext::Store) => { |
| if !self.found_assignment.insert(local) { |
| match &mut self.can_const_prop[local] { |
| // If the local can only get propagated in its own block, then we don't have |
| // to worry about multiple assignments, as we'll nuke the const state at the |
| // end of the block anyway, and inside the block we overwrite previous |
| // states as applicable. |
| ConstPropMode::OnlyInsideOwnBlock => {} |
| ConstPropMode::NoPropagation => {} |
| ConstPropMode::OnlyPropagateInto => {} |
| other @ ConstPropMode::FullConstProp => { |
| trace!( |
| "local {:?} can't be propagated because of multiple assignments", |
| local, |
| ); |
| *other = ConstPropMode::OnlyPropagateInto; |
| } |
| } |
| } |
| } |
| // Reading constants is allowed an arbitrary number of times |
| NonMutatingUse(NonMutatingUseContext::Copy) |
| | NonMutatingUse(NonMutatingUseContext::Move) |
| | NonMutatingUse(NonMutatingUseContext::Inspect) |
| | NonMutatingUse(NonMutatingUseContext::Projection) |
| | NonUse(_) => {} |
| |
| // These could be propagated with a smarter analysis or just some careful thinking about |
| // whether they'd be fine right now. |
| MutatingUse(MutatingUseContext::AsmOutput) |
| | MutatingUse(MutatingUseContext::Yield) |
| | MutatingUse(MutatingUseContext::Drop) |
| | MutatingUse(MutatingUseContext::Retag) |
| // These can't ever be propagated under any scheme, as we can't reason about indirect |
| // mutation. |
| | NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
| | NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
| | NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
| | NonMutatingUse(NonMutatingUseContext::AddressOf) |
| | MutatingUse(MutatingUseContext::Borrow) |
| | MutatingUse(MutatingUseContext::AddressOf) => { |
| trace!("local {:?} can't be propagaged because it's used: {:?}", local, context); |
| self.can_const_prop[local] = ConstPropMode::NoPropagation; |
| } |
| } |
| } |
| } |
| |
| impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> { |
| fn tcx(&self) -> TyCtxt<'tcx> { |
| self.tcx |
| } |
| |
| fn visit_body(&mut self, body: &mut Body<'tcx>) { |
| for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() { |
| self.visit_basic_block_data(bb, data); |
| } |
| } |
| |
| fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) { |
| trace!("visit_constant: {:?}", constant); |
| self.super_constant(constant, location); |
| self.eval_constant(constant, self.source_info.unwrap()); |
| } |
| |
| fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) { |
| trace!("visit_statement: {:?}", statement); |
| let source_info = statement.source_info; |
| self.source_info = Some(source_info); |
| if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind { |
| let place_ty: Ty<'tcx> = place.ty(&self.local_decls, self.tcx).ty; |
| if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) { |
| let can_const_prop = self.can_const_prop[place.local]; |
| if let Some(()) = self.const_prop(rval, place_layout, source_info, place) { |
| // This will return None if the above `const_prop` invocation only "wrote" a |
| // type whose creation requires no write. E.g. a generator whose initial state |
| // consists solely of uninitialized memory (so it doesn't capture any locals). |
| if let Some(value) = self.get_const(place) { |
| if self.should_const_prop(value) { |
| trace!("replacing {:?} with {:?}", rval, value); |
| self.replace_with_const(rval, value, source_info); |
| if can_const_prop == ConstPropMode::FullConstProp |
| || can_const_prop == ConstPropMode::OnlyInsideOwnBlock |
| { |
| trace!("propagated into {:?}", place); |
| } |
| } |
| } |
| match can_const_prop { |
| ConstPropMode::OnlyInsideOwnBlock => { |
| trace!( |
| "found local restricted to its block. \ |
| Will remove it from const-prop after block is finished. Local: {:?}", |
| place.local |
| ); |
| } |
| ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => { |
| trace!("can't propagate into {:?}", place); |
| if place.local != RETURN_PLACE { |
| Self::remove_const(&mut self.ecx, place.local); |
| } |
| } |
| ConstPropMode::FullConstProp => {} |
| } |
| } else { |
| // Const prop failed, so erase the destination, ensuring that whatever happens |
| // from here on, does not know about the previous value. |
| // This is important in case we have |
| // ```rust |
| // let mut x = 42; |
| // x = SOME_MUTABLE_STATIC; |
| // // x must now be undefined |
| // ``` |
| // FIXME: we overzealously erase the entire local, because that's easier to |
| // implement. |
| trace!( |
| "propagation into {:?} failed. |
| Nuking the entire site from orbit, it's the only way to be sure", |
| place, |
| ); |
| Self::remove_const(&mut self.ecx, place.local); |
| } |
| } else { |
| trace!( |
| "cannot propagate into {:?}, because the type of the local is generic.", |
| place, |
| ); |
| Self::remove_const(&mut self.ecx, place.local); |
| } |
| } else { |
| match statement.kind { |
| StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => { |
| let frame = self.ecx.frame_mut(); |
| frame.locals[local].value = |
| if let StatementKind::StorageLive(_) = statement.kind { |
| LocalValue::Uninitialized |
| } else { |
| LocalValue::Dead |
| }; |
| } |
| _ => {} |
| } |
| } |
| |
| self.super_statement(statement, location); |
| } |
| |
| fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) { |
| let source_info = terminator.source_info; |
| self.source_info = Some(source_info); |
| self.super_terminator(terminator, location); |
| match &mut terminator.kind { |
| TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => { |
| if let Some(value) = self.eval_operand(&cond, source_info) { |
| trace!("assertion on {:?} should be {:?}", value, expected); |
| let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected)); |
| let value_const = self.ecx.read_scalar(value).unwrap(); |
| if expected != value_const { |
| let mut eval_to_int = |op| { |
| let op = self |
| .eval_operand(op, source_info) |
| .expect("if we got here, it must be const"); |
| self.ecx.read_immediate(op).unwrap().to_const_int() |
| }; |
| let msg = match msg { |
| AssertKind::DivisionByZero(op) => { |
| Some(AssertKind::DivisionByZero(eval_to_int(op))) |
| } |
| AssertKind::RemainderByZero(op) => { |
| Some(AssertKind::RemainderByZero(eval_to_int(op))) |
| } |
| AssertKind::BoundsCheck { ref len, ref index } => { |
| let len = eval_to_int(len); |
| let index = eval_to_int(index); |
| Some(AssertKind::BoundsCheck { len, index }) |
| } |
| // Overflow is are already covered by checks on the binary operators. |
| AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None, |
| // Need proper const propagator for these. |
| _ => None, |
| }; |
| // Poison all places this operand references so that further code |
| // doesn't use the invalid value |
| match cond { |
| Operand::Move(ref place) | Operand::Copy(ref place) => { |
| Self::remove_const(&mut self.ecx, place.local); |
| } |
| Operand::Constant(_) => {} |
| } |
| if let Some(msg) = msg { |
| self.report_assert_as_lint( |
| lint::builtin::UNCONDITIONAL_PANIC, |
| source_info, |
| "this operation will panic at runtime", |
| msg, |
| ); |
| } |
| } else { |
| if self.should_const_prop(value) { |
| if let ScalarMaybeUninit::Scalar(scalar) = value_const { |
| *cond = self.operand_from_scalar( |
| scalar, |
| self.tcx.types.bool, |
| source_info.span, |
| ); |
| } |
| } |
| } |
| } |
| } |
| TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => { |
| if let Some(value) = self.eval_operand(&discr, source_info) { |
| if self.should_const_prop(value) { |
| if let ScalarMaybeUninit::Scalar(scalar) = |
| self.ecx.read_scalar(value).unwrap() |
| { |
| *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span); |
| } |
| } |
| } |
| } |
| // None of these have Operands to const-propagate |
| TerminatorKind::Goto { .. } |
| | TerminatorKind::Resume |
| | TerminatorKind::Abort |
| | TerminatorKind::Return |
| | TerminatorKind::Unreachable |
| | TerminatorKind::Drop { .. } |
| | TerminatorKind::DropAndReplace { .. } |
| | TerminatorKind::Yield { .. } |
| | TerminatorKind::GeneratorDrop |
| | TerminatorKind::FalseEdge { .. } |
| | TerminatorKind::FalseUnwind { .. } |
| | TerminatorKind::InlineAsm { .. } => {} |
| // Every argument in our function calls can be const propagated. |
| TerminatorKind::Call { ref mut args, .. } => { |
| let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level; |
| // Constant Propagation into function call arguments is gated |
| // under mir-opt-level 2, because LLVM codegen gives performance |
| // regressions with it. |
| if mir_opt_level >= 2 { |
| for opr in args { |
| /* |
| The following code would appear to be incomplete, because |
| the function `Operand::place()` returns `None` if the |
| `Operand` is of the variant `Operand::Constant`. In this |
| context however, that variant will never appear. This is why: |
| |
| When constructing the MIR, all function call arguments are |
| copied into `Locals` of `LocalKind::Temp`. At least, all arguments |
| that are not unsized (Less than 0.1% are unsized. See #71170 |
| to learn more about those). |
| |
| This means that, conversely, all `Operands` found as function call |
| arguments are of the variant `Operand::Copy`. This allows us to |
| simplify our handling of `Operands` in this case. |
| */ |
| if let Some(l) = opr.place() { |
| if let Some(value) = self.get_const(l) { |
| if self.should_const_prop(value) { |
| // FIXME(felix91gr): this code only handles `Scalar` cases. |
| // For now, we're not handling `ScalarPair` cases because |
| // doing so here would require a lot of code duplication. |
| // We should hopefully generalize `Operand` handling into a fn, |
| // and use it to do const-prop here and everywhere else |
| // where it makes sense. |
| if let interpret::Operand::Immediate( |
| interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar( |
| scalar, |
| )), |
| ) = *value |
| { |
| *opr = self.operand_from_scalar( |
| scalar, |
| value.layout.ty, |
| source_info.span, |
| ); |
| } |
| } |
| } |
| } |
| } |
| } |
| } |
| } |
| |
| // We remove all Locals which are restricted in propagation to their containing blocks and |
| // which were modified in the current block. |
| // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const` |
| let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals); |
| for &local in locals.iter() { |
| Self::remove_const(&mut self.ecx, local); |
| } |
| locals.clear(); |
| // Put it back so we reuse the heap of the storage |
| self.ecx.machine.written_only_inside_own_block_locals = locals; |
| if cfg!(debug_assertions) { |
| // Ensure we are correctly erasing locals with the non-debug-assert logic. |
| for local in self.ecx.machine.only_propagate_inside_block_locals.iter() { |
| assert!( |
| self.get_const(local.into()).is_none() |
| || self |
| .layout_of(self.local_decls[local].ty) |
| .map_or(true, |layout| layout.is_zst()) |
| ) |
| } |
| } |
| } |
| } |
