1 //! Propagates constants for early reporting of statically known
6 use rustc_ast::Mutability;
7 use rustc_data_structures::fx::FxHashSet;
8 use rustc_hir::def::DefKind;
9 use rustc_index::bit_set::BitSet;
10 use rustc_index::vec::IndexVec;
11 use rustc_middle::mir::visit::{
12 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
14 use rustc_middle::mir::{
15 BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind, Location,
16 Operand, Place, Rvalue, SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp,
19 use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
20 use rustc_middle::ty::InternalSubsts;
21 use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeVisitable};
22 use rustc_span::{def_id::DefId, Span};
23 use rustc_target::abi::{self, HasDataLayout, Size, TargetDataLayout};
24 use rustc_target::spec::abi::Abi as CallAbi;
25 use rustc_trait_selection::traits;
28 use rustc_const_eval::interpret::{
29 self, compile_time_machine, AllocId, ConstAllocation, ConstValue, CtfeValidationMode, Frame,
30 ImmTy, Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, PlaceTy,
31 Pointer, Scalar, StackPopCleanup, StackPopUnwind,
34 /// The maximum number of bytes that we'll allocate space for a local or the return value.
35 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
36 /// Severely regress performance.
37 const MAX_ALLOC_LIMIT: u64 = 1024;
39 /// Macro for machine-specific `InterpError` without allocation.
40 /// (These will never be shown to the user, but they help diagnose ICEs.)
41 macro_rules! throw_machine_stop_str {
43 // We make a new local type for it. The type itself does not carry any information,
44 // but its vtable (for the `MachineStopType` trait) does.
46 // Printing this type shows the desired string.
47 impl std::fmt::Display for Zst {
48 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
52 impl rustc_middle::mir::interpret::MachineStopType for Zst {}
53 throw_machine_stop!(Zst)
59 impl<'tcx> MirPass<'tcx> for ConstProp {
60 fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
61 sess.mir_opt_level() >= 1
64 #[instrument(skip(self, tcx), level = "debug")]
65 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
66 // will be evaluated by miri and produce its errors there
67 if body.source.promoted.is_some() {
71 let def_id = body.source.def_id().expect_local();
72 let def_kind = tcx.def_kind(def_id);
73 let is_fn_like = def_kind.is_fn_like();
74 let is_assoc_const = def_kind == DefKind::AssocConst;
76 // Only run const prop on functions, methods, closures and associated constants
77 if !is_fn_like && !is_assoc_const {
78 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
79 trace!("ConstProp skipped for {:?}", def_id);
83 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
84 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
85 // computing their layout.
87 trace!("ConstProp skipped for generator {:?}", def_id);
91 // Check if it's even possible to satisfy the 'where' clauses
93 // This branch will never be taken for any normal function.
94 // However, it's possible to `#!feature(trivial_bounds)]` to write
95 // a function with impossible to satisfy clauses, e.g.:
96 // `fn foo() where String: Copy {}`
98 // We don't usually need to worry about this kind of case,
99 // since we would get a compilation error if the user tried
100 // to call it. However, since we can do const propagation
101 // even without any calls to the function, we need to make
102 // sure that it even makes sense to try to evaluate the body.
103 // If there are unsatisfiable where clauses, then all bets are
104 // off, and we just give up.
106 // We manually filter the predicates, skipping anything that's not
107 // "global". We are in a potentially generic context
108 // (e.g. we are evaluating a function without substituting generic
109 // parameters, so this filtering serves two purposes:
111 // 1. We skip evaluating any predicates that we would
112 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
113 // 2. We avoid trying to normalize predicates involving generic
114 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
115 // the normalization code (leading to cycle errors), since
116 // it's usually never invoked in this way.
118 .predicates_of(def_id.to_def_id())
121 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
122 if traits::impossible_predicates(
124 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
126 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
130 trace!("ConstProp starting for {:?}", def_id);
132 let dummy_body = &Body::new(
134 (*body.basic_blocks).clone(),
135 body.source_scopes.clone(),
136 body.local_decls.clone(),
141 body.generator_kind(),
142 body.tainted_by_errors,
145 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
146 // constants, instead of just checking for const-folding succeeding.
147 // That would require a uniform one-def no-mutation analysis
148 // and RPO (or recursing when needing the value of a local).
149 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
150 optimization_finder.visit_body(body);
152 trace!("ConstProp done for {:?}", def_id);
156 pub struct ConstPropMachine<'mir, 'tcx> {
157 /// The virtual call stack.
158 stack: Vec<Frame<'mir, 'tcx>>,
159 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
160 pub written_only_inside_own_block_locals: FxHashSet<Local>,
161 /// Locals that need to be cleared after every block terminates.
162 pub only_propagate_inside_block_locals: BitSet<Local>,
163 pub can_const_prop: IndexVec<Local, ConstPropMode>,
166 impl ConstPropMachine<'_, '_> {
168 only_propagate_inside_block_locals: BitSet<Local>,
169 can_const_prop: IndexVec<Local, ConstPropMode>,
173 written_only_inside_own_block_locals: Default::default(),
174 only_propagate_inside_block_locals,
180 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
181 compile_time_machine!(<'mir, 'tcx>);
182 const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
187 fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
188 // We do not check for alignment to avoid having to carry an `Align`
189 // in `ConstValue::ByRef`.
194 fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
195 false // for now, we don't enforce validity
199 _ecx: &InterpCx<'mir, 'tcx, Self>,
200 _instance: ty::InstanceDef<'tcx>,
201 ) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
202 throw_machine_stop_str!("calling functions isn't supported in ConstProp")
205 fn find_mir_or_eval_fn(
206 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
207 _instance: ty::Instance<'tcx>,
209 _args: &[OpTy<'tcx>],
210 _destination: &PlaceTy<'tcx>,
211 _target: Option<BasicBlock>,
212 _unwind: StackPopUnwind,
213 ) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
218 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
219 _instance: ty::Instance<'tcx>,
220 _args: &[OpTy<'tcx>],
221 _destination: &PlaceTy<'tcx>,
222 _target: Option<BasicBlock>,
223 _unwind: StackPopUnwind,
224 ) -> InterpResult<'tcx> {
225 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
229 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
230 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
231 _unwind: Option<rustc_middle::mir::BasicBlock>,
232 ) -> InterpResult<'tcx> {
233 bug!("panics terminators are not evaluated in ConstProp")
237 _ecx: &InterpCx<'mir, 'tcx, Self>,
240 _right: &ImmTy<'tcx>,
241 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
242 // We can't do this because aliasing of memory can differ between const eval and llvm
243 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
246 fn access_local_mut<'a>(
247 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
250 ) -> InterpResult<'tcx, &'a mut interpret::Operand<Self::Provenance>> {
251 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
252 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
254 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
256 "mutating local {:?} which is restricted to its block. \
257 Will remove it from const-prop after block is finished.",
260 ecx.machine.written_only_inside_own_block_locals.insert(local);
262 ecx.machine.stack[frame].locals[local].access_mut()
265 fn before_access_global(
269 alloc: ConstAllocation<'tcx, Self::Provenance, Self::AllocExtra>,
270 _static_def_id: Option<DefId>,
272 ) -> InterpResult<'tcx> {
274 throw_machine_stop_str!("can't write to global");
276 // If the static allocation is mutable, then we can't const prop it as its content
277 // might be different at runtime.
278 if alloc.inner().mutability == Mutability::Mut {
279 throw_machine_stop_str!("can't access mutable globals in ConstProp");
287 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
288 _ptr: Pointer<AllocId>,
289 ) -> InterpResult<'tcx> {
290 throw_machine_stop_str!("exposing pointers isn't supported in ConstProp")
295 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
296 frame: Frame<'mir, 'tcx>,
297 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
303 ecx: &'a InterpCx<'mir, 'tcx, Self>,
304 ) -> &'a [Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>] {
310 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
311 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>> {
312 &mut ecx.machine.stack
316 /// Finds optimization opportunities on the MIR.
317 struct ConstPropagator<'mir, 'tcx> {
318 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
320 param_env: ParamEnv<'tcx>,
321 local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
322 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
323 // the last known `SourceInfo` here and just keep revisiting it.
324 source_info: Option<SourceInfo>,
327 impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
328 type LayoutOfResult = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
331 fn handle_layout_err(&self, err: LayoutError<'tcx>, _: Span, _: Ty<'tcx>) -> LayoutError<'tcx> {
336 impl HasDataLayout for ConstPropagator<'_, '_> {
338 fn data_layout(&self) -> &TargetDataLayout {
339 &self.tcx.data_layout
343 impl<'tcx> ty::layout::HasTyCtxt<'tcx> for ConstPropagator<'_, 'tcx> {
345 fn tcx(&self) -> TyCtxt<'tcx> {
350 impl<'tcx> ty::layout::HasParamEnv<'tcx> for ConstPropagator<'_, 'tcx> {
352 fn param_env(&self) -> ty::ParamEnv<'tcx> {
357 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
360 dummy_body: &'mir Body<'tcx>,
362 ) -> ConstPropagator<'mir, 'tcx> {
363 let def_id = body.source.def_id();
364 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
365 let param_env = tcx.param_env_reveal_all_normalized(def_id);
367 let can_const_prop = CanConstProp::check(tcx, param_env, body);
368 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
369 for (l, mode) in can_const_prop.iter_enumerated() {
370 if *mode == ConstPropMode::OnlyInsideOwnBlock {
371 only_propagate_inside_block_locals.insert(l);
374 let mut ecx = InterpCx::new(
376 tcx.def_span(def_id),
378 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
382 .layout_of(body.bound_return_ty().subst(tcx, substs))
384 // Don't bother allocating memory for large values.
385 // I don't know how return types can seem to be unsized but this happens in the
386 // `type/type-unsatisfiable.rs` test.
387 .filter(|ret_layout| {
388 ret_layout.is_sized() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
390 .unwrap_or_else(|| ecx.layout_of(tcx.types.unit).unwrap());
393 .allocate(ret_layout, MemoryKind::Stack)
394 .expect("couldn't perform small allocation")
397 ecx.push_stack_frame(
398 Instance::new(def_id, substs),
401 StackPopCleanup::Root { cleanup: false },
403 .expect("failed to push initial stack frame");
409 local_decls: &dummy_body.local_decls,
414 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
415 let op = match self.ecx.eval_place_to_op(place, None) {
417 if matches!(*op, interpret::Operand::Immediate(Immediate::Uninit)) {
418 // Make sure nobody accidentally uses this value.
424 trace!("get_const failed: {}", e);
429 // Try to read the local as an immediate so that if it is representable as a scalar, we can
430 // handle it as such, but otherwise, just return the value as is.
431 Some(match self.ecx.read_immediate_raw(&op) {
432 Ok(Ok(imm)) => imm.into(),
437 /// Remove `local` from the pool of `Locals`. Allows writing to them,
438 /// but not reading from them anymore.
439 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
440 ecx.frame_mut().locals[local] = LocalState {
441 value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
442 layout: Cell::new(None),
446 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
448 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
451 Ok(val) => Some(val),
453 trace!("InterpCx operation failed: {:?}", error);
454 // Some errors shouldn't come up because creating them causes
455 // an allocation, which we should avoid. When that happens,
456 // dedicated error variants should be introduced instead.
458 !error.kind().formatted_string(),
459 "const-prop encountered formatting error: {}",
467 /// Returns the value, if any, of evaluating `c`.
468 fn eval_constant(&mut self, c: &Constant<'tcx>) -> Option<OpTy<'tcx>> {
469 // FIXME we need to revisit this for #67176
474 self.ecx.const_to_op(&c.literal, None).ok()
477 /// Returns the value, if any, of evaluating `place`.
478 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
479 trace!("eval_place(place={:?})", place);
480 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
483 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
484 /// or `eval_place`, depending on the variant of `Operand` used.
485 fn eval_operand(&mut self, op: &Operand<'tcx>) -> Option<OpTy<'tcx>> {
487 Operand::Constant(ref c) => self.eval_constant(c),
488 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
492 fn check_unary_op(&mut self, op: UnOp, arg: &Operand<'tcx>) -> Option<()> {
493 if self.use_ecx(|this| {
494 let val = this.ecx.read_immediate(&this.ecx.eval_operand(arg, None)?)?;
495 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, &val)?;
498 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
499 // appropriate to use.
500 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
510 left: &Operand<'tcx>,
511 right: &Operand<'tcx>,
513 let r = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(right, None)?));
514 let l = self.use_ecx(|this| this.ecx.read_immediate(&this.ecx.eval_operand(left, None)?));
515 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
516 if op == BinOp::Shr || op == BinOp::Shl {
518 // We need the type of the LHS. We cannot use `place_layout` as that is the type
519 // of the result, which for checked binops is not the same!
520 let left_ty = left.ty(self.local_decls, self.tcx);
521 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
522 let right_size = r.layout.size;
523 let r_bits = r.to_scalar().to_bits(right_size).ok();
524 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
529 if let (Some(l), Some(r)) = (&l, &r) {
530 // The remaining operators are handled through `overflowing_binary_op`.
531 if self.use_ecx(|this| {
532 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
541 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
543 Operand::Copy(l) | Operand::Move(l) => {
544 if let Some(value) = self.get_const(l) && self.should_const_prop(&value) {
545 // FIXME(felix91gr): this code only handles `Scalar` cases.
546 // For now, we're not handling `ScalarPair` cases because
547 // doing so here would require a lot of code duplication.
548 // We should hopefully generalize `Operand` handling into a fn,
549 // and use it to do const-prop here and everywhere else
550 // where it makes sense.
551 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
555 *operand = self.operand_from_scalar(
558 self.source_info.unwrap().span,
563 Operand::Constant(_) => (),
567 fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> {
568 // Perform any special handling for specific Rvalue types.
569 // Generally, checks here fall into one of two categories:
570 // 1. Additional checking to provide useful lints to the user
571 // - In this case, we will do some validation and then fall through to the
572 // end of the function which evals the assignment.
573 // 2. Working around bugs in other parts of the compiler
574 // - In this case, we'll return `None` from this function to stop evaluation.
576 // Additional checking: give lints to the user if an overflow would occur.
577 // We do this here and not in the `Assert` terminator as that terminator is
578 // only sometimes emitted (overflow checks can be disabled), but we want to always
580 Rvalue::UnaryOp(op, arg) => {
581 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
582 self.check_unary_op(*op, arg)?;
584 Rvalue::BinaryOp(op, box (left, right)) => {
585 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
586 self.check_binary_op(*op, left, right)?;
588 Rvalue::CheckedBinaryOp(op, box (left, right)) => {
590 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
595 self.check_binary_op(*op, left, right)?;
598 // Do not try creating references (#67862)
599 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
600 trace!("skipping AddressOf | Ref for {:?}", place);
602 // This may be creating mutable references or immutable references to cells.
603 // If that happens, the pointed to value could be mutated via that reference.
604 // Since we aren't tracking references, the const propagator loses track of what
605 // value the local has right now.
606 // Thus, all locals that have their reference taken
607 // must not take part in propagation.
608 Self::remove_const(&mut self.ecx, place.local);
612 Rvalue::ThreadLocalRef(def_id) => {
613 trace!("skipping ThreadLocalRef({:?})", def_id);
618 // There's no other checking to do at this time.
619 Rvalue::Aggregate(..)
621 | Rvalue::CopyForDeref(..)
625 | Rvalue::ShallowInitBox(..)
626 | Rvalue::Discriminant(..)
627 | Rvalue::NullaryOp(..) => {}
630 // FIXME we need to revisit this for #67176
631 if rvalue.needs_subst() {
635 .ty(&self.ecx.frame().body.local_decls, *self.ecx.tcx)
636 .is_sized(*self.ecx.tcx, self.param_env)
638 // the interpreter doesn't support unsized locals (only unsized arguments),
639 // but rustc does (in a kinda broken way), so we have to skip them here
643 if self.tcx.sess.mir_opt_level() >= 4 {
644 self.eval_rvalue_with_identities(rvalue, place)
646 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
650 // Attempt to use algebraic identities to eliminate constant expressions
651 fn eval_rvalue_with_identities(
653 rvalue: &Rvalue<'tcx>,
656 self.use_ecx(|this| match rvalue {
657 Rvalue::BinaryOp(op, box (left, right))
658 | Rvalue::CheckedBinaryOp(op, box (left, right)) => {
659 let l = this.ecx.eval_operand(left, None).and_then(|x| this.ecx.read_immediate(&x));
661 this.ecx.eval_operand(right, None).and_then(|x| this.ecx.read_immediate(&x));
663 let const_arg = match (l, r) {
664 (Ok(x), Err(_)) | (Err(_), Ok(x)) => x, // exactly one side is known
665 (Err(e), Err(_)) => return Err(e), // neither side is known
666 (Ok(_), Ok(_)) => return this.ecx.eval_rvalue_into_place(rvalue, place), // both sides are known
669 if !matches!(const_arg.layout.abi, abi::Abi::Scalar(..)) {
670 // We cannot handle Scalar Pair stuff.
671 // No point in calling `eval_rvalue_into_place`, since only one side is known
672 throw_machine_stop_str!("cannot optimize this")
675 let arg_value = const_arg.to_scalar().to_bits(const_arg.layout.size)?;
676 let dest = this.ecx.eval_place(place)?;
679 BinOp::BitAnd if arg_value == 0 => this.ecx.write_immediate(*const_arg, &dest),
681 if arg_value == const_arg.layout.size.truncate(u128::MAX)
682 || (const_arg.layout.ty.is_bool() && arg_value == 1) =>
684 this.ecx.write_immediate(*const_arg, &dest)
686 BinOp::Mul if const_arg.layout.ty.is_integral() && arg_value == 0 => {
687 if let Rvalue::CheckedBinaryOp(_, _) = rvalue {
688 let val = Immediate::ScalarPair(
689 const_arg.to_scalar().into(),
690 Scalar::from_bool(false).into(),
692 this.ecx.write_immediate(val, &dest)
694 this.ecx.write_immediate(*const_arg, &dest)
697 _ => throw_machine_stop_str!("cannot optimize this"),
700 _ => this.ecx.eval_rvalue_into_place(rvalue, place),
704 /// Creates a new `Operand::Constant` from a `Scalar` value
705 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
706 Operand::Constant(Box::new(Constant {
709 literal: ConstantKind::from_scalar(self.tcx, scalar, ty),
713 fn replace_with_const(
715 rval: &mut Rvalue<'tcx>,
717 source_info: SourceInfo,
719 if let Rvalue::Use(Operand::Constant(c)) = rval {
721 ConstantKind::Ty(c) if matches!(c.kind(), ConstKind::Unevaluated(..)) => {}
723 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
729 trace!("attempting to replace {:?} with {:?}", rval, value);
730 if let Err(e) = self.ecx.const_validate_operand(
733 // FIXME: is ref tracking too expensive?
734 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
735 &mut interpret::RefTracking::empty(),
736 CtfeValidationMode::Regular,
738 trace!("validation error, attempt failed: {:?}", e);
742 // FIXME> figure out what to do when read_immediate_raw fails
743 let imm = self.use_ecx(|this| this.ecx.read_immediate_raw(value));
745 if let Some(Ok(imm)) = imm {
747 interpret::Immediate::Scalar(scalar) => {
748 *rval = Rvalue::Use(self.operand_from_scalar(
754 Immediate::ScalarPair(..) => {
755 // Found a value represented as a pair. For now only do const-prop if the type
756 // of `rvalue` is also a tuple with two scalars.
757 // FIXME: enable the general case stated above ^.
758 let ty = value.layout.ty;
759 // Only do it for tuples
760 if let ty::Tuple(types) = ty.kind() {
761 // Only do it if tuple is also a pair with two scalars
762 if let [ty1, ty2] = types[..] {
763 let alloc = self.use_ecx(|this| {
764 let ty_is_scalar = |ty| {
765 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
768 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
771 .intern_with_temp_alloc(value.layout, |ecx, dest| {
772 ecx.write_immediate(*imm, dest)
781 if let Some(Some(alloc)) = alloc {
782 // Assign entire constant in a single statement.
783 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
784 let const_val = ConstValue::ByRef { alloc, offset: Size::ZERO };
785 let literal = ConstantKind::Val(const_val, ty);
786 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
787 span: source_info.span,
795 // Scalars or scalar pairs that contain undef values are assumed to not have
796 // successfully evaluated and are thus not propagated.
802 /// Returns `true` if and only if this `op` should be const-propagated into.
803 fn should_const_prop(&mut self, op: &OpTy<'tcx>) -> bool {
804 if !self.tcx.consider_optimizing(|| format!("ConstantPropagation - OpTy: {:?}", op)) {
809 interpret::Operand::Immediate(Immediate::Scalar(s)) => s.try_to_int().is_ok(),
810 interpret::Operand::Immediate(Immediate::ScalarPair(l, r)) => {
811 l.try_to_int().is_ok() && r.try_to_int().is_ok()
818 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
819 #[derive(Clone, Copy, Debug, PartialEq)]
820 pub enum ConstPropMode {
821 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
823 /// The `Local` can only be propagated into and from its own block.
825 /// The `Local` can be propagated into but reads cannot be propagated.
827 /// The `Local` cannot be part of propagation at all. Any statement
828 /// referencing it either for reading or writing will not get propagated.
832 pub struct CanConstProp {
833 can_const_prop: IndexVec<Local, ConstPropMode>,
834 // False at the beginning. Once set, no more assignments are allowed to that local.
835 found_assignment: BitSet<Local>,
836 // Cache of locals' information
837 local_kinds: IndexVec<Local, LocalKind>,
841 /// Returns true if `local` can be propagated
844 param_env: ParamEnv<'tcx>,
846 ) -> IndexVec<Local, ConstPropMode> {
847 let mut cpv = CanConstProp {
848 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
849 found_assignment: BitSet::new_empty(body.local_decls.len()),
850 local_kinds: IndexVec::from_fn_n(
851 |local| body.local_kind(local),
852 body.local_decls.len(),
855 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
856 let ty = body.local_decls[local].ty;
857 match tcx.layout_of(param_env.and(ty)) {
858 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
859 // Either the layout fails to compute, then we can't use this local anyway
860 // or the local is too large, then we don't want to.
862 *val = ConstPropMode::NoPropagation;
866 // Cannot use args at all
867 // Cannot use locals because if x < y { y - x } else { x - y } would
869 // FIXME(oli-obk): lint variables until they are used in a condition
870 // FIXME(oli-obk): lint if return value is constant
871 if cpv.local_kinds[local] == LocalKind::Arg {
872 *val = ConstPropMode::OnlyPropagateInto;
874 "local {:?} can't be const propagated because it's a function argument",
877 } else if cpv.local_kinds[local] == LocalKind::Var {
878 *val = ConstPropMode::OnlyInsideOwnBlock;
880 "local {:?} will only be propagated inside its block, because it's a user variable",
885 cpv.visit_body(&body);
890 impl Visitor<'_> for CanConstProp {
891 fn visit_local(&mut self, local: Local, context: PlaceContext, _: Location) {
892 use rustc_middle::mir::visit::PlaceContext::*;
894 // Projections are fine, because `&mut foo.x` will be caught by
895 // `MutatingUseContext::Borrow` elsewhere.
896 MutatingUse(MutatingUseContext::Projection)
897 // These are just stores, where the storing is not propagatable, but there may be later
898 // mutations of the same local via `Store`
899 | MutatingUse(MutatingUseContext::Call)
900 | MutatingUse(MutatingUseContext::AsmOutput)
901 | MutatingUse(MutatingUseContext::Deinit)
902 // Actual store that can possibly even propagate a value
903 | MutatingUse(MutatingUseContext::Store)
904 | MutatingUse(MutatingUseContext::SetDiscriminant) => {
905 if !self.found_assignment.insert(local) {
906 match &mut self.can_const_prop[local] {
907 // If the local can only get propagated in its own block, then we don't have
908 // to worry about multiple assignments, as we'll nuke the const state at the
909 // end of the block anyway, and inside the block we overwrite previous
910 // states as applicable.
911 ConstPropMode::OnlyInsideOwnBlock => {}
912 ConstPropMode::NoPropagation => {}
913 ConstPropMode::OnlyPropagateInto => {}
914 other @ ConstPropMode::FullConstProp => {
916 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
919 *other = ConstPropMode::OnlyInsideOwnBlock;
924 // Reading constants is allowed an arbitrary number of times
925 NonMutatingUse(NonMutatingUseContext::Copy)
926 | NonMutatingUse(NonMutatingUseContext::Move)
927 | NonMutatingUse(NonMutatingUseContext::Inspect)
928 | NonMutatingUse(NonMutatingUseContext::Projection)
931 // These could be propagated with a smarter analysis or just some careful thinking about
932 // whether they'd be fine right now.
933 MutatingUse(MutatingUseContext::Yield)
934 | MutatingUse(MutatingUseContext::Drop)
935 | MutatingUse(MutatingUseContext::Retag)
936 // These can't ever be propagated under any scheme, as we can't reason about indirect
938 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
939 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
940 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
941 | NonMutatingUse(NonMutatingUseContext::AddressOf)
942 | MutatingUse(MutatingUseContext::Borrow)
943 | MutatingUse(MutatingUseContext::AddressOf) => {
944 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
945 self.can_const_prop[local] = ConstPropMode::NoPropagation;
951 impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
952 fn tcx(&self) -> TyCtxt<'tcx> {
956 fn visit_body(&mut self, body: &mut Body<'tcx>) {
957 for (bb, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() {
958 self.visit_basic_block_data(bb, data);
962 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
963 self.super_operand(operand, location);
965 // Only const prop copies and moves on `mir_opt_level=3` as doing so
966 // currently slightly increases compile time in some cases.
967 if self.tcx.sess.mir_opt_level() >= 3 {
968 self.propagate_operand(operand)
972 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
973 trace!("visit_constant: {:?}", constant);
974 self.super_constant(constant, location);
975 self.eval_constant(constant);
978 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
979 trace!("visit_statement: {:?}", statement);
980 let source_info = statement.source_info;
981 self.source_info = Some(source_info);
982 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
983 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
984 if let Some(()) = self.const_prop(rval, place) {
985 // This will return None if the above `const_prop` invocation only "wrote" a
986 // type whose creation requires no write. E.g. a generator whose initial state
987 // consists solely of uninitialized memory (so it doesn't capture any locals).
988 if let Some(ref value) = self.get_const(place) && self.should_const_prop(value) {
989 trace!("replacing {:?} with {:?}", rval, value);
990 self.replace_with_const(rval, value, source_info);
991 if can_const_prop == ConstPropMode::FullConstProp
992 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
994 trace!("propagated into {:?}", place);
997 match can_const_prop {
998 ConstPropMode::OnlyInsideOwnBlock => {
1000 "found local restricted to its block. \
1001 Will remove it from const-prop after block is finished. Local: {:?}",
1005 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1006 trace!("can't propagate into {:?}", place);
1007 if place.local != RETURN_PLACE {
1008 Self::remove_const(&mut self.ecx, place.local);
1011 ConstPropMode::FullConstProp => {}
1014 // Const prop failed, so erase the destination, ensuring that whatever happens
1015 // from here on, does not know about the previous value.
1016 // This is important in case we have
1019 // x = SOME_MUTABLE_STATIC;
1020 // // x must now be uninit
1022 // FIXME: we overzealously erase the entire local, because that's easier to
1025 "propagation into {:?} failed.
1026 Nuking the entire site from orbit, it's the only way to be sure",
1029 Self::remove_const(&mut self.ecx, place.local);
1032 match statement.kind {
1033 StatementKind::SetDiscriminant { ref place, .. } => {
1034 match self.ecx.machine.can_const_prop[place.local] {
1035 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1036 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1037 trace!("propped discriminant into {:?}", place);
1039 Self::remove_const(&mut self.ecx, place.local);
1042 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1043 Self::remove_const(&mut self.ecx, place.local);
1047 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1048 let frame = self.ecx.frame_mut();
1049 frame.locals[local].value =
1050 if let StatementKind::StorageLive(_) = statement.kind {
1051 LocalValue::Live(interpret::Operand::Immediate(
1052 interpret::Immediate::Uninit,
1062 self.super_statement(statement, location);
1065 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1066 let source_info = terminator.source_info;
1067 self.source_info = Some(source_info);
1068 self.super_terminator(terminator, location);
1069 // Do NOT early return in this function, it does some crucial fixup of the state at the end!
1070 match &mut terminator.kind {
1071 TerminatorKind::Assert { expected, ref mut cond, .. } => {
1072 if let Some(ref value) = self.eval_operand(&cond) {
1073 trace!("assertion on {:?} should be {:?}", value, expected);
1074 let expected = Scalar::from_bool(*expected);
1075 // FIXME should be used use_ecx rather than a local match... but we have
1076 // quite a few of these read_scalar/read_immediate that need fixing.
1077 if let Ok(value_const) = self.ecx.read_scalar(&value) {
1078 if expected != value_const {
1079 // Poison all places this operand references so that further code
1080 // doesn't use the invalid value
1082 Operand::Move(ref place) | Operand::Copy(ref place) => {
1083 Self::remove_const(&mut self.ecx, place.local);
1085 Operand::Constant(_) => {}
1088 if self.should_const_prop(value) {
1089 *cond = self.operand_from_scalar(
1091 self.tcx.types.bool,
1099 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1100 // FIXME: This is currently redundant with `visit_operand`, but sadly
1101 // always visiting operands currently causes a perf regression in LLVM codegen, so
1102 // `visit_operand` currently only runs for propagates places for `mir_opt_level=4`.
1103 self.propagate_operand(discr)
1105 // None of these have Operands to const-propagate.
1106 TerminatorKind::Goto { .. }
1107 | TerminatorKind::Resume
1108 | TerminatorKind::Abort
1109 | TerminatorKind::Return
1110 | TerminatorKind::Unreachable
1111 | TerminatorKind::Drop { .. }
1112 | TerminatorKind::DropAndReplace { .. }
1113 | TerminatorKind::Yield { .. }
1114 | TerminatorKind::GeneratorDrop
1115 | TerminatorKind::FalseEdge { .. }
1116 | TerminatorKind::FalseUnwind { .. }
1117 | TerminatorKind::InlineAsm { .. } => {}
1118 // Every argument in our function calls have already been propagated in `visit_operand`.
1120 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1121 // gated on `mir_opt_level=3`.
1122 TerminatorKind::Call { .. } => {}
1125 // We remove all Locals which are restricted in propagation to their containing blocks and
1126 // which were modified in the current block.
1127 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1128 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1129 for &local in locals.iter() {
1130 Self::remove_const(&mut self.ecx, local);
1133 // Put it back so we reuse the heap of the storage
1134 self.ecx.machine.written_only_inside_own_block_locals = locals;
1135 if cfg!(debug_assertions) {
1136 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1137 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1139 self.get_const(local.into()).is_none()
1141 .layout_of(self.local_decls[local].ty)
1142 .map_or(true, |layout| layout.is_zst())