| //! Propagates constants for early reporting of statically known |
| //! assertion failures |
| |
| use std::cell::Cell; |
| |
| use either::Right; |
| |
| use rustc_const_eval::const_eval::CheckAlignment; |
| use rustc_data_structures::fx::FxHashSet; |
| use rustc_hir::def::DefKind; |
| use rustc_index::bit_set::BitSet; |
| use rustc_index::vec::IndexVec; |
| use rustc_middle::mir::visit::{ |
| MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor, |
| }; |
| use rustc_middle::mir::*; |
| use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout}; |
| use rustc_middle::ty::InternalSubsts; |
| use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeVisitableExt}; |
| use rustc_span::{def_id::DefId, Span}; |
| use rustc_target::abi::{self, Align, HasDataLayout, Size, TargetDataLayout}; |
| use rustc_target::spec::abi::Abi as CallAbi; |
| use rustc_trait_selection::traits; |
| |
| use crate::MirPass; |
| use rustc_const_eval::interpret::{ |
| self, compile_time_machine, AllocId, ConstAllocation, ConstValue, CtfeValidationMode, Frame, |
| ImmTy, Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, PlaceTy, |
| Pointer, Scalar, StackPopCleanup, StackPopUnwind, |
| }; |
| |
| /// The maximum number of bytes that we'll allocate space for a local or the return value. |
| /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just |
| /// Severely regress performance. |
| 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 is_enabled(&self, sess: &rustc_session::Session) -> bool { |
| sess.mir_opt_level() >= 1 |
| } |
| |
| #[instrument(skip(self, tcx), level = "debug")] |
| fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { |
| // will be evaluated by miri and produce its errors there |
| if body.source.promoted.is_some() { |
| return; |
| } |
| |
| let def_id = body.source.def_id().expect_local(); |
| let def_kind = tcx.def_kind(def_id); |
| let is_fn_like = def_kind.is_fn_like(); |
| let is_assoc_const = def_kind == 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 {:?}", def_id); |
| return; |
| } |
| |
| let is_generator = tcx.type_of(def_id.to_def_id()).subst_identity().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 {:?}", 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(def_id.to_def_id()) |
| .predicates |
| .iter() |
| .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None }); |
| if traits::impossible_predicates( |
| tcx, |
| traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(), |
| ) { |
| trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id); |
| return; |
| } |
| |
| trace!("ConstProp starting for {:?}", def_id); |
| |
| let dummy_body = &Body::new( |
| body.source, |
| (*body.basic_blocks).clone(), |
| body.source_scopes.clone(), |
| body.local_decls.clone(), |
| Default::default(), |
| body.arg_count, |
| Default::default(), |
| body.span, |
| body.generator_kind(), |
| body.tainted_by_errors, |
| ); |
| |
| // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold |
| // constants, instead of just checking for const-folding succeeding. |
| // That would require a 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); |
| optimization_finder.visit_body(body); |
| |
| trace!("ConstProp done for {:?}", def_id); |
| } |
| } |
| |
| pub 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. |
| pub written_only_inside_own_block_locals: FxHashSet<Local>, |
| /// Locals that need to be cleared after every block terminates. |
| pub only_propagate_inside_block_locals: BitSet<Local>, |
| pub can_const_prop: IndexVec<Local, ConstPropMode>, |
| } |
| |
| impl ConstPropMachine<'_, '_> { |
| pub fn new( |
| only_propagate_inside_block_locals: BitSet<Local>, |
| can_const_prop: IndexVec<Local, ConstPropMode>, |
| ) -> Self { |
| Self { |
| stack: Vec::new(), |
| written_only_inside_own_block_locals: Default::default(), |
| only_propagate_inside_block_locals, |
| can_const_prop, |
| } |
| } |
| } |
| |
| impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> { |
| compile_time_machine!(<'mir, 'tcx>); |
| const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`) |
| |
| type MemoryKind = !; |
| |
| #[inline(always)] |
| fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> CheckAlignment { |
| // We do not check for alignment to avoid having to carry an `Align` |
| // in `ConstValue::ByRef`. |
| CheckAlignment::No |
| } |
| |
| #[inline(always)] |
| fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool { |
| false // for now, we don't enforce validity |
| } |
| fn alignment_check_failed( |
| ecx: &InterpCx<'mir, 'tcx, Self>, |
| _has: Align, |
| _required: Align, |
| _check: CheckAlignment, |
| ) -> InterpResult<'tcx, ()> { |
| span_bug!( |
| ecx.cur_span(), |
| "`alignment_check_failed` called when no alignment check requested" |
| ) |
| } |
| |
| fn load_mir( |
| _ecx: &InterpCx<'mir, 'tcx, Self>, |
| _instance: ty::InstanceDef<'tcx>, |
| ) -> InterpResult<'tcx, &'tcx Body<'tcx>> { |
| throw_machine_stop_str!("calling functions isn't supported in ConstProp") |
| } |
| |
| fn find_mir_or_eval_fn( |
| _ecx: &mut InterpCx<'mir, 'tcx, Self>, |
| _instance: ty::Instance<'tcx>, |
| _abi: CallAbi, |
| _args: &[OpTy<'tcx>], |
| _destination: &PlaceTy<'tcx>, |
| _target: Option<BasicBlock>, |
| _unwind: StackPopUnwind, |
| ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> { |
| Ok(None) |
| } |
| |
| fn call_intrinsic( |
| _ecx: &mut InterpCx<'mir, 'tcx, Self>, |
| _instance: ty::Instance<'tcx>, |
| _args: &[OpTy<'tcx>], |
| _destination: &PlaceTy<'tcx>, |
| _target: Option<BasicBlock>, |
| _unwind: StackPopUnwind, |
| ) -> 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 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 access_local_mut<'a>( |
| ecx: &'a mut InterpCx<'mir, 'tcx, Self>, |
| frame: usize, |
| local: Local, |
| ) -> InterpResult<'tcx, &'a mut interpret::Operand<Self::Provenance>> { |
| if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation { |
| throw_machine_stop_str!("tried to write to a local that is marked as not propagatable") |
| } |
| if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) { |
| trace!( |
| "mutating local {:?} which is restricted to its block. \ |
| Will remove it from const-prop after block is finished.", |
| local |
| ); |
| ecx.machine.written_only_inside_own_block_locals.insert(local); |
| } |
| ecx.machine.stack[frame].locals[local].access_mut() |
| } |
| |
| fn before_access_global( |
| _tcx: TyCtxt<'tcx>, |
| _machine: &Self, |
| _alloc_id: AllocId, |
| alloc: ConstAllocation<'tcx>, |
| _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 alloc.inner().mutability.is_mut() { |
| throw_machine_stop_str!("can't access mutable globals in ConstProp"); |
| } |
| |
| Ok(()) |
| } |
| |
| #[inline(always)] |
| fn expose_ptr( |
| _ecx: &mut InterpCx<'mir, 'tcx, Self>, |
| _ptr: Pointer<AllocId>, |
| ) -> InterpResult<'tcx> { |
| throw_machine_stop_str!("exposing pointers isn't supported in ConstProp") |
| } |
| |
| #[inline(always)] |
| fn init_frame_extra( |
| _ecx: &mut InterpCx<'mir, 'tcx, Self>, |
| frame: Frame<'mir, 'tcx>, |
| ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> { |
| Ok(frame) |
| } |
| |
| #[inline(always)] |
| fn stack<'a>( |
| ecx: &'a InterpCx<'mir, 'tcx, Self>, |
| ) -> &'a [Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>] { |
| &ecx.machine.stack |
| } |
| |
| #[inline(always)] |
| fn stack_mut<'a>( |
| ecx: &'a mut InterpCx<'mir, 'tcx, Self>, |
| ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::Provenance, 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>, |
| param_env: ParamEnv<'tcx>, |
| local_decls: &'mir 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<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> { |
| type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>; |
| |
| #[inline] |
| fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> { |
| err |
| } |
| } |
| |
| impl HasDataLayout for ConstPropagator<'_, '_> { |
| #[inline] |
| fn data_layout(&self) -> &TargetDataLayout { |
| &self.tcx.data_layout |
| } |
| } |
| |
| impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> { |
| #[inline] |
| fn tcx(&self) -> TyCtxt<'tcx> { |
| self.tcx |
| } |
| } |
| |
| impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> { |
| #[inline] |
| fn param_env(&self) -> ty::ParamEnv<'tcx> { |
| self.param_env |
| } |
| } |
| |
| impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> { |
| fn new( |
| body: &Body<'tcx>, |
| dummy_body: &'mir Body<'tcx>, |
| tcx: TyCtxt<'tcx>, |
| ) -> ConstPropagator<'mir, 'tcx> { |
| let def_id = body.source.def_id(); |
| let substs = &InternalSubsts::identity_for_item(tcx, def_id); |
| let param_env = tcx.param_env_reveal_all_normalized(def_id); |
| |
| let can_const_prop = CanConstProp::check(tcx, param_env, 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, |
| tcx.def_span(def_id), |
| param_env, |
| ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop), |
| ); |
| |
| let ret_layout = ecx |
| .layout_of(body.bound_return_ty().subst(tcx, substs)) |
| .ok() |
| // Don't bother allocating memory for large values. |
| // I don't know how return types can seem to be unsized but this happens in the |
| // `type/type-unsatisfiable.rs` test. |
| .filter(|ret_layout| { |
| ret_layout.is_sized() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) |
| }) |
| .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap()); |
| |
| let ret = ecx |
| .allocate(ret_layout, MemoryKind::Stack) |
| .expect("couldn't perform small allocation") |
| .into(); |
| |
| ecx.push_stack_frame( |
| Instance::new(def_id, substs), |
| dummy_body, |
| &ret, |
| StackPopCleanup::Root { cleanup: false }, |
| ) |
| .expect("failed to push initial stack frame"); |
| |
| ConstPropagator { |
| ecx, |
| tcx, |
| param_env, |
| local_decls: &dummy_body.local_decls, |
| 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) => { |
| if matches!(*op, interpret::Operand::Immediate(Immediate::Uninit)) { |
| // Make sure nobody accidentally uses this value. |
| return None; |
| } |
| 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.read_immediate_raw(&op) { |
| Ok(Right(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::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)), |
| layout: Cell::new(None), |
| }; |
| } |
| |
| /// Returns the value, if any, of evaluating `c`. |
| fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> { |
| // FIXME we need to revisit this for #67176 |
| if c.needs_subst() { |
| return None; |
| } |
| |
| // No span, we don't want errors to be shown. |
| self.ecx.eval_mir_constant(&c.literal, None, None).ok() |
| } |
| |
| /// 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.ecx.eval_place_to_op(place, None).ok() |
| } |
| |
| /// 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>) -> Option<OpTy<'tcx>> { |
| match *op { |
| Operand::Constant(ref c) => self.eval_constant(c), |
| Operand::Move(place) | Operand::Copy(place) => self.eval_place(place), |
| } |
| } |
| |
| fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) { |
| match *operand { |
| Operand::Copy(l) | Operand::Move(l) => { |
| if let Some(value) = self.get_const(l) && 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( |
| scalar, |
| )) = *value |
| { |
| *operand = self.operand_from_scalar( |
| scalar, |
| value.layout.ty, |
| self.source_info.unwrap().span, |
| ); |
| } |
| } |
| } |
| Operand::Constant(_) => (), |
| } |
| } |
| |
| fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> { |
| // 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 { |
| // 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::CopyForDeref(..) |
| | Rvalue::Repeat(..) |
| | Rvalue::Len(..) |
| | Rvalue::Cast(..) |
| | Rvalue::ShallowInitBox(..) |
| | Rvalue::Discriminant(..) |
| | Rvalue::NullaryOp(..) |
| | Rvalue::UnaryOp(..) |
| | Rvalue::BinaryOp(..) |
| | Rvalue::CheckedBinaryOp(..) => {} |
| } |
| |
| // FIXME we need to revisit this for #67176 |
| if rvalue.needs_subst() { |
| return None; |
| } |
| if !rvalue |
| .ty(&self.ecx.frame().body.local_decls, *self.ecx.tcx) |
| .is_sized(*self.ecx.tcx, self.param_env) |
| { |
| // the interpreter doesn't support unsized locals (only unsized arguments), |
| // but rustc does (in a kinda broken way), so we have to skip them here |
| return None; |
| } |
| |
| self.eval_rvalue_with_identities(rvalue, place) |
| } |
| |
| // Attempt to use algebraic identities to eliminate constant expressions |
| fn eval_rvalue_with_identities( |
| &mut self, |
| rvalue: &Rvalue<'tcx>, |
| place: Place<'tcx>, |
| ) -> Option<()> { |
| match rvalue { |
| Rvalue::BinaryOp(op, box (left, right)) |
| | Rvalue::CheckedBinaryOp(op, box (left, right)) => { |
| let l = self.ecx.eval_operand(left, None).and_then(|x| self.ecx.read_immediate(&x)); |
| let r = |
| self.ecx.eval_operand(right, None).and_then(|x| self.ecx.read_immediate(&x)); |
| |
| let const_arg = match (l, r) { |
| (Ok(x), Err(_)) | (Err(_), Ok(x)) => x, // exactly one side is known |
| (Err(_), Err(_)) => return None, // neither side is known |
| (Ok(_), Ok(_)) => return self.ecx.eval_rvalue_into_place(rvalue, place).ok(), // both sides are known |
| }; |
| |
| if !matches!(const_arg.layout.abi, abi::Abi::Scalar(..)) { |
| // We cannot handle Scalar Pair stuff. |
| // No point in calling `eval_rvalue_into_place`, since only one side is known |
| return None; |
| } |
| |
| let arg_value = const_arg.to_scalar().to_bits(const_arg.layout.size).ok()?; |
| let dest = self.ecx.eval_place(place).ok()?; |
| |
| match op { |
| BinOp::BitAnd if arg_value == 0 => { |
| self.ecx.write_immediate(*const_arg, &dest).ok() |
| } |
| BinOp::BitOr |
| if arg_value == const_arg.layout.size.truncate(u128::MAX) |
| || (const_arg.layout.ty.is_bool() && arg_value == 1) => |
| { |
| self.ecx.write_immediate(*const_arg, &dest).ok() |
| } |
| BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => { |
| if let Rvalue::CheckedBinaryOp(_, _) = rvalue { |
| let val = Immediate::ScalarPair( |
| const_arg.to_scalar(), |
| Scalar::from_bool(false), |
| ); |
| self.ecx.write_immediate(val, &dest).ok() |
| } else { |
| self.ecx.write_immediate(*const_arg, &dest).ok() |
| } |
| } |
| _ => None, |
| } |
| } |
| _ => self.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: ConstantKind::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 { |
| match c.literal { |
| ConstantKind::Ty(c) if matches!(c.kind(), ConstKind::Unevaluated(..)) => {} |
| _ => { |
| trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c); |
| return; |
| } |
| } |
| } |
| |
| trace!("attempting to replace {:?} with {:?}", rval, value); |
| if let Err(e) = self.ecx.const_validate_operand( |
| value, |
| vec![], |
| // FIXME: is ref tracking too expensive? |
| // FIXME: what is the point of ref tracking if we do not even check the tracked refs? |
| &mut interpret::RefTracking::empty(), |
| CtfeValidationMode::Regular, |
| ) { |
| trace!("validation error, attempt failed: {:?}", e); |
| return; |
| } |
| |
| // FIXME> figure out what to do when read_immediate_raw fails |
| let imm = self.ecx.read_immediate_raw(value).ok(); |
| |
| if let Some(Right(imm)) = imm { |
| match *imm { |
| interpret::Immediate::Scalar(scalar) => { |
| *rval = Rvalue::Use(self.operand_from_scalar( |
| scalar, |
| value.layout.ty, |
| source_info.span, |
| )); |
| } |
| Immediate::ScalarPair(..) => { |
| // Found a value represented as a pair. For now only do const-prop if the type |
| // of `rvalue` is also a tuple with two scalars. |
| // FIXME: enable the general case stated above ^. |
| let ty = value.layout.ty; |
| // Only do it for tuples |
| if let ty::Tuple(types) = ty.kind() { |
| // Only do it if tuple is also a pair with two scalars |
| if let [ty1, ty2] = types[..] { |
| let ty_is_scalar = |ty| { |
| self.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar()) |
| == Some(true) |
| }; |
| let alloc = if ty_is_scalar(ty1) && ty_is_scalar(ty2) { |
| let alloc = self |
| .ecx |
| .intern_with_temp_alloc(value.layout, |ecx, dest| { |
| ecx.write_immediate(*imm, dest) |
| }) |
| .unwrap(); |
| Some(alloc) |
| } else { |
| None |
| }; |
| |
| if let Some(alloc) = alloc { |
| // Assign entire constant in a single statement. |
| // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`. |
| let const_val = ConstValue::ByRef { alloc, offset: Size::ZERO }; |
| let literal = ConstantKind::Val(const_val, ty); |
| *rval = Rvalue::Use(Operand::Constant(Box::new(Constant { |
| span: source_info.span, |
| user_ty: None, |
| literal, |
| }))); |
| } |
| } |
| } |
| } |
| // Scalars or scalar pairs that contain undef values are assumed to not have |
| // successfully evaluated and are thus not propagated. |
| _ => {} |
| } |
| } |
| } |
| |
| /// Returns `true` if and only if this `op` should be const-propagated into. |
| fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool { |
| if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) { |
| return false; |
| } |
| |
| match **op { |
| interpret::Operand::Immediate(Immediate::Scalar(s)) => s.try_to_int().is_ok(), |
| interpret::Operand::Immediate(Immediate::ScalarPair(l, r)) => { |
| l.try_to_int().is_ok() && r.try_to_int().is_ok() |
| } |
| _ => false, |
| } |
| } |
| } |
| |
| /// The mode that `ConstProp` is allowed to run in for a given `Local`. |
| #[derive(Clone, Copy, Debug, PartialEq)] |
| pub 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, |
| } |
| |
| pub 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 |
| pub fn check<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| param_env: ParamEnv<'tcx>, |
| body: &Body<'tcx>, |
| ) -> 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() { |
| let ty = body.local_decls[local].ty; |
| match tcx.layout_of(param_env.and(ty)) { |
| Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {} |
| // Either the layout fails to compute, then we can't use this local anyway |
| // or the local is too large, then we don't want to. |
| _ => { |
| *val = ConstPropMode::NoPropagation; |
| continue; |
| } |
| } |
| // 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 Visitor<'_> 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) |
| | MutatingUse(MutatingUseContext::AsmOutput) |
| | MutatingUse(MutatingUseContext::Deinit) |
| // Actual store that can possibly even propagate a value |
| | MutatingUse(MutatingUseContext::Store) |
| | MutatingUse(MutatingUseContext::SetDiscriminant) => { |
| 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. Previous state: {:?}", |
| local, other, |
| ); |
| *other = ConstPropMode::OnlyInsideOwnBlock; |
| } |
| } |
| } |
| } |
| // 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::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<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> { |
| fn tcx(&self) -> TyCtxt<'tcx> { |
| self.tcx |
| } |
| |
| fn visit_body(&mut self, body: &mut Body<'tcx>) { |
| for (bb, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() { |
| self.visit_basic_block_data(bb, data); |
| } |
| } |
| |
| fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) { |
| self.super_operand(operand, location); |
| |
| // Only const prop copies and moves on `mir_opt_level=3` as doing so |
| // currently slightly increases compile time in some cases. |
| if self.tcx.sess.mir_opt_level() >= 3 { |
| self.propagate_operand(operand) |
| } |
| } |
| |
| fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) { |
| trace!("visit_constant: {:?}", constant); |
| self.super_constant(constant, location); |
| self.eval_constant(constant); |
| } |
| |
| 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); |
| match statement.kind { |
| StatementKind::Assign(box (place, ref mut rval)) => { |
| let can_const_prop = self.ecx.machine.can_const_prop[place.local]; |
| if let Some(()) = self.const_prop(rval, 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(ref value) = self.get_const(place) && 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 uninit |
| // ``` |
| // 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); |
| } |
| } |
| StatementKind::SetDiscriminant { ref place, .. } => { |
| match self.ecx.machine.can_const_prop[place.local] { |
| ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => { |
| if self.ecx.statement(statement).is_ok() { |
| trace!("propped discriminant into {:?}", place); |
| } else { |
| Self::remove_const(&mut self.ecx, place.local); |
| } |
| } |
| ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => { |
| Self::remove_const(&mut self.ecx, place.local); |
| } |
| } |
| } |
| StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => { |
| let frame = self.ecx.frame_mut(); |
| frame.locals[local].value = if let StatementKind::StorageLive(_) = statement.kind { |
| LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)) |
| } 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 mut cond, .. } => { |
| if let Some(ref value) = self.eval_operand(&cond) |
| && let Ok(value_const) = self.ecx.read_scalar(&value) |
| && self.should_const_prop(value) |
| { |
| trace!("assertion on {:?} should be {:?}", value, expected); |
| *cond = self.operand_from_scalar( |
| value_const, |
| self.tcx.types.bool, |
| source_info.span, |
| ); |
| } |
| } |
| TerminatorKind::SwitchInt { ref mut discr, .. } => { |
| // FIXME: This is currently redundant with `visit_operand`, but sadly |
| // always visiting operands currently causes a perf regression in LLVM codegen, so |
| // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`. |
| self.propagate_operand(discr) |
| } |
| // 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 have already been propagated in `visit_operand`. |
| // |
| // NOTE: because LLVM codegen gives slight performance regressions with it, so this is |
| // gated on `mir_opt_level=3`. |
| TerminatorKind::Call { .. } => {} |
| } |
| } |
| |
| fn visit_basic_block_data(&mut self, block: BasicBlock, data: &mut BasicBlockData<'tcx>) { |
| self.super_basic_block_data(block, data); |
| |
| // 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()) |
| ) |
| } |
| } |
| } |
| } |
