rust/src/librustc_mir/transform/const_prop.rs

// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Propagates constants for early reporting of statically known
//! assertion failures


use rustc::hir::def::Def;
use rustc::mir::{Constant, Location, Place, Mir, Operand, Rvalue, Local};
use rustc::mir::{NullOp, UnOp, StatementKind, Statement, BasicBlock, LocalKind};
use rustc::mir::{TerminatorKind, ClearCrossCrate, SourceInfo, BinOp, ProjectionElem};
use rustc::mir::visit::{Visitor, PlaceContext};
use rustc::mir::interpret::{
    ConstEvalErr, EvalErrorKind, Scalar, GlobalId, EvalResult,
};
use rustc::ty::{TyCtxt, self, Instance};
use interpret::{self, EvalContext, Value, OpTy, MemoryKind, ScalarMaybeUndef};
use const_eval::{CompileTimeInterpreter, eval_promoted, mk_borrowck_eval_cx};
use transform::{MirPass, MirSource};
use syntax::source_map::{Span, DUMMY_SP};
use rustc::ty::subst::Substs;
use rustc_data_structures::indexed_vec::IndexVec;
use rustc::ty::ParamEnv;
use rustc::ty::layout::{
    LayoutOf, TyLayout, LayoutError,
    HasTyCtxt, TargetDataLayout, HasDataLayout,
};

pub struct ConstProp;

impl MirPass for ConstProp {
    fn run_pass<'a, 'tcx>(&self,
                          tcx: TyCtxt<'a, 'tcx, 'tcx>,
                          source: MirSource,
                          mir: &mut Mir<'tcx>) {
        // will be evaluated by miri and produce its errors there
        if source.promoted.is_some() {
            return;
        }

        use rustc::hir::map::blocks::FnLikeNode;
        let node_id = tcx.hir.as_local_node_id(source.def_id)
                             .expect("Non-local call to local provider is_const_fn");

        let is_fn_like = FnLikeNode::from_node(tcx.hir.get(node_id)).is_some();
        let is_assoc_const = match tcx.describe_def(source.def_id) {
            Some(Def::AssociatedConst(_)) => true,
            _ => false,
        };

        // 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
        }

        trace!("ConstProp starting for {:?}", source.def_id);

        // 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(mir, tcx, source);
        optimization_finder.visit_mir(mir);

        trace!("ConstProp done for {:?}", source.def_id);
    }
}

type Const<'tcx> = (OpTy<'tcx>, Span);

/// Finds optimization opportunities on the MIR.
struct ConstPropagator<'a, 'mir, 'tcx:'a+'mir> {
    ecx: EvalContext<'a, 'mir, 'tcx, CompileTimeInterpreter<'a, 'mir, 'tcx>>,
    mir: &'mir Mir<'tcx>,
    tcx: TyCtxt<'a, 'tcx, 'tcx>,
    source: MirSource,
    places: IndexVec<Local, Option<Const<'tcx>>>,
    can_const_prop: IndexVec<Local, bool>,
    param_env: ParamEnv<'tcx>,
}

impl<'a, 'b, 'tcx> LayoutOf for &'a ConstPropagator<'a, 'b, 'tcx> {
    type Ty = ty::Ty<'tcx>;
    type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;

    fn layout_of(self, ty: ty::Ty<'tcx>) -> Self::TyLayout {
        self.tcx.layout_of(self.param_env.and(ty))
    }
}

impl<'a, 'b, 'tcx> HasDataLayout for &'a ConstPropagator<'a, 'b, 'tcx> {
    #[inline]
    fn data_layout(&self) -> &TargetDataLayout {
        &self.tcx.data_layout
    }
}

impl<'a, 'b, 'tcx> HasTyCtxt<'tcx> for &'a ConstPropagator<'a, 'b, 'tcx> {
    #[inline]
    fn tcx<'c>(&'c self) -> TyCtxt<'c, 'tcx, 'tcx> {
        self.tcx
    }
}

impl<'a, 'mir, 'tcx> ConstPropagator<'a, 'mir, 'tcx> {
    fn new(
        mir: &'mir Mir<'tcx>,
        tcx: TyCtxt<'a, 'tcx, 'tcx>,
        source: MirSource,
    ) -> ConstPropagator<'a, 'mir, 'tcx> {
        let param_env = tcx.param_env(source.def_id);
        let substs = Substs::identity_for_item(tcx, source.def_id);
        let instance = Instance::new(source.def_id, substs);
        let ecx = mk_borrowck_eval_cx(tcx, instance, mir, DUMMY_SP).unwrap();
        ConstPropagator {
            ecx,
            mir,
            tcx,
            source,
            param_env,
            can_const_prop: CanConstProp::check(mir),
            places: IndexVec::from_elem(None, &mir.local_decls),
        }
    }

    fn use_ecx<F, T>(
        &mut self,
        source_info: SourceInfo,
        f: F
    ) -> Option<T>
    where
        F: FnOnce(&mut Self) -> EvalResult<'tcx, T>,
    {
        self.ecx.tcx.span = source_info.span;
        let lint_root = match self.mir.source_scope_local_data {
            ClearCrossCrate::Set(ref ivs) => {
                //FIXME(#51314): remove this check
                if source_info.scope.index() >= ivs.len() {
                    return None;
                }
                ivs[source_info.scope].lint_root
            },
            ClearCrossCrate::Clear => return None,
        };
        let r = match f(self) {
            Ok(val) => Some(val),
            Err(error) => {
                let stacktrace = self.ecx.generate_stacktrace(None);
                let diagnostic = ConstEvalErr { span: source_info.span, error, stacktrace };
                use rustc::mir::interpret::EvalErrorKind::*;
                match diagnostic.error.kind {
                    // don't report these, they make no sense in a const prop context
                    | MachineError(_)
                    // at runtime these transformations might make sense
                    // FIXME: figure out the rules and start linting
                    | FunctionAbiMismatch(..)
                    | FunctionArgMismatch(..)
                    | FunctionRetMismatch(..)
                    | FunctionArgCountMismatch
                    // fine at runtime, might be a register address or sth
                    | ReadBytesAsPointer
                    // fine at runtime
                    | ReadForeignStatic
                    | Unimplemented(_)
                    // don't report const evaluator limits
                    | StackFrameLimitReached
                    | NoMirFor(..)
                    | InlineAsm
                    => {},

                    | InvalidMemoryAccess
                    | DanglingPointerDeref
                    | DoubleFree
                    | InvalidFunctionPointer
                    | InvalidBool
                    | InvalidDiscriminant(..)
                    | PointerOutOfBounds { .. }
                    | InvalidNullPointerUsage
                    | MemoryLockViolation { .. }
                    | MemoryAcquireConflict { .. }
                    | ValidationFailure(..)
                    | InvalidMemoryLockRelease { .. }
                    | DeallocatedLockedMemory { .. }
                    | InvalidPointerMath
                    | ReadUndefBytes(_)
                    | DeadLocal
                    | InvalidBoolOp(_)
                    | DerefFunctionPointer
                    | ExecuteMemory
                    | Intrinsic(..)
                    | InvalidChar(..)
                    | AbiViolation(_)
                    | AlignmentCheckFailed{..}
                    | CalledClosureAsFunction
                    | VtableForArgumentlessMethod
                    | ModifiedConstantMemory
                    | AssumptionNotHeld
                    // FIXME: should probably be removed and turned into a bug! call
                    | TypeNotPrimitive(_)
                    | ReallocatedWrongMemoryKind(_, _)
                    | DeallocatedWrongMemoryKind(_, _)
                    | ReallocateNonBasePtr
                    | DeallocateNonBasePtr
                    | IncorrectAllocationInformation(..)
                    | UnterminatedCString(_)
                    | HeapAllocZeroBytes
                    | HeapAllocNonPowerOfTwoAlignment(_)
                    | Unreachable
                    | ReadFromReturnPointer
                    | GeneratorResumedAfterReturn
                    | GeneratorResumedAfterPanic
                    | ReferencedConstant
                    | InfiniteLoop
                    => {
                        // FIXME: report UB here
                    },

                    | OutOfTls
                    | TlsOutOfBounds
                    | PathNotFound(_)
                    => bug!("these should not be in rustc, but in miri's machine errors"),

                    | Layout(_)
                    | UnimplementedTraitSelection
                    | TypeckError
                    | TooGeneric
                    // these are just noise
                    => {},

                    // non deterministic
                    | ReadPointerAsBytes
                    // FIXME: implement
                    => {},

                    | Panic { .. }
                    | BoundsCheck{..}
                    | Overflow(_)
                    | OverflowNeg
                    | DivisionByZero
                    | RemainderByZero
                    => {
                        diagnostic.report_as_lint(
                            self.ecx.tcx,
                            "this expression will panic at runtime",
                            lint_root,
                        );
                    }
                }
                None
            },
        };
        self.ecx.tcx.span = DUMMY_SP;
        r
    }

    fn eval_constant(
        &mut self,
        c: &Constant<'tcx>,
        source_info: SourceInfo,
    ) -> Option<Const<'tcx>> {
        self.ecx.tcx.span = source_info.span;
        match self.ecx.const_to_op(c.literal) {
            Ok(op) => {
                Some((op, c.span))
            },
            Err(error) => {
                let stacktrace = self.ecx.generate_stacktrace(None);
                let err = ::rustc::mir::interpret::ConstEvalErr {
                    error, stacktrace, span: source_info.span,
                };
                err.report_as_error(self.ecx.tcx, "erroneous constant used");
                None
            },
        }
    }

    fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
        match *place {
            Place::Local(loc) => self.places[loc].clone(),
            Place::Projection(ref proj) => match proj.elem {
                ProjectionElem::Field(field, _) => {
                    trace!("field proj on {:?}", proj.base);
                    let (base, span) = self.eval_place(&proj.base, source_info)?;
                    let res = self.use_ecx(source_info, |this| {
                        this.ecx.operand_field(base, field.index() as u64)
                    })?;
                    Some((res, span))
                },
                // We could get more projections by using e.g. `operand_projection`,
                // but we do not even have the stack frame set up properly so
                // an `Index` projection would throw us off-track.
                _ => None,
            },
            Place::Promoted(ref promoted) => {
                let generics = self.tcx.generics_of(self.source.def_id);
                if generics.requires_monomorphization(self.tcx) {
                    // FIXME: can't handle code with generics
                    return None;
                }
                let substs = Substs::identity_for_item(self.tcx, self.source.def_id);
                let instance = Instance::new(self.source.def_id, substs);
                let cid = GlobalId {
                    instance,
                    promoted: Some(promoted.0),
                };
                // cannot use `const_eval` here, because that would require having the MIR
                // for the current function available, but we're producing said MIR right now
                let res = self.use_ecx(source_info, |this| {
                    eval_promoted(this.tcx, cid, this.mir, this.param_env)
                })?;
                trace!("evaluated promoted {:?} to {:?}", promoted, res);
                Some((res, source_info.span))
            },
            _ => None,
        }
    }

    fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
        match *op {
            Operand::Constant(ref c) => self.eval_constant(c, source_info),
            | Operand::Move(ref place)
            | Operand::Copy(ref place) => self.eval_place(place, source_info),
        }
    }

    fn const_prop(
        &mut self,
        rvalue: &Rvalue<'tcx>,
        place_layout: TyLayout<'tcx>,
        source_info: SourceInfo,
    ) -> Option<Const<'tcx>> {
        let span = source_info.span;
        match *rvalue {
            Rvalue::Use(ref op) => {
                self.eval_operand(op, source_info)
            },
            Rvalue::Repeat(..) |
            Rvalue::Ref(..) |
            Rvalue::Aggregate(..) |
            Rvalue::NullaryOp(NullOp::Box, _) |
            Rvalue::Discriminant(..) => None,

            Rvalue::Cast(kind, ref operand, _) => {
                let (op, span) = self.eval_operand(operand, source_info)?;
                self.use_ecx(source_info, |this| {
                    let dest = this.ecx.allocate(place_layout, MemoryKind::Stack)?;
                    this.ecx.cast(op, kind, dest.into())?;
                    Ok((dest.into(), span))
                })
            }

            // FIXME(oli-obk): evaluate static/constant slice lengths
            Rvalue::Len(_) => None,
            Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
                type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some((
                    OpTy {
                        op: interpret::Operand::Immediate(Value::Scalar(
                            Scalar::Bits {
                                bits: n as u128,
                                size: self.tcx.data_layout.pointer_size.bytes() as u8,
                            }.into()
                        )),
                        layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
                    },
                    span,
                )))
            }
            Rvalue::UnaryOp(op, ref arg) => {
                let def_id = if self.tcx.is_closure(self.source.def_id) {
                    self.tcx.closure_base_def_id(self.source.def_id)
                } else {
                    self.source.def_id
                };
                let generics = self.tcx.generics_of(def_id);
                if generics.requires_monomorphization(self.tcx) {
                    // FIXME: can't handle code with generics
                    return None;
                }

                let (arg, _) = self.eval_operand(arg, source_info)?;
                let val = self.use_ecx(source_info, |this| {
                    let prim = this.ecx.read_scalar(arg)?.not_undef()?;
                    match op {
                        UnOp::Neg => {
                            // Need to do overflow check here: For actual CTFE, MIR
                            // generation emits code that does this before calling the op.
                            let size = arg.layout.size;
                            if prim.to_bits(size)? == (1 << (size.bits() - 1)) {
                                return err!(OverflowNeg);
                            }
                        }
                        UnOp::Not => {
                            // Cannot overflow
                        }
                    }
                    // Now run the actual operation.
                    this.ecx.unary_op(op, prim, arg.layout)
                })?;
                let res = OpTy {
                    op: interpret::Operand::Immediate(Value::Scalar(val.into())),
                    layout: place_layout,
                };
                Some((res, span))
            }
            Rvalue::CheckedBinaryOp(op, ref left, ref right) |
            Rvalue::BinaryOp(op, ref left, ref right) => {
                trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
                let right = self.eval_operand(right, source_info)?;
                let def_id = if self.tcx.is_closure(self.source.def_id) {
                    self.tcx.closure_base_def_id(self.source.def_id)
                } else {
                    self.source.def_id
                };
                let generics = self.tcx.generics_of(def_id);
                if generics.requires_monomorphization(self.tcx) {
                    // FIXME: can't handle code with generics
                    return None;
                }

                let r = self.use_ecx(source_info, |this| {
                    this.ecx.read_value(right.0)
                })?;
                if op == BinOp::Shr || op == BinOp::Shl {
                    let left_ty = left.ty(self.mir, self.tcx);
                    let left_bits = self
                        .tcx
                        .layout_of(self.param_env.and(left_ty))
                        .unwrap()
                        .size
                        .bits();
                    let right_size = right.0.layout.size;
                    let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
                    if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
                        let source_scope_local_data = match self.mir.source_scope_local_data {
                            ClearCrossCrate::Set(ref data) => data,
                            ClearCrossCrate::Clear => return None,
                        };
                        let dir = if op == BinOp::Shr {
                            "right"
                        } else {
                            "left"
                        };
                        let node_id = source_scope_local_data[source_info.scope].lint_root;
                        self.tcx.lint_node(
                            ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
                            node_id,
                            span,
                            &format!("attempt to shift {} with overflow", dir));
                        return None;
                    }
                }
                let left = self.eval_operand(left, source_info)?;
                let l = self.use_ecx(source_info, |this| {
                    this.ecx.read_value(left.0)
                })?;
                trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
                let (val, overflow) = self.use_ecx(source_info, |this| {
                    this.ecx.binary_op_val(op, l, r)
                })?;
                let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
                    Value::ScalarPair(
                        val.into(),
                        Scalar::from_bool(overflow).into(),
                    )
                } else {
                    if overflow {
                        let err = EvalErrorKind::Overflow(op).into();
                        let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
                        return None;
                    }
                    Value::Scalar(val.into())
                };
                let res = OpTy {
                    op: interpret::Operand::Immediate(val),
                    layout: place_layout,
                };
                Some((res, span))
            },
        }
    }
}

fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                          param_env: ty::ParamEnv<'tcx>,
                          ty: ty::Ty<'tcx>) -> Option<u64> {
    tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
}

struct CanConstProp {
    can_const_prop: IndexVec<Local, bool>,
    // false at the beginning, once set, there are not allowed to be any more assignments
    found_assignment: IndexVec<Local, bool>,
}

impl CanConstProp {
    /// returns true if `local` can be propagated
    fn check(mir: &Mir) -> IndexVec<Local, bool> {
        let mut cpv = CanConstProp {
            can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
            found_assignment: IndexVec::from_elem(false, &mir.local_decls),
        };
        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
            *val = mir.local_kind(local) == LocalKind::Temp;
        }
        cpv.visit_mir(mir);
        cpv.can_const_prop
    }
}

impl<'tcx> Visitor<'tcx> for CanConstProp {
    fn visit_local(
        &mut self,
        &local: &Local,
        context: PlaceContext<'tcx>,
        _: Location,
    ) {
        use rustc::mir::visit::PlaceContext::*;
        match context {
            // Constants must have at most one write
            // FIXME(oli-obk): we could be more powerful here, if the multiple writes
            // only occur in independent execution paths
            Store => if self.found_assignment[local] {
                self.can_const_prop[local] = false;
            } else {
                self.found_assignment[local] = true
            },
            // Reading constants is allowed an arbitrary number of times
            Copy | Move |
            StorageDead | StorageLive |
            Validate |
            Projection(_) |
            Inspect => {},
            _ => self.can_const_prop[local] = false,
        }
    }
}

impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
    fn visit_constant(
        &mut self,
        constant: &Constant<'tcx>,
        location: Location,
    ) {
        trace!("visit_constant: {:?}", constant);
        self.super_constant(constant, location);
        let source_info = *self.mir.source_info(location);
        self.eval_constant(constant, source_info);
    }

    fn visit_statement(
        &mut self,
        block: BasicBlock,
        statement: &Statement<'tcx>,
        location: Location,
    ) {
        trace!("visit_statement: {:?}", statement);
        if let StatementKind::Assign(ref place, ref rval) = statement.kind {
            let place_ty: ty::Ty<'tcx> = place
                .ty(&self.mir.local_decls, self.tcx)
                .to_ty(self.tcx);
            if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
                if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
                    if let Place::Local(local) = *place {
                        trace!("checking whether {:?} can be stored to {:?}", value, local);
                        if self.can_const_prop[local] {
                            trace!("storing {:?} to {:?}", value, local);
                            assert!(self.places[local].is_none());
                            self.places[local] = Some(value);
                        }
                    }
                }
            }
        }
        self.super_statement(block, statement, location);
    }

    fn visit_terminator_kind(
        &mut self,
        block: BasicBlock,
        kind: &TerminatorKind<'tcx>,
        location: Location,
    ) {
        self.super_terminator_kind(block, kind, location);
        let source_info = *self.mir.source_info(location);
        if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
            if let Some(value) = self.eval_operand(cond, source_info) {
                trace!("assertion on {:?} should be {:?}", value, expected);
                let expected = Value::Scalar(Scalar::from_bool(*expected).into());
                if expected != value.0.to_immediate() {
                    // 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) => {
                            let mut place = place;
                            while let Place::Projection(ref proj) = *place {
                                place = &proj.base;
                            }
                            if let Place::Local(local) = *place {
                                self.places[local] = None;
                            }
                        },
                        Operand::Constant(_) => {}
                    }
                    let span = self.mir[block]
                        .terminator
                        .as_ref()
                        .unwrap()
                        .source_info
                        .span;
                    let node_id = self
                        .tcx
                        .hir
                        .as_local_node_id(self.source.def_id)
                        .expect("some part of a failing const eval must be local");
                    use rustc::mir::interpret::EvalErrorKind::*;
                    let msg = match msg {
                        Overflow(_) |
                        OverflowNeg |
                        DivisionByZero |
                        RemainderByZero => msg.description().to_owned(),
                        BoundsCheck { ref len, ref index } => {
                            let len = self
                                .eval_operand(len, source_info)
                                .expect("len must be const");
                            let len = match len.0.to_immediate() {
                                Value::Scalar(ScalarMaybeUndef::Scalar(Scalar::Bits {
                                    bits, ..
                                })) => bits,
                                _ => bug!("const len not primitive: {:?}", len),
                            };
                            let index = self
                                .eval_operand(index, source_info)
                                .expect("index must be const");
                            let index = match index.0.to_immediate() {
                                Value::Scalar(ScalarMaybeUndef::Scalar(Scalar::Bits {
                                    bits, ..
                                })) => bits,
                                _ => bug!("const index not primitive: {:?}", index),
                            };
                            format!(
                                "index out of bounds: \
                                the len is {} but the index is {}",
                                len,
                                index,
                            )
                        },
                        // Need proper const propagator for these
                        _ => return,
                    };
                    self.tcx.lint_node(
                        ::rustc::lint::builtin::CONST_ERR,
                        node_id,
                        span,
                        &msg,
                    );
                }
            }
        }
    }
}