1 //! Propagates constants for early reporting of statically known
6 use rustc::hir::def::DefKind;
8 AggregateKind, Constant, Location, Place, PlaceBase, Body, Operand, Rvalue,
9 Local, NullOp, UnOp, StatementKind, Statement, LocalKind, Static, StaticKind,
10 TerminatorKind, Terminator, ClearCrossCrate, SourceInfo, BinOp, ProjectionElem,
11 SourceScope, SourceScopeLocalData, LocalDecl,
13 use rustc::mir::visit::{
14 Visitor, PlaceContext, MutatingUseContext, MutVisitor, NonMutatingUseContext,
16 use rustc::mir::interpret::{Scalar, GlobalId, InterpResult, PanicInfo};
17 use rustc::ty::{self, Instance, ParamEnv, Ty, TyCtxt};
18 use syntax_pos::{Span, DUMMY_SP};
19 use rustc::ty::subst::InternalSubsts;
20 use rustc_data_structures::indexed_vec::IndexVec;
21 use rustc::ty::layout::{
22 LayoutOf, TyLayout, LayoutError, HasTyCtxt, TargetDataLayout, HasDataLayout,
25 use crate::interpret::{
26 self, InterpCx, ScalarMaybeUndef, Immediate, OpTy,
27 ImmTy, MemoryKind, StackPopCleanup, LocalValue, LocalState,
29 use crate::const_eval::{
30 CompileTimeInterpreter, error_to_const_error, mk_eval_cx,
32 use crate::transform::{MirPass, MirSource};
36 impl<'tcx> MirPass<'tcx> for ConstProp {
37 fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
38 // will be evaluated by miri and produce its errors there
39 if source.promoted.is_some() {
43 use rustc::hir::map::blocks::FnLikeNode;
44 let hir_id = tcx.hir().as_local_hir_id(source.def_id())
45 .expect("Non-local call to local provider is_const_fn");
47 let is_fn_like = FnLikeNode::from_node(tcx.hir().get(hir_id)).is_some();
48 let is_assoc_const = match tcx.def_kind(source.def_id()) {
49 Some(DefKind::AssocConst) => true,
53 // Only run const prop on functions, methods, closures and associated constants
54 if !is_fn_like && !is_assoc_const {
55 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
56 trace!("ConstProp skipped for {:?}", source.def_id());
60 trace!("ConstProp starting for {:?}", source.def_id());
62 // Steal some data we need from `body`.
63 let source_scope_local_data = std::mem::replace(
64 &mut body.source_scope_local_data,
65 ClearCrossCrate::Clear
70 body.basic_blocks().clone(),
72 ClearCrossCrate::Clear,
74 body.local_decls.clone(),
78 tcx.def_span(source.def_id()),
82 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
83 // constants, instead of just checking for const-folding succeeding.
84 // That would require an uniform one-def no-mutation analysis
85 // and RPO (or recursing when needing the value of a local).
86 let mut optimization_finder = ConstPropagator::new(
89 source_scope_local_data,
93 optimization_finder.visit_body(body);
95 // put back the data we stole from `mir`
96 let source_scope_local_data = optimization_finder.release_stolen_data();
98 &mut body.source_scope_local_data,
99 source_scope_local_data
102 trace!("ConstProp done for {:?}", source.def_id());
106 type Const<'tcx> = OpTy<'tcx>;
108 /// Finds optimization opportunities on the MIR.
109 struct ConstPropagator<'mir, 'tcx> {
110 ecx: InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>>,
112 source: MirSource<'tcx>,
113 can_const_prop: IndexVec<Local, bool>,
114 param_env: ParamEnv<'tcx>,
115 source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
116 local_decls: IndexVec<Local, LocalDecl<'tcx>>,
119 impl<'mir, 'tcx> LayoutOf for ConstPropagator<'mir, 'tcx> {
121 type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
123 fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyLayout {
124 self.tcx.layout_of(self.param_env.and(ty))
128 impl<'mir, 'tcx> HasDataLayout for ConstPropagator<'mir, 'tcx> {
130 fn data_layout(&self) -> &TargetDataLayout {
131 &self.tcx.data_layout
135 impl<'mir, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'mir, 'tcx> {
137 fn tcx(&self) -> TyCtxt<'tcx> {
142 impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
145 dummy_body: &'mir Body<'tcx>,
146 source_scope_local_data: ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
148 source: MirSource<'tcx>,
149 ) -> ConstPropagator<'mir, 'tcx> {
150 let def_id = source.def_id();
151 let param_env = tcx.param_env(def_id);
152 let span = tcx.def_span(def_id);
153 let mut ecx = mk_eval_cx(tcx, span, param_env);
154 let can_const_prop = CanConstProp::check(body);
156 ecx.push_stack_frame(
157 Instance::new(def_id, &InternalSubsts::identity_for_item(tcx, def_id)),
161 StackPopCleanup::None {
164 ).expect("failed to push initial stack frame");
172 source_scope_local_data,
173 //FIXME(wesleywiser) we can't steal this because `Visitor::super_visit_body()` needs it
174 local_decls: body.local_decls.clone(),
178 fn release_stolen_data(self) -> ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>> {
179 self.source_scope_local_data
182 fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
183 let l = &self.ecx.frame().locals[local];
185 // If the local is `Unitialized` or `Dead` then we haven't propagated a value into it.
187 // `InterpCx::access_local()` mostly takes care of this for us however, for ZSTs,
188 // it will synthesize a value for us. In doing so, that will cause the
189 // `get_const(l).is_empty()` assert right before we call `set_const()` in `visit_statement`
191 if let LocalValue::Uninitialized | LocalValue::Dead = l.value {
195 self.ecx.access_local(self.ecx.frame(), local, None).ok()
198 fn set_const(&mut self, local: Local, c: Const<'tcx>) {
199 let frame = self.ecx.frame_mut();
201 if let Some(layout) = frame.locals[local].layout.get() {
202 debug_assert_eq!(c.layout, layout);
205 frame.locals[local] = LocalState {
206 value: LocalValue::Live(*c),
207 layout: Cell::new(Some(c.layout)),
211 fn remove_const(&mut self, local: Local) {
212 self.ecx.frame_mut().locals[local] = LocalState {
213 value: LocalValue::Uninitialized,
214 layout: Cell::new(None),
220 source_info: SourceInfo,
224 F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
226 self.ecx.tcx.span = source_info.span;
227 let lint_root = match self.source_scope_local_data {
228 ClearCrossCrate::Set(ref ivs) => {
229 //FIXME(#51314): remove this check
230 if source_info.scope.index() >= ivs.len() {
233 ivs[source_info.scope].lint_root
235 ClearCrossCrate::Clear => return None,
237 let r = match f(self) {
238 Ok(val) => Some(val),
240 let diagnostic = error_to_const_error(&self.ecx, error);
241 use rustc::mir::interpret::InterpError::*;
242 match diagnostic.error {
243 Exit(_) => bug!("the CTFE program cannot exit"),
245 | UndefinedBehavior(_)
247 | ResourceExhaustion(_) => {
248 // Ignore these errors.
251 diagnostic.report_as_lint(
253 "this expression will panic at runtime",
262 self.ecx.tcx.span = DUMMY_SP;
269 ) -> Option<Const<'tcx>> {
270 self.ecx.tcx.span = c.span;
271 match self.ecx.eval_const_to_op(c.literal, None) {
276 let err = error_to_const_error(&self.ecx, error);
277 err.report_as_error(self.ecx.tcx, "erroneous constant used");
283 fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
284 trace!("eval_place(place={:?})", place);
285 place.iterate(|place_base, place_projection| {
286 let mut eval = match place_base {
287 PlaceBase::Local(loc) => self.get_const(*loc).clone()?,
288 PlaceBase::Static(box Static {kind: StaticKind::Promoted(promoted), ..}) => {
289 let generics = self.tcx.generics_of(self.source.def_id());
290 if generics.requires_monomorphization(self.tcx) {
291 // FIXME: can't handle code with generics
294 let substs = InternalSubsts::identity_for_item(self.tcx, self.source.def_id());
295 let instance = Instance::new(self.source.def_id(), substs);
298 promoted: Some(*promoted),
300 let res = self.use_ecx(source_info, |this| {
301 this.ecx.const_eval_raw(cid)
303 trace!("evaluated promoted {:?} to {:?}", promoted, res);
309 for proj in place_projection {
311 ProjectionElem::Field(field, _) => {
312 trace!("field proj on {:?}", proj.base);
313 eval = self.use_ecx(source_info, |this| {
314 this.ecx.operand_field(eval, field.index() as u64)
317 ProjectionElem::Deref => {
318 trace!("processing deref");
319 eval = self.use_ecx(source_info, |this| {
320 this.ecx.deref_operand(eval)
323 // We could get more projections by using e.g., `operand_projection`,
324 // but we do not even have the stack frame set up properly so
325 // an `Index` projection would throw us off-track.
334 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
336 Operand::Constant(ref c) => self.eval_constant(c),
337 | Operand::Move(ref place)
338 | Operand::Copy(ref place) => self.eval_place(place, source_info),
344 rvalue: &Rvalue<'tcx>,
345 place_layout: TyLayout<'tcx>,
346 source_info: SourceInfo,
347 ) -> Option<Const<'tcx>> {
348 let span = source_info.span;
350 Rvalue::Use(ref op) => {
351 self.eval_operand(op, source_info)
353 Rvalue::Ref(_, _, ref place) => {
354 let src = self.eval_place(place, source_info)?;
355 let mplace = src.try_as_mplace().ok()?;
356 Some(ImmTy::from_scalar(mplace.ptr.into(), place_layout).into())
359 Rvalue::Aggregate(..) |
360 Rvalue::NullaryOp(NullOp::Box, _) |
361 Rvalue::Discriminant(..) => None,
363 Rvalue::Cast(kind, ref operand, _) => {
364 let op = self.eval_operand(operand, source_info)?;
365 self.use_ecx(source_info, |this| {
366 let dest = this.ecx.allocate(place_layout, MemoryKind::Stack);
367 this.ecx.cast(op, kind, dest.into())?;
371 Rvalue::Len(ref place) => {
372 let place = self.eval_place(&place, source_info)?;
373 let mplace = place.try_as_mplace().ok()?;
375 if let ty::Slice(_) = mplace.layout.ty.sty {
376 let len = mplace.meta.unwrap().to_usize(&self.ecx).unwrap();
378 Some(ImmTy::from_uint(
380 self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
383 trace!("not slice: {:?}", mplace.layout.ty.sty);
387 Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
388 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some(
391 self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
395 Rvalue::UnaryOp(op, ref arg) => {
396 let def_id = if self.tcx.is_closure(self.source.def_id()) {
397 self.tcx.closure_base_def_id(self.source.def_id())
401 let generics = self.tcx.generics_of(def_id);
402 if generics.requires_monomorphization(self.tcx) {
403 // FIXME: can't handle code with generics
407 let arg = self.eval_operand(arg, source_info)?;
408 let val = self.use_ecx(source_info, |this| {
409 let prim = this.ecx.read_immediate(arg)?;
412 // Need to do overflow check here: For actual CTFE, MIR
413 // generation emits code that does this before calling the op.
414 if prim.to_bits()? == (1 << (prim.layout.size.bits() - 1)) {
415 throw_panic!(OverflowNeg)
422 // Now run the actual operation.
423 this.ecx.unary_op(op, prim)
427 Rvalue::CheckedBinaryOp(op, ref left, ref right) |
428 Rvalue::BinaryOp(op, ref left, ref right) => {
429 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
430 let right = self.eval_operand(right, source_info)?;
431 let def_id = if self.tcx.is_closure(self.source.def_id()) {
432 self.tcx.closure_base_def_id(self.source.def_id())
436 let generics = self.tcx.generics_of(def_id);
437 if generics.requires_monomorphization(self.tcx) {
438 // FIXME: can't handle code with generics
442 let r = self.use_ecx(source_info, |this| {
443 this.ecx.read_immediate(right)
445 if op == BinOp::Shr || op == BinOp::Shl {
446 let left_ty = left.ty(&self.local_decls, self.tcx);
449 .layout_of(self.param_env.and(left_ty))
453 let right_size = right.layout.size;
454 let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
455 if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
456 let source_scope_local_data = match self.source_scope_local_data {
457 ClearCrossCrate::Set(ref data) => data,
458 ClearCrossCrate::Clear => return None,
460 let dir = if op == BinOp::Shr {
465 let hir_id = source_scope_local_data[source_info.scope].lint_root;
467 ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
470 &format!("attempt to shift {} with overflow", dir));
474 let left = self.eval_operand(left, source_info)?;
475 let l = self.use_ecx(source_info, |this| {
476 this.ecx.read_immediate(left)
478 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
479 let (val, overflow, _ty) = self.use_ecx(source_info, |this| {
480 this.ecx.overflowing_binary_op(op, l, r)
482 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
483 Immediate::ScalarPair(
485 Scalar::from_bool(overflow).into(),
489 let err = err_panic!(Overflow(op)).into();
490 let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
493 Immediate::Scalar(val.into())
497 layout: place_layout,
504 fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
505 Operand::Constant(Box::new(
509 literal: self.tcx.mk_const(*ty::Const::from_scalar(
518 fn replace_with_const(
520 rval: &mut Rvalue<'tcx>,
522 source_info: SourceInfo,
524 trace!("attepting to replace {:?} with {:?}", rval, value);
525 if let Err(e) = self.ecx.validate_operand(
528 // FIXME: is ref tracking too expensive?
529 Some(&mut interpret::RefTracking::empty()),
531 trace!("validation error, attempt failed: {:?}", e);
535 // FIXME> figure out what tho do when try_read_immediate fails
536 let imm = self.use_ecx(source_info, |this| {
537 this.ecx.try_read_immediate(value)
540 if let Some(Ok(imm)) = imm {
542 interpret::Immediate::Scalar(ScalarMaybeUndef::Scalar(scalar)) => {
544 self.operand_from_scalar(scalar, value.layout.ty, source_info.span));
546 Immediate::ScalarPair(
547 ScalarMaybeUndef::Scalar(one),
548 ScalarMaybeUndef::Scalar(two)
550 let ty = &value.layout.ty.sty;
551 if let ty::Tuple(substs) = ty {
552 *rval = Rvalue::Aggregate(
553 Box::new(AggregateKind::Tuple),
555 self.operand_from_scalar(
556 one, substs[0].expect_ty(), source_info.span
558 self.operand_from_scalar(
559 two, substs[1].expect_ty(), source_info.span
570 fn should_const_prop(&self) -> bool {
571 self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2
575 fn type_size_of<'tcx>(
577 param_env: ty::ParamEnv<'tcx>,
580 tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
583 struct CanConstProp {
584 can_const_prop: IndexVec<Local, bool>,
585 // false at the beginning, once set, there are not allowed to be any more assignments
586 found_assignment: IndexVec<Local, bool>,
590 /// returns true if `local` can be propagated
591 fn check(body: &Body<'_>) -> IndexVec<Local, bool> {
592 let mut cpv = CanConstProp {
593 can_const_prop: IndexVec::from_elem(true, &body.local_decls),
594 found_assignment: IndexVec::from_elem(false, &body.local_decls),
596 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
597 // cannot use args at all
598 // cannot use locals because if x < y { y - x } else { x - y } would
600 // FIXME(oli-obk): lint variables until they are used in a condition
601 // FIXME(oli-obk): lint if return value is constant
602 *val = body.local_kind(local) == LocalKind::Temp;
605 trace!("local {:?} can't be propagated because it's not a temporary", local);
608 cpv.visit_body(body);
613 impl<'tcx> Visitor<'tcx> for CanConstProp {
617 context: PlaceContext,
620 use rustc::mir::visit::PlaceContext::*;
622 // Constants must have at most one write
623 // FIXME(oli-obk): we could be more powerful here, if the multiple writes
624 // only occur in independent execution paths
625 MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
626 trace!("local {:?} can't be propagated because of multiple assignments", local);
627 self.can_const_prop[local] = false;
629 self.found_assignment[local] = true
631 // Reading constants is allowed an arbitrary number of times
632 NonMutatingUse(NonMutatingUseContext::Copy) |
633 NonMutatingUse(NonMutatingUseContext::Move) |
634 NonMutatingUse(NonMutatingUseContext::Inspect) |
635 NonMutatingUse(NonMutatingUseContext::Projection) |
636 MutatingUse(MutatingUseContext::Projection) |
639 trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
640 self.can_const_prop[local] = false;
646 impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
649 constant: &mut Constant<'tcx>,
652 trace!("visit_constant: {:?}", constant);
653 self.super_constant(constant, location);
654 self.eval_constant(constant);
659 statement: &mut Statement<'tcx>,
662 trace!("visit_statement: {:?}", statement);
663 if let StatementKind::Assign(ref place, ref mut rval) = statement.kind {
664 let place_ty: Ty<'tcx> = place
665 .ty(&self.local_decls, self.tcx)
667 if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
668 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
670 base: PlaceBase::Local(local),
673 trace!("checking whether {:?} can be stored to {:?}", value, local);
674 if self.can_const_prop[local] {
675 trace!("storing {:?} to {:?}", value, local);
676 assert!(self.get_const(local).is_none());
677 self.set_const(local, value);
679 if self.should_const_prop() {
680 self.replace_with_const(
683 statement.source_info,
691 self.super_statement(statement, location);
696 terminator: &mut Terminator<'tcx>,
699 self.super_terminator(terminator, location);
700 let source_info = terminator.source_info;
701 match &mut terminator.kind {
702 TerminatorKind::Assert { expected, ref msg, ref mut cond, .. } => {
703 if let Some(value) = self.eval_operand(&cond, source_info) {
704 trace!("assertion on {:?} should be {:?}", value, expected);
705 let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
706 let value_const = self.ecx.read_scalar(value).unwrap();
707 if expected != value_const {
708 // poison all places this operand references so that further code
709 // doesn't use the invalid value
711 Operand::Move(ref place) | Operand::Copy(ref place) => {
712 if let PlaceBase::Local(local) = place.base {
713 self.remove_const(local);
716 Operand::Constant(_) => {}
718 let span = terminator.source_info.span;
722 .as_local_hir_id(self.source.def_id())
723 .expect("some part of a failing const eval must be local");
724 let msg = match msg {
725 PanicInfo::Overflow(_) |
726 PanicInfo::OverflowNeg |
727 PanicInfo::DivisionByZero |
728 PanicInfo::RemainderByZero =>
729 msg.description().to_owned(),
730 PanicInfo::BoundsCheck { ref len, ref index } => {
732 .eval_operand(len, source_info)
733 .expect("len must be const");
734 let len = match self.ecx.read_scalar(len) {
735 Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
738 other => bug!("const len not primitive: {:?}", other),
741 .eval_operand(index, source_info)
742 .expect("index must be const");
743 let index = match self.ecx.read_scalar(index) {
744 Ok(ScalarMaybeUndef::Scalar(Scalar::Raw {
747 other => bug!("const index not primitive: {:?}", other),
750 "index out of bounds: \
751 the len is {} but the index is {}",
756 // Need proper const propagator for these
760 ::rustc::lint::builtin::CONST_ERR,
766 if self.should_const_prop() {
767 if let ScalarMaybeUndef::Scalar(scalar) = value_const {
768 *cond = self.operand_from_scalar(
778 TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
779 if self.should_const_prop() {
780 if let Some(value) = self.eval_operand(&discr, source_info) {
781 if let ScalarMaybeUndef::Scalar(scalar) =
782 self.ecx.read_scalar(value).unwrap() {
783 *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);
788 //none of these have Operands to const-propagate
789 TerminatorKind::Goto { .. } |
790 TerminatorKind::Resume |
791 TerminatorKind::Abort |
792 TerminatorKind::Return |
793 TerminatorKind::Unreachable |
794 TerminatorKind::Drop { .. } |
795 TerminatorKind::DropAndReplace { .. } |
796 TerminatorKind::Yield { .. } |
797 TerminatorKind::GeneratorDrop |
798 TerminatorKind::FalseEdges { .. } |
799 TerminatorKind::FalseUnwind { .. } => { }
800 //FIXME(wesleywiser) Call does have Operands that could be const-propagated
801 TerminatorKind::Call { .. } => { }