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;
10 use rustc_index::bit_set::BitSet;
11 use rustc_index::vec::IndexVec;
12 use rustc_middle::mir::visit::{
13 MutVisitor, MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor,
15 use rustc_middle::mir::{
16 AssertKind, BasicBlock, BinOp, Body, ClearCrossCrate, Constant, Local, LocalDecl, LocalKind,
17 Location, Operand, Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement,
18 StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
20 use rustc_middle::ty::layout::{HasTyCtxt, LayoutError, TyAndLayout};
21 use rustc_middle::ty::subst::{InternalSubsts, Subst};
22 use rustc_middle::ty::{
23 self, ConstInt, ConstKind, Instance, ParamEnv, ScalarInt, Ty, TyCtxt, TypeFoldable,
25 use rustc_session::lint;
26 use rustc_span::{def_id::DefId, Span};
27 use rustc_target::abi::{HasDataLayout, LayoutOf, Size, TargetDataLayout};
28 use rustc_trait_selection::traits;
30 use crate::const_eval::ConstEvalErr;
31 use crate::interpret::{
32 self, compile_time_machine, AllocId, Allocation, ConstValue, CtfeValidationMode, Frame, ImmTy,
33 Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemPlace, Memory, MemoryKind, OpTy,
34 Operand as InterpOperand, PlaceTy, Pointer, Scalar, ScalarMaybeUninit, StackPopCleanup,
36 use crate::transform::MirPass;
38 /// The maximum number of bytes that we'll allocate space for a local or the return value.
39 /// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
40 /// Severely regress performance.
41 const MAX_ALLOC_LIMIT: u64 = 1024;
43 /// Macro for machine-specific `InterpError` without allocation.
44 /// (These will never be shown to the user, but they help diagnose ICEs.)
45 macro_rules! throw_machine_stop_str {
47 // We make a new local type for it. The type itself does not carry any information,
48 // but its vtable (for the `MachineStopType` trait) does.
50 // Printing this type shows the desired string.
51 impl std::fmt::Display for Zst {
52 fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
56 impl rustc_middle::mir::interpret::MachineStopType for Zst {}
57 throw_machine_stop!(Zst)
63 impl<'tcx> MirPass<'tcx> for ConstProp {
64 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
65 // will be evaluated by miri and produce its errors there
66 if body.source.promoted.is_some() {
70 use rustc_middle::hir::map::blocks::FnLikeNode;
71 let def_id = body.source.def_id().expect_local();
72 let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
74 let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
75 let is_assoc_const = tcx.def_kind(def_id.to_def_id()) == DefKind::AssocConst;
77 // Only run const prop on functions, methods, closures and associated constants
78 if !is_fn_like && !is_assoc_const {
79 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
80 trace!("ConstProp skipped for {:?}", def_id);
84 let is_generator = tcx.type_of(def_id.to_def_id()).is_generator();
85 // FIXME(welseywiser) const prop doesn't work on generators because of query cycles
86 // computing their layout.
88 trace!("ConstProp skipped for generator {:?}", def_id);
92 if !tcx.consider_optimizing(|| format!("ConstantPropagation {:?} {:?}", def_id, hir_id)) {
96 // Check if it's even possible to satisfy the 'where' clauses
98 // This branch will never be taken for any normal function.
99 // However, it's possible to `#!feature(trivial_bounds)]` to write
100 // a function with impossible to satisfy clauses, e.g.:
101 // `fn foo() where String: Copy {}`
103 // We don't usually need to worry about this kind of case,
104 // since we would get a compilation error if the user tried
105 // to call it. However, since we can do const propagation
106 // even without any calls to the function, we need to make
107 // sure that it even makes sense to try to evaluate the body.
108 // If there are unsatisfiable where clauses, then all bets are
109 // off, and we just give up.
111 // We manually filter the predicates, skipping anything that's not
112 // "global". We are in a potentially generic context
113 // (e.g. we are evaluating a function without substituting generic
114 // parameters, so this filtering serves two purposes:
116 // 1. We skip evaluating any predicates that we would
117 // never be able prove are unsatisfiable (e.g. `<T as Foo>`
118 // 2. We avoid trying to normalize predicates involving generic
119 // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
120 // the normalization code (leading to cycle errors), since
121 // it's usually never invoked in this way.
123 .predicates_of(def_id.to_def_id())
126 .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
127 if traits::impossible_predicates(
129 traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
131 trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
135 trace!("ConstProp starting for {:?}", def_id);
137 let dummy_body = &Body::new(
139 body.basic_blocks().clone(),
140 body.source_scopes.clone(),
141 body.local_decls.clone(),
145 tcx.def_span(def_id),
149 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
150 // constants, instead of just checking for const-folding succeeding.
151 // That would require an uniform one-def no-mutation analysis
152 // and RPO (or recursing when needing the value of a local).
153 let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
154 optimization_finder.visit_body(body);
156 trace!("ConstProp done for {:?}", def_id);
160 struct ConstPropMachine<'mir, 'tcx> {
161 /// The virtual call stack.
162 stack: Vec<Frame<'mir, 'tcx, (), ()>>,
163 /// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
164 written_only_inside_own_block_locals: FxHashSet<Local>,
165 /// Locals that need to be cleared after every block terminates.
166 only_propagate_inside_block_locals: BitSet<Local>,
167 can_const_prop: IndexVec<Local, ConstPropMode>,
170 impl<'mir, 'tcx> ConstPropMachine<'mir, 'tcx> {
172 only_propagate_inside_block_locals: BitSet<Local>,
173 can_const_prop: IndexVec<Local, ConstPropMode>,
177 written_only_inside_own_block_locals: Default::default(),
178 only_propagate_inside_block_locals,
184 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
185 compile_time_machine!(<'mir, 'tcx>);
187 type MemoryExtra = ();
189 fn find_mir_or_eval_fn(
190 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
191 _instance: ty::Instance<'tcx>,
192 _args: &[OpTy<'tcx>],
193 _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
194 _unwind: Option<BasicBlock>,
195 ) -> InterpResult<'tcx, Option<&'mir Body<'tcx>>> {
200 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
201 _instance: ty::Instance<'tcx>,
202 _args: &[OpTy<'tcx>],
203 _ret: Option<(PlaceTy<'tcx>, BasicBlock)>,
204 _unwind: Option<BasicBlock>,
205 ) -> InterpResult<'tcx> {
206 throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
210 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
211 _msg: &rustc_middle::mir::AssertMessage<'tcx>,
212 _unwind: Option<rustc_middle::mir::BasicBlock>,
213 ) -> InterpResult<'tcx> {
214 bug!("panics terminators are not evaluated in ConstProp")
217 fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
218 throw_unsup!(ReadPointerAsBytes)
222 _ecx: &InterpCx<'mir, 'tcx, Self>,
226 ) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
227 // We can't do this because aliasing of memory can differ between const eval and llvm
228 throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
232 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
233 _dest: PlaceTy<'tcx>,
234 ) -> InterpResult<'tcx> {
235 throw_machine_stop_str!("can't const prop heap allocations")
239 _ecx: &InterpCx<'mir, 'tcx, Self>,
240 frame: &Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>,
242 ) -> InterpResult<'tcx, InterpOperand<Self::PointerTag>> {
243 let l = &frame.locals[local];
245 if l.value == LocalValue::Uninitialized {
246 throw_machine_stop_str!("tried to access an uninitialized local")
252 fn access_local_mut<'a>(
253 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
256 ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
258 if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
259 throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
261 if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
263 "mutating local {:?} which is restricted to its block. \
264 Will remove it from const-prop after block is finished.",
267 ecx.machine.written_only_inside_own_block_locals.insert(local);
269 ecx.machine.stack[frame].locals[local].access_mut()
272 fn before_access_global(
275 allocation: &Allocation<Self::PointerTag, Self::AllocExtra>,
276 _static_def_id: Option<DefId>,
278 ) -> InterpResult<'tcx> {
280 throw_machine_stop_str!("can't write to global");
282 // If the static allocation is mutable, then we can't const prop it as its content
283 // might be different at runtime.
284 if allocation.mutability == Mutability::Mut {
285 throw_machine_stop_str!("can't access mutable globals in ConstProp");
293 _ecx: &mut InterpCx<'mir, 'tcx, Self>,
294 frame: Frame<'mir, 'tcx>,
295 ) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
301 ecx: &'a InterpCx<'mir, 'tcx, Self>,
302 ) -> &'a [Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>] {
308 ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
309 ) -> &'a mut Vec<Frame<'mir, 'tcx, Self::PointerTag, Self::FrameExtra>> {
310 &mut ecx.machine.stack
314 /// Finds optimization opportunities on the MIR.
315 struct ConstPropagator<'mir, 'tcx> {
316 ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
318 param_env: ParamEnv<'tcx>,
319 // FIXME(eddyb) avoid cloning these two fields more than once,
320 // by accessing them through `ecx` instead.
321 source_scopes: IndexVec<SourceScope, SourceScopeData<'tcx>>,
322 local_decls: IndexVec<Local, LocalDecl<'tcx>>,
323 // Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
324 // the last known `SourceInfo` here and just keep revisiting it.
325 source_info: Option<SourceInfo>,
328 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
330 type TyAndLayout = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;
332 fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyAndLayout {
333 self.tcx.layout_of(self.param_env.and(ty))
337 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
339 fn data_layout(&self) -> &TargetDataLayout {
340 &self.tcx.data_layout
344 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
346 fn tcx(&self) -> TyCtxt<'tcx> {
351 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
354 dummy_body: &'mir Body<'tcx>,
356 ) -> ConstPropagator<'mir, 'tcx> {
357 let def_id = body.source.def_id();
358 let substs = &InternalSubsts::identity_for_item(tcx, def_id);
359 let param_env = tcx.param_env_reveal_all_normalized(def_id);
361 let span = tcx.def_span(def_id);
362 // FIXME: `CanConstProp::check` computes the layout of all locals, return those layouts
363 // so we can write them to `ecx.frame_mut().locals.layout, reducing the duplication in
364 // `layout_of` query invocations.
365 let can_const_prop = CanConstProp::check(tcx, param_env, body);
366 let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
367 for (l, mode) in can_const_prop.iter_enumerated() {
368 if *mode == ConstPropMode::OnlyInsideOwnBlock {
369 only_propagate_inside_block_locals.insert(l);
372 let mut ecx = InterpCx::new(
376 ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
381 .layout_of(body.return_ty().subst(tcx, substs))
383 // Don't bother allocating memory for ZST types which have no values
384 // or for large values.
385 .filter(|ret_layout| {
386 !ret_layout.is_zst() && ret_layout.size < Size::from_bytes(MAX_ALLOC_LIMIT)
388 .map(|ret_layout| ecx.allocate(ret_layout, MemoryKind::Stack));
390 ecx.push_stack_frame(
391 Instance::new(def_id, substs),
394 StackPopCleanup::None { cleanup: false },
396 .expect("failed to push initial stack frame");
402 // FIXME(eddyb) avoid cloning these two fields more than once,
403 // by accessing them through `ecx` instead.
404 source_scopes: body.source_scopes.clone(),
405 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
406 local_decls: body.local_decls.clone(),
411 fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
412 let op = match self.ecx.eval_place_to_op(place, None) {
415 trace!("get_const failed: {}", e);
420 // Try to read the local as an immediate so that if it is representable as a scalar, we can
421 // handle it as such, but otherwise, just return the value as is.
422 Some(match self.ecx.try_read_immediate(op) {
423 Ok(Ok(imm)) => imm.into(),
428 /// Remove `local` from the pool of `Locals`. Allows writing to them,
429 /// but not reading from them anymore.
430 fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
431 ecx.frame_mut().locals[local] =
432 LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) };
435 fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
436 match &self.source_scopes[source_info.scope].local_data {
437 ClearCrossCrate::Set(data) => Some(data.lint_root),
438 ClearCrossCrate::Clear => None,
442 fn use_ecx<F, T>(&mut self, f: F) -> Option<T>
444 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
447 Ok(val) => Some(val),
449 trace!("InterpCx operation failed: {:?}", error);
450 // Some errors shouldn't come up because creating them causes
451 // an allocation, which we should avoid. When that happens,
452 // dedicated error variants should be introduced instead.
454 !error.kind.allocates(),
455 "const-prop encountered allocating error: {}",
463 /// Returns the value, if any, of evaluating `c`.
464 fn eval_constant(&mut self, c: &Constant<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
465 // FIXME we need to revisit this for #67176
470 match self.ecx.const_to_op(c.literal, None) {
473 let tcx = self.ecx.tcx.at(c.span);
474 let err = ConstEvalErr::new(&self.ecx, error, Some(c.span));
475 if let Some(lint_root) = self.lint_root(source_info) {
476 let lint_only = match c.literal.val {
477 // Promoteds must lint and not error as the user didn't ask for them
478 ConstKind::Unevaluated(_, _, Some(_)) => true,
479 // Out of backwards compatibility we cannot report hard errors in unused
480 // generic functions using associated constants of the generic parameters.
481 _ => c.literal.needs_subst(),
484 // Out of backwards compatibility we cannot report hard errors in unused
485 // generic functions using associated constants of the generic parameters.
486 err.report_as_lint(tcx, "erroneous constant used", lint_root, Some(c.span));
488 err.report_as_error(tcx, "erroneous constant used");
491 err.report_as_error(tcx, "erroneous constant used");
498 /// Returns the value, if any, of evaluating `place`.
499 fn eval_place(&mut self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
500 trace!("eval_place(place={:?})", place);
501 self.use_ecx(|this| this.ecx.eval_place_to_op(place, None))
504 /// Returns the value, if any, of evaluating `op`. Calls upon `eval_constant`
505 /// or `eval_place`, depending on the variant of `Operand` used.
506 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<OpTy<'tcx>> {
508 Operand::Constant(ref c) => self.eval_constant(c, source_info),
509 Operand::Move(place) | Operand::Copy(place) => self.eval_place(place),
513 fn report_assert_as_lint(
515 lint: &'static lint::Lint,
516 source_info: SourceInfo,
517 message: &'static str,
518 panic: AssertKind<impl std::fmt::Debug>,
520 let lint_root = self.lint_root(source_info)?;
521 self.tcx.struct_span_lint_hir(lint, lint_root, source_info.span, |lint| {
522 let mut err = lint.build(message);
523 err.span_label(source_info.span, format!("{:?}", panic));
533 source_info: SourceInfo,
535 if let (val, true) = self.use_ecx(|this| {
536 let val = this.ecx.read_immediate(this.ecx.eval_operand(arg, None)?)?;
537 let (_res, overflow, _ty) = this.ecx.overflowing_unary_op(op, val)?;
540 // `AssertKind` only has an `OverflowNeg` variant, so make sure that is
541 // appropriate to use.
542 assert_eq!(op, UnOp::Neg, "Neg is the only UnOp that can overflow");
543 self.report_assert_as_lint(
544 lint::builtin::ARITHMETIC_OVERFLOW,
546 "this arithmetic operation will overflow",
547 AssertKind::OverflowNeg(val.to_const_int()),
557 left: &Operand<'tcx>,
558 right: &Operand<'tcx>,
559 source_info: SourceInfo,
561 let r = self.use_ecx(|this| this.ecx.read_immediate(this.ecx.eval_operand(right, None)?));
562 let l = self.use_ecx(|this| this.ecx.read_immediate(this.ecx.eval_operand(left, None)?));
563 // Check for exceeding shifts *even if* we cannot evaluate the LHS.
564 if op == BinOp::Shr || op == BinOp::Shl {
566 // We need the type of the LHS. We cannot use `place_layout` as that is the type
567 // of the result, which for checked binops is not the same!
568 let left_ty = left.ty(&self.local_decls, self.tcx);
569 let left_size = self.ecx.layout_of(left_ty).ok()?.size;
570 let right_size = r.layout.size;
571 let r_bits = r.to_scalar().ok();
572 // This is basically `force_bits`.
573 let r_bits = r_bits.and_then(|r| r.to_bits_or_ptr(right_size, &self.tcx).ok());
574 if r_bits.map_or(false, |b| b >= left_size.bits() as u128) {
575 debug!("check_binary_op: reporting assert for {:?}", source_info);
576 self.report_assert_as_lint(
577 lint::builtin::ARITHMETIC_OVERFLOW,
579 "this arithmetic operation will overflow",
580 AssertKind::Overflow(
583 Some(l) => l.to_const_int(),
584 // Invent a dummy value, the diagnostic ignores it anyway
585 None => ConstInt::new(
586 ScalarInt::try_from_uint(1_u8, left_size).unwrap(),
588 left_ty.is_ptr_sized_integral(),
597 if let (Some(l), Some(r)) = (l, r) {
598 // The remaining operators are handled through `overflowing_binary_op`.
599 if self.use_ecx(|this| {
600 let (_res, overflow, _ty) = this.ecx.overflowing_binary_op(op, l, r)?;
603 self.report_assert_as_lint(
604 lint::builtin::ARITHMETIC_OVERFLOW,
606 "this arithmetic operation will overflow",
607 AssertKind::Overflow(op, l.to_const_int(), r.to_const_int()),
614 fn propagate_operand(&mut self, operand: &mut Operand<'tcx>) {
616 Operand::Copy(l) | Operand::Move(l) => {
617 if let Some(value) = self.get_const(l) {
618 if self.should_const_prop(value) {
619 // FIXME(felix91gr): this code only handles `Scalar` cases.
620 // For now, we're not handling `ScalarPair` cases because
621 // doing so here would require a lot of code duplication.
622 // We should hopefully generalize `Operand` handling into a fn,
623 // and use it to do const-prop here and everywhere else
624 // where it makes sense.
625 if let interpret::Operand::Immediate(interpret::Immediate::Scalar(
626 ScalarMaybeUninit::Scalar(scalar),
629 *operand = self.operand_from_scalar(
632 self.source_info.unwrap().span,
638 Operand::Constant(_) => (),
644 rvalue: &Rvalue<'tcx>,
645 source_info: SourceInfo,
648 // Perform any special handling for specific Rvalue types.
649 // Generally, checks here fall into one of two categories:
650 // 1. Additional checking to provide useful lints to the user
651 // - In this case, we will do some validation and then fall through to the
652 // end of the function which evals the assignment.
653 // 2. Working around bugs in other parts of the compiler
654 // - In this case, we'll return `None` from this function to stop evaluation.
656 // Additional checking: give lints to the user if an overflow would occur.
657 // We do this here and not in the `Assert` terminator as that terminator is
658 // only sometimes emitted (overflow checks can be disabled), but we want to always
660 Rvalue::UnaryOp(op, arg) => {
661 trace!("checking UnaryOp(op = {:?}, arg = {:?})", op, arg);
662 self.check_unary_op(*op, arg, source_info)?;
664 Rvalue::BinaryOp(op, left, right) => {
665 trace!("checking BinaryOp(op = {:?}, left = {:?}, right = {:?})", op, left, right);
666 self.check_binary_op(*op, left, right, source_info)?;
668 Rvalue::CheckedBinaryOp(op, left, right) => {
670 "checking CheckedBinaryOp(op = {:?}, left = {:?}, right = {:?})",
675 self.check_binary_op(*op, left, right, source_info)?;
678 // Do not try creating references (#67862)
679 Rvalue::AddressOf(_, place) | Rvalue::Ref(_, _, place) => {
680 trace!("skipping AddressOf | Ref for {:?}", place);
682 // This may be creating mutable references or immutable references to cells.
683 // If that happens, the pointed to value could be mutated via that reference.
684 // Since we aren't tracking references, the const propagator loses track of what
685 // value the local has right now.
686 // Thus, all locals that have their reference taken
687 // must not take part in propagation.
688 Self::remove_const(&mut self.ecx, place.local);
692 Rvalue::ThreadLocalRef(def_id) => {
693 trace!("skipping ThreadLocalRef({:?})", def_id);
698 // There's no other checking to do at this time.
699 Rvalue::Aggregate(..)
704 | Rvalue::Discriminant(..)
705 | Rvalue::NullaryOp(..) => {}
708 // FIXME we need to revisit this for #67176
709 if rvalue.needs_subst() {
713 if self.tcx.sess.opts.debugging_opts.mir_opt_level >= 3 {
714 self.eval_rvalue_with_identities(rvalue, place)
716 self.use_ecx(|this| this.ecx.eval_rvalue_into_place(rvalue, place))
720 // Attempt to use albegraic identities to eliminate constant expressions
721 fn eval_rvalue_with_identities(
723 rvalue: &Rvalue<'tcx>,
726 self.use_ecx(|this| {
728 Rvalue::BinaryOp(op, left, right) | Rvalue::CheckedBinaryOp(op, left, right) => {
729 let l = this.ecx.eval_operand(left, None);
730 let r = this.ecx.eval_operand(right, None);
732 let const_arg = match (l, r) {
733 (Ok(x), Err(_)) | (Err(_), Ok(x)) => this.ecx.read_immediate(x)?,
734 (Err(e), Err(_)) => return Err(e),
736 this.ecx.eval_rvalue_into_place(rvalue, place)?;
742 this.ecx.force_bits(const_arg.to_scalar()?, const_arg.layout.size)?;
743 let dest = this.ecx.eval_place(place)?;
748 this.ecx.write_immediate(*const_arg, dest)?;
752 if arg_value == const_arg.layout.size.truncate(u128::MAX)
753 || (const_arg.layout.ty.is_bool() && arg_value == 1)
755 this.ecx.write_immediate(*const_arg, dest)?;
759 if const_arg.layout.ty.is_integral() && arg_value == 0 {
760 if let Rvalue::CheckedBinaryOp(_, _, _) = rvalue {
761 let val = Immediate::ScalarPair(
762 const_arg.to_scalar()?.into(),
763 Scalar::from_bool(false).into(),
765 this.ecx.write_immediate(val, dest)?;
767 this.ecx.write_immediate(*const_arg, dest)?;
772 this.ecx.eval_rvalue_into_place(rvalue, place)?;
777 this.ecx.eval_rvalue_into_place(rvalue, place)?;
785 /// Creates a new `Operand::Constant` from a `Scalar` value
786 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
787 Operand::Constant(Box::new(Constant {
790 literal: ty::Const::from_scalar(self.tcx, scalar, ty),
794 fn replace_with_const(
796 rval: &mut Rvalue<'tcx>,
798 source_info: SourceInfo,
800 if let Rvalue::Use(Operand::Constant(c)) = rval {
801 if !matches!(c.literal.val, ConstKind::Unevaluated(..)) {
802 trace!("skipping replace of Rvalue::Use({:?} because it is already a const", c);
807 trace!("attepting to replace {:?} with {:?}", rval, value);
808 if let Err(e) = self.ecx.const_validate_operand(
811 // FIXME: is ref tracking too expensive?
812 // FIXME: what is the point of ref tracking if we do not even check the tracked refs?
813 &mut interpret::RefTracking::empty(),
814 CtfeValidationMode::Regular,
816 trace!("validation error, attempt failed: {:?}", e);
820 // FIXME> figure out what to do when try_read_immediate fails
821 let imm = self.use_ecx(|this| this.ecx.try_read_immediate(value));
823 if let Some(Ok(imm)) = imm {
825 interpret::Immediate::Scalar(ScalarMaybeUninit::Scalar(scalar)) => {
826 *rval = Rvalue::Use(self.operand_from_scalar(
832 Immediate::ScalarPair(
833 ScalarMaybeUninit::Scalar(_),
834 ScalarMaybeUninit::Scalar(_),
836 // Found a value represented as a pair. For now only do const-prop if the type
837 // of `rvalue` is also a tuple with two scalars.
838 // FIXME: enable the general case stated above ^.
839 let ty = &value.layout.ty;
840 // Only do it for tuples
841 if let ty::Tuple(substs) = ty.kind() {
842 // Only do it if tuple is also a pair with two scalars
843 if substs.len() == 2 {
844 let alloc = self.use_ecx(|this| {
845 let ty1 = substs[0].expect_ty();
846 let ty2 = substs[1].expect_ty();
847 let ty_is_scalar = |ty| {
848 this.ecx.layout_of(ty).ok().map(|layout| layout.abi.is_scalar())
851 if ty_is_scalar(ty1) && ty_is_scalar(ty2) {
854 .intern_with_temp_alloc(value.layout, |ecx, dest| {
855 ecx.write_immediate_to_mplace(*imm, dest)
864 if let Some(Some(alloc)) = alloc {
865 // Assign entire constant in a single statement.
866 // We can't use aggregates, as we run after the aggregate-lowering `MirPhase`.
867 *rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
868 span: source_info.span,
870 literal: self.ecx.tcx.mk_const(ty::Const {
872 val: ty::ConstKind::Value(ConstValue::ByRef {
882 // Scalars or scalar pairs that contain undef values are assumed to not have
883 // successfully evaluated and are thus not propagated.
889 /// Returns `true` if and only if this `op` should be const-propagated into.
890 fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
891 let mir_opt_level = self.tcx.sess.opts.debugging_opts.mir_opt_level;
893 if mir_opt_level == 0 {
898 interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUninit::Scalar(s))) => {
901 interpret::Operand::Immediate(Immediate::ScalarPair(
902 ScalarMaybeUninit::Scalar(l),
903 ScalarMaybeUninit::Scalar(r),
904 )) => l.is_bits() && r.is_bits(),
910 /// The mode that `ConstProp` is allowed to run in for a given `Local`.
911 #[derive(Clone, Copy, Debug, PartialEq)]
913 /// The `Local` can be propagated into and reads of this `Local` can also be propagated.
915 /// The `Local` can only be propagated into and from its own block.
917 /// The `Local` can be propagated into but reads cannot be propagated.
919 /// The `Local` cannot be part of propagation at all. Any statement
920 /// referencing it either for reading or writing will not get propagated.
924 struct CanConstProp {
925 can_const_prop: IndexVec<Local, ConstPropMode>,
926 // False at the beginning. Once set, no more assignments are allowed to that local.
927 found_assignment: BitSet<Local>,
928 // Cache of locals' information
929 local_kinds: IndexVec<Local, LocalKind>,
933 /// Returns true if `local` can be propagated
936 param_env: ParamEnv<'tcx>,
938 ) -> IndexVec<Local, ConstPropMode> {
939 let mut cpv = CanConstProp {
940 can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
941 found_assignment: BitSet::new_empty(body.local_decls.len()),
942 local_kinds: IndexVec::from_fn_n(
943 |local| body.local_kind(local),
944 body.local_decls.len(),
947 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
948 let ty = body.local_decls[local].ty;
949 match tcx.layout_of(param_env.and(ty)) {
950 Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
951 // Either the layout fails to compute, then we can't use this local anyway
952 // or the local is too large, then we don't want to.
954 *val = ConstPropMode::NoPropagation;
958 // Cannot use args at all
959 // Cannot use locals because if x < y { y - x } else { x - y } would
961 // FIXME(oli-obk): lint variables until they are used in a condition
962 // FIXME(oli-obk): lint if return value is constant
963 if cpv.local_kinds[local] == LocalKind::Arg {
964 *val = ConstPropMode::OnlyPropagateInto;
966 "local {:?} can't be const propagated because it's a function argument",
969 } else if cpv.local_kinds[local] == LocalKind::Var {
970 *val = ConstPropMode::OnlyInsideOwnBlock;
972 "local {:?} will only be propagated inside its block, because it's a user variable",
977 cpv.visit_body(&body);
982 impl<'tcx> Visitor<'tcx> for CanConstProp {
983 fn visit_local(&mut self, &local: &Local, context: PlaceContext, _: Location) {
984 use rustc_middle::mir::visit::PlaceContext::*;
986 // Projections are fine, because `&mut foo.x` will be caught by
987 // `MutatingUseContext::Borrow` elsewhere.
988 MutatingUse(MutatingUseContext::Projection)
989 // These are just stores, where the storing is not propagatable, but there may be later
990 // mutations of the same local via `Store`
991 | MutatingUse(MutatingUseContext::Call)
992 // Actual store that can possibly even propagate a value
993 | MutatingUse(MutatingUseContext::Store) => {
994 if !self.found_assignment.insert(local) {
995 match &mut self.can_const_prop[local] {
996 // If the local can only get propagated in its own block, then we don't have
997 // to worry about multiple assignments, as we'll nuke the const state at the
998 // end of the block anyway, and inside the block we overwrite previous
999 // states as applicable.
1000 ConstPropMode::OnlyInsideOwnBlock => {}
1001 ConstPropMode::NoPropagation => {}
1002 ConstPropMode::OnlyPropagateInto => {}
1003 other @ ConstPropMode::FullConstProp => {
1005 "local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
1008 *other = ConstPropMode::OnlyInsideOwnBlock;
1013 // Reading constants is allowed an arbitrary number of times
1014 NonMutatingUse(NonMutatingUseContext::Copy)
1015 | NonMutatingUse(NonMutatingUseContext::Move)
1016 | NonMutatingUse(NonMutatingUseContext::Inspect)
1017 | NonMutatingUse(NonMutatingUseContext::Projection)
1020 // These could be propagated with a smarter analysis or just some careful thinking about
1021 // whether they'd be fine right now.
1022 MutatingUse(MutatingUseContext::AsmOutput)
1023 | MutatingUse(MutatingUseContext::Yield)
1024 | MutatingUse(MutatingUseContext::Drop)
1025 | MutatingUse(MutatingUseContext::Retag)
1026 // These can't ever be propagated under any scheme, as we can't reason about indirect
1028 | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
1029 | NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
1030 | NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
1031 | NonMutatingUse(NonMutatingUseContext::AddressOf)
1032 | MutatingUse(MutatingUseContext::Borrow)
1033 | MutatingUse(MutatingUseContext::AddressOf) => {
1034 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
1035 self.can_const_prop[local] = ConstPropMode::NoPropagation;
1041 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
1042 fn tcx(&self) -> TyCtxt<'tcx> {
1046 fn visit_body(&mut self, body: &mut Body<'tcx>) {
1047 for (bb, data) in body.basic_blocks_mut().iter_enumerated_mut() {
1048 self.visit_basic_block_data(bb, data);
1052 fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
1053 self.super_operand(operand, location);
1055 // Only const prop copies and moves on `mir_opt_level=2` as doing so
1056 // currently slightly increases compile time in some cases.
1057 if self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2 {
1058 self.propagate_operand(operand)
1062 fn visit_constant(&mut self, constant: &mut Constant<'tcx>, location: Location) {
1063 trace!("visit_constant: {:?}", constant);
1064 self.super_constant(constant, location);
1065 self.eval_constant(constant, self.source_info.unwrap());
1068 fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
1069 trace!("visit_statement: {:?}", statement);
1070 let source_info = statement.source_info;
1071 self.source_info = Some(source_info);
1072 if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
1073 let can_const_prop = self.ecx.machine.can_const_prop[place.local];
1074 if let Some(()) = self.const_prop(rval, source_info, place) {
1075 // This will return None if the above `const_prop` invocation only "wrote" a
1076 // type whose creation requires no write. E.g. a generator whose initial state
1077 // consists solely of uninitialized memory (so it doesn't capture any locals).
1078 if let Some(value) = self.get_const(place) {
1079 if self.should_const_prop(value) {
1080 trace!("replacing {:?} with {:?}", rval, value);
1081 self.replace_with_const(rval, value, source_info);
1082 if can_const_prop == ConstPropMode::FullConstProp
1083 || can_const_prop == ConstPropMode::OnlyInsideOwnBlock
1085 trace!("propagated into {:?}", place);
1089 match can_const_prop {
1090 ConstPropMode::OnlyInsideOwnBlock => {
1092 "found local restricted to its block. \
1093 Will remove it from const-prop after block is finished. Local: {:?}",
1097 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1098 trace!("can't propagate into {:?}", place);
1099 if place.local != RETURN_PLACE {
1100 Self::remove_const(&mut self.ecx, place.local);
1103 ConstPropMode::FullConstProp => {}
1106 // Const prop failed, so erase the destination, ensuring that whatever happens
1107 // from here on, does not know about the previous value.
1108 // This is important in case we have
1111 // x = SOME_MUTABLE_STATIC;
1112 // // x must now be uninit
1114 // FIXME: we overzealously erase the entire local, because that's easier to
1117 "propagation into {:?} failed.
1118 Nuking the entire site from orbit, it's the only way to be sure",
1121 Self::remove_const(&mut self.ecx, place.local);
1124 match statement.kind {
1125 StatementKind::SetDiscriminant { ref place, .. } => {
1126 match self.ecx.machine.can_const_prop[place.local] {
1127 ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
1128 if self.use_ecx(|this| this.ecx.statement(statement)).is_some() {
1129 trace!("propped discriminant into {:?}", place);
1131 Self::remove_const(&mut self.ecx, place.local);
1134 ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
1135 Self::remove_const(&mut self.ecx, place.local);
1139 StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
1140 let frame = self.ecx.frame_mut();
1141 frame.locals[local].value =
1142 if let StatementKind::StorageLive(_) = statement.kind {
1143 LocalValue::Uninitialized
1152 self.super_statement(statement, location);
1155 fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
1156 let source_info = terminator.source_info;
1157 self.source_info = Some(source_info);
1158 self.super_terminator(terminator, location);
1159 match &mut terminator.kind {
1160 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
1161 if let Some(value) = self.eval_operand(&cond, source_info) {
1162 trace!("assertion on {:?} should be {:?}", value, expected);
1163 let expected = ScalarMaybeUninit::from(Scalar::from_bool(*expected));
1164 let value_const = self.ecx.read_scalar(value).unwrap();
1165 if expected != value_const {
1170 impl<T: std::fmt::Debug> std::fmt::Debug for DbgVal<T> {
1171 fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
1173 Self::Val(val) => val.fmt(fmt),
1174 Self::Underscore => fmt.write_str("_"),
1178 let mut eval_to_int = |op| {
1179 // This can be `None` if the lhs wasn't const propagated and we just
1180 // triggered the assert on the value of the rhs.
1181 match self.eval_operand(op, source_info) {
1183 DbgVal::Val(self.ecx.read_immediate(op).unwrap().to_const_int())
1185 None => DbgVal::Underscore,
1188 let msg = match msg {
1189 AssertKind::DivisionByZero(op) => {
1190 Some(AssertKind::DivisionByZero(eval_to_int(op)))
1192 AssertKind::RemainderByZero(op) => {
1193 Some(AssertKind::RemainderByZero(eval_to_int(op)))
1195 AssertKind::BoundsCheck { ref len, ref index } => {
1196 let len = eval_to_int(len);
1197 let index = eval_to_int(index);
1198 Some(AssertKind::BoundsCheck { len, index })
1200 // Overflow is are already covered by checks on the binary operators.
1201 AssertKind::Overflow(..) | AssertKind::OverflowNeg(_) => None,
1202 // Need proper const propagator for these.
1205 // Poison all places this operand references so that further code
1206 // doesn't use the invalid value
1208 Operand::Move(ref place) | Operand::Copy(ref place) => {
1209 Self::remove_const(&mut self.ecx, place.local);
1211 Operand::Constant(_) => {}
1213 if let Some(msg) = msg {
1214 self.report_assert_as_lint(
1215 lint::builtin::UNCONDITIONAL_PANIC,
1217 "this operation will panic at runtime",
1222 if self.should_const_prop(value) {
1223 if let ScalarMaybeUninit::Scalar(scalar) = value_const {
1224 *cond = self.operand_from_scalar(
1226 self.tcx.types.bool,
1234 TerminatorKind::SwitchInt { ref mut discr, .. } => {
1235 // FIXME: This is currently redundant with `visit_operand`, but sadly
1236 // always visiting operands currently causes a perf regression in LLVM codegen, so
1237 // `visit_operand` currently only runs for propagates places for `mir_opt_level=3`.
1238 self.propagate_operand(discr)
1240 // None of these have Operands to const-propagate.
1241 TerminatorKind::Goto { .. }
1242 | TerminatorKind::Resume
1243 | TerminatorKind::Abort
1244 | TerminatorKind::Return
1245 | TerminatorKind::Unreachable
1246 | TerminatorKind::Drop { .. }
1247 | TerminatorKind::DropAndReplace { .. }
1248 | TerminatorKind::Yield { .. }
1249 | TerminatorKind::GeneratorDrop
1250 | TerminatorKind::FalseEdge { .. }
1251 | TerminatorKind::FalseUnwind { .. }
1252 | TerminatorKind::InlineAsm { .. } => {}
1253 // Every argument in our function calls have already been propagated in `visit_operand`.
1255 // NOTE: because LLVM codegen gives slight performance regressions with it, so this is
1256 // gated on `mir_opt_level=2`.
1257 TerminatorKind::Call { .. } => {}
1260 // We remove all Locals which are restricted in propagation to their containing blocks and
1261 // which were modified in the current block.
1262 // Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
1263 let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
1264 for &local in locals.iter() {
1265 Self::remove_const(&mut self.ecx, local);
1268 // Put it back so we reuse the heap of the storage
1269 self.ecx.machine.written_only_inside_own_block_locals = locals;
1270 if cfg!(debug_assertions) {
1271 // Ensure we are correctly erasing locals with the non-debug-assert logic.
1272 for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
1274 self.get_const(local.into()).is_none()
1276 .layout_of(self.local_decls[local].ty)
1277 .map_or(true, |layout| layout.is_zst())