1 //! Propagates constants for early reporting of statically known
5 use rustc::hir::def::Def;
6 use rustc::mir::{Constant, Location, Place, Mir, Operand, Rvalue, Local};
7 use rustc::mir::{NullOp, UnOp, StatementKind, Statement, BasicBlock, LocalKind};
8 use rustc::mir::{TerminatorKind, ClearCrossCrate, SourceInfo, BinOp, ProjectionElem};
9 use rustc::mir::visit::{Visitor, PlaceContext, MutatingUseContext, NonMutatingUseContext};
10 use rustc::mir::interpret::{EvalErrorKind, Scalar, GlobalId, EvalResult};
11 use rustc::ty::{TyCtxt, self, Instance};
12 use syntax::source_map::{Span, DUMMY_SP};
13 use rustc::ty::subst::Substs;
14 use rustc_data_structures::indexed_vec::IndexVec;
15 use rustc::ty::ParamEnv;
16 use rustc::ty::layout::{
17 LayoutOf, TyLayout, LayoutError,
18 HasTyCtxt, TargetDataLayout, HasDataLayout,
21 use interpret::{self, EvalContext, ScalarMaybeUndef, Immediate, OpTy, MemoryKind};
23 CompileTimeInterpreter, error_to_const_error, eval_promoted, mk_eval_cx,
26 use transform::{MirPass, MirSource};
30 impl MirPass for ConstProp {
31 fn run_pass<'a, 'tcx>(&self,
32 tcx: TyCtxt<'a, 'tcx, 'tcx>,
34 mir: &mut Mir<'tcx>) {
35 // will be evaluated by miri and produce its errors there
36 if source.promoted.is_some() {
40 use rustc::hir::map::blocks::FnLikeNode;
41 let node_id = tcx.hir().as_local_node_id(source.def_id)
42 .expect("Non-local call to local provider is_const_fn");
44 let is_fn_like = FnLikeNode::from_node(tcx.hir().get(node_id)).is_some();
45 let is_assoc_const = match tcx.describe_def(source.def_id) {
46 Some(Def::AssociatedConst(_)) => true,
50 // Only run const prop on functions, methods, closures and associated constants
51 if !is_fn_like && !is_assoc_const {
52 // skip anon_const/statics/consts because they'll be evaluated by miri anyway
53 trace!("ConstProp skipped for {:?}", source.def_id);
57 trace!("ConstProp starting for {:?}", source.def_id);
59 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
60 // constants, instead of just checking for const-folding succeeding.
61 // That would require an uniform one-def no-mutation analysis
62 // and RPO (or recursing when needing the value of a local).
63 let mut optimization_finder = ConstPropagator::new(mir, tcx, source);
64 optimization_finder.visit_mir(mir);
66 trace!("ConstProp done for {:?}", source.def_id);
70 type Const<'tcx> = (OpTy<'tcx>, Span);
72 /// Finds optimization opportunities on the MIR.
73 struct ConstPropagator<'a, 'mir, 'tcx:'a+'mir> {
74 ecx: EvalContext<'a, 'mir, 'tcx, CompileTimeInterpreter<'a, 'mir, 'tcx>>,
76 tcx: TyCtxt<'a, 'tcx, 'tcx>,
78 places: IndexVec<Local, Option<Const<'tcx>>>,
79 can_const_prop: IndexVec<Local, bool>,
80 param_env: ParamEnv<'tcx>,
83 impl<'a, 'b, 'tcx> LayoutOf for ConstPropagator<'a, 'b, 'tcx> {
84 type Ty = ty::Ty<'tcx>;
85 type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
87 fn layout_of(&self, ty: ty::Ty<'tcx>) -> Self::TyLayout {
88 self.tcx.layout_of(self.param_env.and(ty))
92 impl<'a, 'b, 'tcx> HasDataLayout for ConstPropagator<'a, 'b, 'tcx> {
94 fn data_layout(&self) -> &TargetDataLayout {
99 impl<'a, 'b, 'tcx> HasTyCtxt<'tcx> for ConstPropagator<'a, 'b, 'tcx> {
101 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'tcx, 'tcx> {
106 impl<'a, 'mir, 'tcx> ConstPropagator<'a, 'mir, 'tcx> {
108 mir: &'mir Mir<'tcx>,
109 tcx: TyCtxt<'a, 'tcx, 'tcx>,
111 ) -> ConstPropagator<'a, 'mir, 'tcx> {
112 let param_env = tcx.param_env(source.def_id);
113 let ecx = mk_eval_cx(tcx, tcx.def_span(source.def_id), param_env);
120 can_const_prop: CanConstProp::check(mir),
121 places: IndexVec::from_elem(None, &mir.local_decls),
127 source_info: SourceInfo,
131 F: FnOnce(&mut Self) -> EvalResult<'tcx, T>,
133 self.ecx.tcx.span = source_info.span;
134 let lint_root = match self.mir.source_scope_local_data {
135 ClearCrossCrate::Set(ref ivs) => {
136 //FIXME(#51314): remove this check
137 if source_info.scope.index() >= ivs.len() {
140 ivs[source_info.scope].lint_root
142 ClearCrossCrate::Clear => return None,
144 let r = match f(self) {
145 Ok(val) => Some(val),
147 let diagnostic = error_to_const_error(&self.ecx, error);
148 use rustc::mir::interpret::EvalErrorKind::*;
149 match diagnostic.error {
150 // don't report these, they make no sense in a const prop context
152 // at runtime these transformations might make sense
153 // FIXME: figure out the rules and start linting
154 | FunctionAbiMismatch(..)
155 | FunctionArgMismatch(..)
156 | FunctionRetMismatch(..)
157 | FunctionArgCountMismatch
158 // fine at runtime, might be a register address or sth
163 // don't report const evaluator limits
164 | StackFrameLimitReached
169 | InvalidMemoryAccess
170 | DanglingPointerDeref
172 | InvalidFunctionPointer
174 | InvalidDiscriminant(..)
175 | PointerOutOfBounds { .. }
176 | InvalidNullPointerUsage
177 | ValidationFailure(..)
182 | DerefFunctionPointer
187 | AlignmentCheckFailed{..}
188 | CalledClosureAsFunction
189 | VtableForArgumentlessMethod
190 | ModifiedConstantMemory
193 // FIXME: should probably be removed and turned into a bug! call
194 | TypeNotPrimitive(_)
195 | ReallocatedWrongMemoryKind(_, _)
196 | DeallocatedWrongMemoryKind(_, _)
197 | ReallocateNonBasePtr
198 | DeallocateNonBasePtr
199 | IncorrectAllocationInformation(..)
200 | UnterminatedCString(_)
202 | HeapAllocNonPowerOfTwoAlignment(_)
204 | ReadFromReturnPointer
205 | GeneratorResumedAfterReturn
206 | GeneratorResumedAfterPanic
210 // FIXME: report UB here
216 => bug!("these should not be in rustc, but in miri's machine errors"),
219 | UnimplementedTraitSelection
222 // these are just noise
237 diagnostic.report_as_lint(
239 "this expression will panic at runtime",
247 self.ecx.tcx.span = DUMMY_SP;
254 source_info: SourceInfo,
255 ) -> Option<Const<'tcx>> {
256 self.ecx.tcx.span = source_info.span;
257 match lazy_const_to_op(&self.ecx, *c.literal, c.ty) {
262 let err = error_to_const_error(&self.ecx, error);
263 err.report_as_error(self.ecx.tcx, "erroneous constant used");
269 fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
271 Place::Local(loc) => self.places[loc].clone(),
272 Place::Projection(ref proj) => match proj.elem {
273 ProjectionElem::Field(field, _) => {
274 trace!("field proj on {:?}", proj.base);
275 let (base, span) = self.eval_place(&proj.base, source_info)?;
276 let res = self.use_ecx(source_info, |this| {
277 this.ecx.operand_field(base, field.index() as u64)
281 // We could get more projections by using e.g., `operand_projection`,
282 // but we do not even have the stack frame set up properly so
283 // an `Index` projection would throw us off-track.
286 Place::Promoted(ref promoted) => {
287 let generics = self.tcx.generics_of(self.source.def_id);
288 if generics.requires_monomorphization(self.tcx) {
289 // FIXME: can't handle code with generics
292 let substs = Substs::identity_for_item(self.tcx, self.source.def_id);
293 let instance = Instance::new(self.source.def_id, substs);
296 promoted: Some(promoted.0),
298 // cannot use `const_eval` here, because that would require having the MIR
299 // for the current function available, but we're producing said MIR right now
300 let res = self.use_ecx(source_info, |this| {
301 eval_promoted(this.tcx, cid, this.mir, this.param_env)
303 trace!("evaluated promoted {:?} to {:?}", promoted, res);
304 Some((res.into(), source_info.span))
310 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
312 Operand::Constant(ref c) => self.eval_constant(c, source_info),
313 | Operand::Move(ref place)
314 | Operand::Copy(ref place) => self.eval_place(place, source_info),
320 rvalue: &Rvalue<'tcx>,
321 place_layout: TyLayout<'tcx>,
322 source_info: SourceInfo,
323 ) -> Option<Const<'tcx>> {
324 let span = source_info.span;
326 Rvalue::Use(ref op) => {
327 self.eval_operand(op, source_info)
331 Rvalue::Aggregate(..) |
332 Rvalue::NullaryOp(NullOp::Box, _) |
333 Rvalue::Discriminant(..) => None,
335 Rvalue::Cast(kind, ref operand, _) => {
336 let (op, span) = self.eval_operand(operand, source_info)?;
337 self.use_ecx(source_info, |this| {
338 let dest = this.ecx.allocate(place_layout, MemoryKind::Stack);
339 this.ecx.cast(op, kind, dest.into())?;
340 Ok((dest.into(), span))
344 // FIXME(oli-obk): evaluate static/constant slice lengths
345 Rvalue::Len(_) => None,
346 Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
347 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some((
349 op: interpret::Operand::Immediate(Immediate::Scalar(
352 size: self.tcx.data_layout.pointer_size.bytes() as u8,
355 layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
360 Rvalue::UnaryOp(op, ref arg) => {
361 let def_id = if self.tcx.is_closure(self.source.def_id) {
362 self.tcx.closure_base_def_id(self.source.def_id)
366 let generics = self.tcx.generics_of(def_id);
367 if generics.requires_monomorphization(self.tcx) {
368 // FIXME: can't handle code with generics
372 let (arg, _) = self.eval_operand(arg, source_info)?;
373 let val = self.use_ecx(source_info, |this| {
374 let prim = this.ecx.read_scalar(arg)?.not_undef()?;
377 // Need to do overflow check here: For actual CTFE, MIR
378 // generation emits code that does this before calling the op.
379 let size = arg.layout.size;
380 if prim.to_bits(size)? == (1 << (size.bits() - 1)) {
381 return err!(OverflowNeg);
388 // Now run the actual operation.
389 this.ecx.unary_op(op, prim, arg.layout)
392 op: interpret::Operand::Immediate(Immediate::Scalar(val.into())),
393 layout: place_layout,
397 Rvalue::CheckedBinaryOp(op, ref left, ref right) |
398 Rvalue::BinaryOp(op, ref left, ref right) => {
399 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
400 let right = self.eval_operand(right, source_info)?;
401 let def_id = if self.tcx.is_closure(self.source.def_id) {
402 self.tcx.closure_base_def_id(self.source.def_id)
406 let generics = self.tcx.generics_of(def_id);
407 if generics.requires_monomorphization(self.tcx) {
408 // FIXME: can't handle code with generics
412 let r = self.use_ecx(source_info, |this| {
413 this.ecx.read_immediate(right.0)
415 if op == BinOp::Shr || op == BinOp::Shl {
416 let left_ty = left.ty(self.mir, self.tcx);
419 .layout_of(self.param_env.and(left_ty))
423 let right_size = right.0.layout.size;
424 let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
425 if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
426 let source_scope_local_data = match self.mir.source_scope_local_data {
427 ClearCrossCrate::Set(ref data) => data,
428 ClearCrossCrate::Clear => return None,
430 let dir = if op == BinOp::Shr {
435 let node_id = source_scope_local_data[source_info.scope].lint_root;
437 ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
440 &format!("attempt to shift {} with overflow", dir));
444 let left = self.eval_operand(left, source_info)?;
445 let l = self.use_ecx(source_info, |this| {
446 this.ecx.read_immediate(left.0)
448 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
449 let (val, overflow) = self.use_ecx(source_info, |this| {
450 this.ecx.binary_op_imm(op, l, r)
452 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
453 Immediate::ScalarPair(
455 Scalar::from_bool(overflow).into(),
459 let err = EvalErrorKind::Overflow(op).into();
460 let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
463 Immediate::Scalar(val.into())
466 op: interpret::Operand::Immediate(val),
467 layout: place_layout,
475 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
476 param_env: ty::ParamEnv<'tcx>,
477 ty: ty::Ty<'tcx>) -> Option<u64> {
478 tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
481 struct CanConstProp {
482 can_const_prop: IndexVec<Local, bool>,
483 // false at the beginning, once set, there are not allowed to be any more assignments
484 found_assignment: IndexVec<Local, bool>,
488 /// returns true if `local` can be propagated
489 fn check(mir: &Mir) -> IndexVec<Local, bool> {
490 let mut cpv = CanConstProp {
491 can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
492 found_assignment: IndexVec::from_elem(false, &mir.local_decls),
494 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
495 // cannot use args at all
496 // cannot use locals because if x < y { y - x } else { x - y } would
498 // FIXME(oli-obk): lint variables until they are used in a condition
499 // FIXME(oli-obk): lint if return value is constant
500 *val = mir.local_kind(local) == LocalKind::Temp;
507 impl<'tcx> Visitor<'tcx> for CanConstProp {
511 context: PlaceContext<'tcx>,
514 use rustc::mir::visit::PlaceContext::*;
516 // Constants must have at most one write
517 // FIXME(oli-obk): we could be more powerful here, if the multiple writes
518 // only occur in independent execution paths
519 MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
520 self.can_const_prop[local] = false;
522 self.found_assignment[local] = true
524 // Reading constants is allowed an arbitrary number of times
525 NonMutatingUse(NonMutatingUseContext::Copy) |
526 NonMutatingUse(NonMutatingUseContext::Move) |
527 NonMutatingUse(NonMutatingUseContext::Inspect) |
528 NonMutatingUse(NonMutatingUseContext::Projection) |
529 MutatingUse(MutatingUseContext::Projection) |
531 _ => self.can_const_prop[local] = false,
536 impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
539 constant: &Constant<'tcx>,
542 trace!("visit_constant: {:?}", constant);
543 self.super_constant(constant, location);
544 let source_info = *self.mir.source_info(location);
545 self.eval_constant(constant, source_info);
551 statement: &Statement<'tcx>,
554 trace!("visit_statement: {:?}", statement);
555 if let StatementKind::Assign(ref place, ref rval) = statement.kind {
556 let place_ty: ty::Ty<'tcx> = place
557 .ty(&self.mir.local_decls, self.tcx)
559 if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
560 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
561 if let Place::Local(local) = *place {
562 trace!("checking whether {:?} can be stored to {:?}", value, local);
563 if self.can_const_prop[local] {
564 trace!("storing {:?} to {:?}", value, local);
565 assert!(self.places[local].is_none());
566 self.places[local] = Some(value);
572 self.super_statement(block, statement, location);
575 fn visit_terminator_kind(
578 kind: &TerminatorKind<'tcx>,
581 self.super_terminator_kind(block, kind, location);
582 let source_info = *self.mir.source_info(location);
583 if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
584 if let Some(value) = self.eval_operand(cond, source_info) {
585 trace!("assertion on {:?} should be {:?}", value, expected);
586 let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
587 if expected != self.ecx.read_scalar(value.0).unwrap() {
588 // poison all places this operand references so that further code
589 // doesn't use the invalid value
591 Operand::Move(ref place) | Operand::Copy(ref place) => {
592 let mut place = place;
593 while let Place::Projection(ref proj) = *place {
596 if let Place::Local(local) = *place {
597 self.places[local] = None;
600 Operand::Constant(_) => {}
602 let span = self.mir[block]
611 .as_local_node_id(self.source.def_id)
612 .expect("some part of a failing const eval must be local");
613 use rustc::mir::interpret::EvalErrorKind::*;
614 let msg = match msg {
618 RemainderByZero => msg.description().to_owned(),
619 BoundsCheck { ref len, ref index } => {
621 .eval_operand(len, source_info)
622 .expect("len must be const");
623 let len = match self.ecx.read_scalar(len.0) {
624 Ok(ScalarMaybeUndef::Scalar(Scalar::Bits {
627 other => bug!("const len not primitive: {:?}", other),
630 .eval_operand(index, source_info)
631 .expect("index must be const");
632 let index = match self.ecx.read_scalar(index.0) {
633 Ok(ScalarMaybeUndef::Scalar(Scalar::Bits {
636 other => bug!("const index not primitive: {:?}", other),
639 "index out of bounds: \
640 the len is {} but the index is {}",
645 // Need proper const propagator for these
649 ::rustc::lint::builtin::CONST_ERR,