1036 lines
43 KiB
Rust
1036 lines
43 KiB
Rust
//! Propagates constants for early reporting of statically known
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//! assertion failures
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use std::cell::Cell;
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use rustc_ast::Mutability;
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use rustc_data_structures::fx::FxHashSet;
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use rustc_hir::def::DefKind;
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use rustc_hir::HirId;
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use rustc_index::bit_set::BitSet;
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use rustc_index::vec::IndexVec;
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use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
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use rustc_middle::mir::{
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AssertKind, BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind,
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Location, Operand, Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement,
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StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
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};
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use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
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use rustc_middle::ty::subst::{InternalSubsts, Subst};
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use rustc_middle::ty::{
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self, ConstInt, ConstKind, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeFoldable,
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};
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use rustc_session::lint;
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use rustc_span::{def_id::DefId, Span};
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use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
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use rustc_target::spec::abi::Abi;
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use rustc_trait_selection::traits;
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use crate::MirLint;
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use rustc_const_eval::const_eval::ConstEvalErr;
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use rustc_const_eval::interpret::{
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self, compile_time_machine, AllocId, ConstAllocation, Frame, ImmTy, InterpCx, InterpResult,
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LocalState, LocalValue, MemPlace, MemoryKind, OpTy, Operand as InterpOperand, PlaceTy, Scalar,
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ScalarMaybeUninit, StackPopCleanup, StackPopUnwind,
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};
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/// The maximum number of bytes that we'll allocate space for a local or the return value.
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/// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
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/// Severely regress performance.
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const MAX_ALLOC_LIMIT: u64 = 1024;
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/// Macro for machine-specific `InterpError` without allocation.
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/// (These will never be shown to the user, but they help diagnose ICEs.)
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macro_rules! throw_machine_stop_str {
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($($tt:tt)*) => {{
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// We make a new local type for it. The type itself does not carry any information,
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// but its vtable (for the `MachineStopType` trait) does.
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struct Zst;
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// Printing this type shows the desired string.
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impl std::fmt::Display for Zst {
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fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
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write!(f, $($tt)*)
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}
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}
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impl rustc_middle::mir::interpret::MachineStopType for Zst {}
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throw_machine_stop!(Zst)
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}};
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}
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pub struct ConstProp;
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impl<'tcx> MirLint<'tcx> for ConstProp {
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fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
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// will be evaluated by miri and produce its errors there
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if body.source.promoted.is_some() {
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return;
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}
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let def_id = body.source.def_id().expect_local();
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let is_fn_like = tcx.hir().get_by_def_id(def_id).fn_kind().is_some();
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let is_assoc_const = tcx.def_kind(def_id) == DefKind::AssocConst;
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// Only run const prop on functions, methods, closures and associated constants
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if !is_fn_like && !is_assoc_const {
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// skip anon_const/statics/consts because they'll be evaluated by miri anyway
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trace!("ConstProp skipped for {:?}", def_id);
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return;
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}
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let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
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// FIXME(welseywiser) const prop doesn't work on generators because of query cycles
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// computing their layout.
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if is_generator {
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trace!("ConstProp skipped for generator {:?}", def_id);
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return;
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}
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// Check if it's even possible to satisfy the 'where' clauses
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// for this item.
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// This branch will never be taken for any normal function.
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// However, it's possible to `#!feature(trivial_bounds)]` to write
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// a function with impossible to satisfy clauses, e.g.:
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// `fn foo() where String: Copy {}`
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//
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// We don't usually need to worry about this kind of case,
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// since we would get a compilation error if the user tried
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// to call it. However, since we can do const propagation
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// even without any calls to the function, we need to make
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// sure that it even makes sense to try to evaluate the body.
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// If there are unsatisfiable where clauses, then all bets are
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// off, and we just give up.
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//
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// We manually filter the predicates, skipping anything that's not
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// "global". We are in a potentially generic context
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// (e.g. we are evaluating a function without substituting generic
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// parameters, so this filtering serves two purposes:
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//
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// 1. We skip evaluating any predicates that we would
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// never be able prove are unsatisfiable (e.g. `<T as Foo>`
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// 2. We avoid trying to normalize predicates involving generic
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// parameters (e.g. `<T as Foo>::MyItem`). This can confuse
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// the normalization code (leading to cycle errors), since
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// it's usually never invoked in this way.
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let predicates = tcx
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.predicates_of(def_id.to_def_id())
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.predicates
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.iter()
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.filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
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if traits::impossible_predicates(
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tcx,
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traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
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) {
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trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
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return;
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}
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trace!("ConstProp starting for {:?}", def_id);
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let dummy_body = &Body::new(
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body.source,
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body.basic_blocks().clone(),
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body.source_scopes.clone(),
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body.local_decls.clone(),
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Default::default(),
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body.arg_count,
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Default::default(),
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body.span,
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body.generator_kind(),
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body.tainted_by_errors,
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);
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// FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
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// constants, instead of just checking for const-folding succeeding.
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// That would require a uniform one-def no-mutation analysis
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// and RPO (or recursing when needing the value of a local).
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let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
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optimization_finder.visit_body(body);
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trace!("ConstProp done for {:?}", def_id);
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}
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}
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struct ConstPropMachine<'mir, 'tcx> {
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/// The virtual call stack.
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stack: Vec<Frame<'mir, 'tcx>>,
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/// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
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written_only_inside_own_block_locals: FxHashSet<Local>,
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/// Locals that need to be cleared after every block terminates.
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only_propagate_inside_block_locals: BitSet<Local>,
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can_const_prop: IndexVec<Local, ConstPropMode>,
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}
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impl ConstPropMachine<'_, '_> {
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fn new(
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only_propagate_inside_block_locals: BitSet<Local>,
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can_const_prop: IndexVec<Local, ConstPropMode>,
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) -> Self {
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Self {
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stack: Vec::new(),
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written_only_inside_own_block_locals: Default::default(),
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only_propagate_inside_block_locals,
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can_const_prop,
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}
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}
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}
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impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
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compile_time_machine!(<'mir, 'tcx>);
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const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
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type MemoryKind = !;
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type MemoryExtra = ();
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fn load_mir(
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_ecx: &InterpCx<'mir, 'tcx, Self>,
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_instance: ty::InstanceDef<'tcx>,
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) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
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throw_machine_stop_str!("calling functions isn't supported in ConstProp")
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}
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fn find_mir_or_eval_fn(
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_ecx: &mut InterpCx<'mir, 'tcx, Self>,
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_instance: ty::Instance<'tcx>,
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_abi: Abi,
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_args: &[OpTy<'tcx>],
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_ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
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_unwind: StackPopUnwind,
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) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
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Ok(None)
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}
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fn call_intrinsic(
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_ecx: &mut InterpCx<'mir, 'tcx, Self>,
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_instance: ty::Instance<'tcx>,
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_args: &[OpTy<'tcx>],
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_ret: Option<(&PlaceTy<'tcx>, BasicBlock)>,
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_unwind: StackPopUnwind,
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) -> InterpResult<'tcx> {
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throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
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}
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fn assert_panic(
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_ecx: &mut InterpCx<'mir, 'tcx, Self>,
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_msg: &rustc_middle::mir::AssertMessage<'tcx>,
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_unwind: Option<rustc_middle::mir::BasicBlock>,
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) -> InterpResult<'tcx> {
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bug!("panics terminators are not evaluated in ConstProp")
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}
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fn binary_ptr_op(
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_ecx: &InterpCx<'mir, 'tcx, Self>,
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_bin_op: BinOp,
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_left: &ImmTy<'tcx>,
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_right: &ImmTy<'tcx>,
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) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
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// We can't do this because aliasing of memory can differ between const eval and llvm
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throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
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}
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fn access_local(
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_ecx: &InterpCx<'mir, 'tcx, Self>,
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frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
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local: Local,
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) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
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let l = &frame.locals[local];
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if l.value == LocalValue::Unallocated {
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throw_machine_stop_str!("tried to access an uninitialized local")
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}
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l.access()
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}
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fn access_local_mut<'a>(
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ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
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frame: usize,
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local: Local,
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) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
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{
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if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
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throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
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}
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if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
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trace!(
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"mutating local {:?} which is restricted to its block. \
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Will remove it from const-prop after block is finished.",
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local
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);
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ecx.machine.written_only_inside_own_block_locals.insert(local);
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}
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ecx.machine.stack[frame].locals[local].access_mut()
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}
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fn before_access_global(
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_memory_extra: &(),
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_alloc_id: AllocId,
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alloc: ConstAllocation<'tcx, Self::PointerTag, Self::AllocExtra>,
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_static_def_id: Option<DefId>,
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is_write: bool,
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) -> InterpResult<'tcx> {
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if is_write {
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throw_machine_stop_str!("can't write to global");
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}
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// If the static allocation is mutable, then we can't const prop it as its content
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// might be different at runtime.
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if alloc.inner().mutability == Mutability::Mut {
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throw_machine_stop_str!("can't access mutable globals in ConstProp");
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}
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Ok(())
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}
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#[inline(always)]
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fn init_frame_extra(
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_ecx: &mut InterpCx<'mir, 'tcx, Self>,
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frame: Frame<'mir, 'tcx>,
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) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
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Ok(frame)
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}
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#[inline(always)]
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fn stack<'a>(
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ecx: &'a InterpCx<'mir, 'tcx, Self>,
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) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
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&ecx.machine.stack
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}
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#[inline(always)]
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fn stack_mut<'a>(
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ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
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) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
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&mut ecx.machine.stack
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}
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}
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/// Finds optimization opportunities on the MIR.
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struct ConstPropagator<'mir, 'tcx> {
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ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
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tcx: TyCtxt<'tcx>,
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param_env: ParamEnv<'tcx>,
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// FIXME(eddyb) avoid cloning these two fields more than once,
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// by accessing them through `ecx` instead.
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source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
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local_decls: IndexVec<Local, LocalDecl<'tcx>>,
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// Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
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// the last known `SourceInfo` here and just keep revisiting it.
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source_info: Option<SourceInfo>,
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}
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impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
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type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
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#[inline]
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fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
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err
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}
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}
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impl HasDataLayout for ConstPropagator<'_, '_> {
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#[inline]
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fn data_layout(&self) -> &TargetDataLayout {
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&self.tcx.data_layout
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}
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}
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impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
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#[inline]
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fn tcx(&self) -> TyCtxt<'tcx> {
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self.tcx
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}
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}
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impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
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#[inline]
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fn param_env(&self) -> ty::ParamEnv<'tcx> {
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self.param_env
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}
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}
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impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
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fn new(
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body: &Body<'tcx>,
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dummy_body: &'mir Body<'tcx>,
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tcx: TyCtxt<'tcx>,
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) -> ConstPropagator<'mir, 'tcx> {
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let def_id = body.source.def_id();
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let substs = &InternalSubsts::identity_for_item(tcx, def_id);
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let param_env = tcx.param_env_reveal_all_normalized(def_id);
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let span = tcx.def_span(def_id);
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// FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
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// so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
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// `layout_of` query invocations.
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let can_const_prop = CanConstProp::check(tcx, param_env, body);
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let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
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for (l, mode) in can_const_prop.iter_enumerated() {
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if *mode == ConstPropMode::OnlyInsideOwnBlock {
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only_propagate_inside_block_locals.insert(l);
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}
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}
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let mut ecx = InterpCx::new(
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tcx,
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span,
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param_env,
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ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
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(),
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);
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let ret = ecx
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.layout_of(body.return_ty().subst(tcx, substs))
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.ok()
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// Don't bother allocating memory for ZST types which have no values
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// or for large values.
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.filter(|ret_layout| {
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!ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
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})
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.map(|ret_layout| {
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ecx.allocate(ret_layout, MemoryKind::Stack)
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.expect("couldn't perform small allocation")
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.into()
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});
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ecx.push_stack_frame(
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Instance::new(def_id, substs),
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dummy_body,
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ret.as_ref(),
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StackPopCleanup::Root { cleanup: false },
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)
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.expect("failed to push initial stack frame");
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ConstPropagator {
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ecx,
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tcx,
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param_env,
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// FIXME(eddyb) avoid cloning these two fields more than once,
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// by accessing them through `ecx` instead.
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source_scopes: body.source_scopes.clone(),
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//FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
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local_decls: body.local_decls.clone(),
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source_info: None,
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}
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}
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fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
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let op = match self.ecx.eval_place_to_op(place, None) {
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Ok(op) => op,
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Err(e) => {
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trace!("get_const failed: {}", e);
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return None;
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}
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};
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|
|
// Try to read the local as an immediate so that if it is representable as a scalar, we can
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|
// handle it as such, but otherwise, just return the value as is.
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Some(match self.ecx.try_read_immediate(&op) {
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Ok(Ok(imm)) => imm.into(),
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_ => op,
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})
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}
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|
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/// Remove `local` from the pool of `Locals`. Allows writing to them,
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/// but not reading from them anymore.
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fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
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ecx.frame_mut().locals[local] =
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LocalState { value: LocalValue::Unallocated, layout: Cell::new(None) };
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}
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fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
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source_info.scope.lint_root(&self.source_scopes)
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|
}
|
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|
|
fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
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|
where
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|
F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
|
|
{
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|
match f(self) {
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Ok(val) => Some(val),
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|
Err(error) => {
|
|
trace!("InterpCx operation failed: {:?}", error);
|
|
// Some errors shouldn't come up because creating them causes
|
|
// an allocation, which we should avoid. When that happens,
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|
// dedicated error variants should be introduced instead.
|
|
assert!(
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|
!error.kind().formatted_string(),
|
|
"const-prop encountered formatting 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.mir_const_to_op(&c.literal, None) {
|
|
Ok(op) => Some(op),
|
|
Err(error) => {
|
|
let tcx = self.ecx.tcx.at(c.span);
|
|
let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
|
|
if let Some(lint_root) = self.lint_root(source_info) {
|
|
let lint_only = match c.literal {
|
|
ConstantKind::Ty(ct) => match ct.val() {
|
|
// Promoteds must lint and not error as the user didn't ask for them
|
|
ConstKind::Unevaluated(ty::Unevaluated {
|
|
def: _,
|
|
substs: _,
|
|
promoted: 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(),
|
|
},
|
|
ConstantKind::Val(_, ty) => ty.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<impl std::fmt::Debug>,
|
|
) {
|
|
if let Some(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();
|
|
});
|
|
}
|
|
}
|
|
|
|
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()),
|
|
);
|
|
return None;
|
|
}
|
|
|
|
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 {
|
|
let r = r?;
|
|
// 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();
|
|
let r_bits = r_bits.and_then(|r| r.to_bits(right_size).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(
|
|
ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
|
|
left_ty.is_signed(),
|
|
left_ty.is_ptr_sized_integral(),
|
|
),
|
|
},
|
|
r.to_const_int(),
|
|
),
|
|
);
|
|
return None;
|
|
}
|
|
}
|
|
|
|
if let (Some(l), Some(r)) = (&l, &r) {
|
|
// 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()),
|
|
);
|
|
return None;
|
|
}
|
|
}
|
|
Some(())
|
|
}
|
|
|
|
fn const_prop(
|
|
&mut self,
|
|
rvalue: &Rvalue<'tcx>,
|
|
source_info: SourceInfo,
|
|
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 {
|
|
// 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, box (left, right)) => {
|
|
trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
|
|
self.check_binary_op(*op, left, right, source_info)?;
|
|
}
|
|
Rvalue::CheckedBinaryOp(op, box (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::ShallowInitBox(..)
|
|
| Rvalue::Discriminant(..)
|
|
| Rvalue::NullaryOp(..) => {}
|
|
}
|
|
|
|
// FIXME we need to revisit this for #67176
|
|
if rvalue.needs_subst() {
|
|
return None;
|
|
}
|
|
|
|
self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
|
|
}
|
|
}
|
|
|
|
/// 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<'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)
|
|
// 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. 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> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
|
|
fn visit_body(&mut self, body: &Body<'tcx>) {
|
|
for (bb, data) in body.basic_blocks().iter_enumerated() {
|
|
self.visit_basic_block_data(bb, data);
|
|
}
|
|
}
|
|
|
|
fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
|
|
self.super_operand(operand, location);
|
|
}
|
|
|
|
fn visit_constant(&mut self, constant: &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: &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 rval)) = statement.kind {
|
|
let can_const_prop = self.ecx.machine.can_const_prop[place.local];
|
|
if let Some(()) = self.const_prop(rval, source_info, 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);
|
|
}
|
|
} else {
|
|
match statement.kind {
|
|
StatementKind::SetDiscriminant { ref place, .. } => {
|
|
match self.ecx.machine.can_const_prop[place.local] {
|
|
ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
|
|
if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
|
|
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::Unallocated
|
|
} else {
|
|
LocalValue::Dead
|
|
};
|
|
}
|
|
_ => {}
|
|
}
|
|
}
|
|
|
|
self.super_statement(statement, location);
|
|
}
|
|
|
|
fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
|
|
let source_info = terminator.source_info;
|
|
self.source_info = Some(source_info);
|
|
self.super_terminator(terminator, location);
|
|
match &terminator.kind {
|
|
TerminatorKind::Assert { expected, ref msg, ref cond, .. } => {
|
|
if let Some(ref 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 {
|
|
enum DbgVal<T> {
|
|
Val(T),
|
|
Underscore,
|
|
}
|
|
impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
|
|
fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
|
|
match self {
|
|
Self::Val(val) => val.fmt(fmt),
|
|
Self::Underscore => fmt.write_str("_"),
|
|
}
|
|
}
|
|
}
|
|
let mut eval_to_int = |op| {
|
|
// This can be `None` if the lhs wasn't const propagated and we just
|
|
// triggered the assert on the value of the rhs.
|
|
self.eval_operand(op, source_info).map_or(DbgVal::Underscore, |op| {
|
|
DbgVal::Val(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::Overflow(bin_op @ (BinOp::Div | BinOp::Rem), op1, op2) => {
|
|
// Division overflow is *UB* in the MIR, and different than the
|
|
// other overflow checks.
|
|
Some(AssertKind::Overflow(
|
|
*bin_op,
|
|
eval_to_int(op1),
|
|
eval_to_int(op2),
|
|
))
|
|
}
|
|
AssertKind::BoundsCheck { ref len, ref index } => {
|
|
let len = eval_to_int(len);
|
|
let index = eval_to_int(index);
|
|
Some(AssertKind::BoundsCheck { len, index })
|
|
}
|
|
// Remaining overflow errors 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,
|
|
);
|
|
}
|
|
}
|
|
}
|
|
}
|
|
// 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::SwitchInt { .. }
|
|
| TerminatorKind::Call { .. }
|
|
| TerminatorKind::InlineAsm { .. } => {}
|
|
}
|
|
|
|
// 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())
|
|
)
|
|
}
|
|
}
|
|
}
|
|
}
|