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