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_borrowck_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 substs = Substs::identity_for_item(tcx, source.def_id);
114 let instance = Instance::new(source.def_id, substs);
115 let ecx = mk_borrowck_eval_cx(tcx, instance, mir, DUMMY_SP).unwrap();
122 can_const_prop: CanConstProp::check(mir),
123 places: IndexVec::from_elem(None, &mir.local_decls),
129 source_info: SourceInfo,
133 F: FnOnce(&mut Self) -> EvalResult<'tcx, T>,
135 self.ecx.tcx.span = source_info.span;
136 let lint_root = match self.mir.source_scope_local_data {
137 ClearCrossCrate::Set(ref ivs) => {
138 //FIXME(#51314): remove this check
139 if source_info.scope.index() >= ivs.len() {
142 ivs[source_info.scope].lint_root
144 ClearCrossCrate::Clear => return None,
146 let r = match f(self) {
147 Ok(val) => Some(val),
149 let diagnostic = error_to_const_error(&self.ecx, error);
150 use rustc::mir::interpret::EvalErrorKind::*;
151 match diagnostic.error {
152 // don't report these, they make no sense in a const prop context
154 // at runtime these transformations might make sense
155 // FIXME: figure out the rules and start linting
156 | FunctionAbiMismatch(..)
157 | FunctionArgMismatch(..)
158 | FunctionRetMismatch(..)
159 | FunctionArgCountMismatch
160 // fine at runtime, might be a register address or sth
165 // don't report const evaluator limits
166 | StackFrameLimitReached
171 | InvalidMemoryAccess
172 | DanglingPointerDeref
174 | InvalidFunctionPointer
176 | InvalidDiscriminant(..)
177 | PointerOutOfBounds { .. }
178 | InvalidNullPointerUsage
179 | ValidationFailure(..)
184 | DerefFunctionPointer
189 | AlignmentCheckFailed{..}
190 | CalledClosureAsFunction
191 | VtableForArgumentlessMethod
192 | ModifiedConstantMemory
195 // FIXME: should probably be removed and turned into a bug! call
196 | TypeNotPrimitive(_)
197 | ReallocatedWrongMemoryKind(_, _)
198 | DeallocatedWrongMemoryKind(_, _)
199 | ReallocateNonBasePtr
200 | DeallocateNonBasePtr
201 | IncorrectAllocationInformation(..)
202 | UnterminatedCString(_)
204 | HeapAllocNonPowerOfTwoAlignment(_)
206 | ReadFromReturnPointer
207 | GeneratorResumedAfterReturn
208 | GeneratorResumedAfterPanic
212 // FIXME: report UB here
218 => bug!("these should not be in rustc, but in miri's machine errors"),
221 | UnimplementedTraitSelection
224 // these are just noise
239 diagnostic.report_as_lint(
241 "this expression will panic at runtime",
249 self.ecx.tcx.span = DUMMY_SP;
256 source_info: SourceInfo,
257 ) -> Option<Const<'tcx>> {
258 self.ecx.tcx.span = source_info.span;
259 match lazy_const_to_op(&self.ecx, *c.literal, c.ty) {
264 let err = error_to_const_error(&self.ecx, error);
265 err.report_as_error(self.ecx.tcx, "erroneous constant used");
271 fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
273 Place::Local(loc) => self.places[loc].clone(),
274 Place::Projection(ref proj) => match proj.elem {
275 ProjectionElem::Field(field, _) => {
276 trace!("field proj on {:?}", proj.base);
277 let (base, span) = self.eval_place(&proj.base, source_info)?;
278 let res = self.use_ecx(source_info, |this| {
279 this.ecx.operand_field(base, field.index() as u64)
283 // We could get more projections by using e.g., `operand_projection`,
284 // but we do not even have the stack frame set up properly so
285 // an `Index` projection would throw us off-track.
288 Place::Promoted(ref 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 = Substs::identity_for_item(self.tcx, self.source.def_id);
295 let instance = Instance::new(self.source.def_id, substs);
298 promoted: Some(promoted.0),
300 // cannot use `const_eval` here, because that would require having the MIR
301 // for the current function available, but we're producing said MIR right now
302 let res = self.use_ecx(source_info, |this| {
303 eval_promoted(this.tcx, cid, this.mir, this.param_env)
305 trace!("evaluated promoted {:?} to {:?}", promoted, res);
306 Some((res.into(), source_info.span))
312 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
314 Operand::Constant(ref c) => self.eval_constant(c, source_info),
315 | Operand::Move(ref place)
316 | Operand::Copy(ref place) => self.eval_place(place, source_info),
322 rvalue: &Rvalue<'tcx>,
323 place_layout: TyLayout<'tcx>,
324 source_info: SourceInfo,
325 ) -> Option<Const<'tcx>> {
326 let span = source_info.span;
328 Rvalue::Use(ref op) => {
329 self.eval_operand(op, source_info)
333 Rvalue::Aggregate(..) |
334 Rvalue::NullaryOp(NullOp::Box, _) |
335 Rvalue::Discriminant(..) => None,
337 Rvalue::Cast(kind, ref operand, _) => {
338 let (op, span) = self.eval_operand(operand, source_info)?;
339 self.use_ecx(source_info, |this| {
340 let dest = this.ecx.allocate(place_layout, MemoryKind::Stack);
341 this.ecx.cast(op, kind, dest.into())?;
342 Ok((dest.into(), span))
346 // FIXME(oli-obk): evaluate static/constant slice lengths
347 Rvalue::Len(_) => None,
348 Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
349 type_size_of(self.tcx, self.param_env, ty).and_then(|n| Some((
351 op: interpret::Operand::Immediate(Immediate::Scalar(
354 size: self.tcx.data_layout.pointer_size.bytes() as u8,
357 layout: self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).ok()?,
362 Rvalue::UnaryOp(op, ref arg) => {
363 let def_id = if self.tcx.is_closure(self.source.def_id) {
364 self.tcx.closure_base_def_id(self.source.def_id)
368 let generics = self.tcx.generics_of(def_id);
369 if generics.requires_monomorphization(self.tcx) {
370 // FIXME: can't handle code with generics
374 let (arg, _) = self.eval_operand(arg, source_info)?;
375 let val = self.use_ecx(source_info, |this| {
376 let prim = this.ecx.read_scalar(arg)?.not_undef()?;
379 // Need to do overflow check here: For actual CTFE, MIR
380 // generation emits code that does this before calling the op.
381 let size = arg.layout.size;
382 if prim.to_bits(size)? == (1 << (size.bits() - 1)) {
383 return err!(OverflowNeg);
390 // Now run the actual operation.
391 this.ecx.unary_op(op, prim, arg.layout)
394 op: interpret::Operand::Immediate(Immediate::Scalar(val.into())),
395 layout: place_layout,
399 Rvalue::CheckedBinaryOp(op, ref left, ref right) |
400 Rvalue::BinaryOp(op, ref left, ref right) => {
401 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
402 let right = self.eval_operand(right, source_info)?;
403 let def_id = if self.tcx.is_closure(self.source.def_id) {
404 self.tcx.closure_base_def_id(self.source.def_id)
408 let generics = self.tcx.generics_of(def_id);
409 if generics.requires_monomorphization(self.tcx) {
410 // FIXME: can't handle code with generics
414 let r = self.use_ecx(source_info, |this| {
415 this.ecx.read_immediate(right.0)
417 if op == BinOp::Shr || op == BinOp::Shl {
418 let left_ty = left.ty(self.mir, self.tcx);
421 .layout_of(self.param_env.and(left_ty))
425 let right_size = right.0.layout.size;
426 let r_bits = r.to_scalar().and_then(|r| r.to_bits(right_size));
427 if r_bits.ok().map_or(false, |b| b >= left_bits as u128) {
428 let source_scope_local_data = match self.mir.source_scope_local_data {
429 ClearCrossCrate::Set(ref data) => data,
430 ClearCrossCrate::Clear => return None,
432 let dir = if op == BinOp::Shr {
437 let node_id = source_scope_local_data[source_info.scope].lint_root;
439 ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
442 &format!("attempt to shift {} with overflow", dir));
446 let left = self.eval_operand(left, source_info)?;
447 let l = self.use_ecx(source_info, |this| {
448 this.ecx.read_immediate(left.0)
450 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
451 let (val, overflow) = self.use_ecx(source_info, |this| {
452 this.ecx.binary_op_imm(op, l, r)
454 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
455 Immediate::ScalarPair(
457 Scalar::from_bool(overflow).into(),
461 let err = EvalErrorKind::Overflow(op).into();
462 let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
465 Immediate::Scalar(val.into())
468 op: interpret::Operand::Immediate(val),
469 layout: place_layout,
477 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
478 param_env: ty::ParamEnv<'tcx>,
479 ty: ty::Ty<'tcx>) -> Option<u64> {
480 tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
483 struct CanConstProp {
484 can_const_prop: IndexVec<Local, bool>,
485 // false at the beginning, once set, there are not allowed to be any more assignments
486 found_assignment: IndexVec<Local, bool>,
490 /// returns true if `local` can be propagated
491 fn check(mir: &Mir) -> IndexVec<Local, bool> {
492 let mut cpv = CanConstProp {
493 can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
494 found_assignment: IndexVec::from_elem(false, &mir.local_decls),
496 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
497 // cannot use args at all
498 // cannot use locals because if x < y { y - x } else { x - y } would
500 // FIXME(oli-obk): lint variables until they are used in a condition
501 // FIXME(oli-obk): lint if return value is constant
502 *val = mir.local_kind(local) == LocalKind::Temp;
509 impl<'tcx> Visitor<'tcx> for CanConstProp {
513 context: PlaceContext<'tcx>,
516 use rustc::mir::visit::PlaceContext::*;
518 // Constants must have at most one write
519 // FIXME(oli-obk): we could be more powerful here, if the multiple writes
520 // only occur in independent execution paths
521 MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
522 self.can_const_prop[local] = false;
524 self.found_assignment[local] = true
526 // Reading constants is allowed an arbitrary number of times
527 NonMutatingUse(NonMutatingUseContext::Copy) |
528 NonMutatingUse(NonMutatingUseContext::Move) |
529 NonMutatingUse(NonMutatingUseContext::Inspect) |
530 NonMutatingUse(NonMutatingUseContext::Projection) |
531 MutatingUse(MutatingUseContext::Projection) |
533 _ => self.can_const_prop[local] = false,
538 impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
541 constant: &Constant<'tcx>,
544 trace!("visit_constant: {:?}", constant);
545 self.super_constant(constant, location);
546 let source_info = *self.mir.source_info(location);
547 self.eval_constant(constant, source_info);
553 statement: &Statement<'tcx>,
556 trace!("visit_statement: {:?}", statement);
557 if let StatementKind::Assign(ref place, ref rval) = statement.kind {
558 let place_ty: ty::Ty<'tcx> = place
559 .ty(&self.mir.local_decls, self.tcx)
561 if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
562 if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
563 if let Place::Local(local) = *place {
564 trace!("checking whether {:?} can be stored to {:?}", value, local);
565 if self.can_const_prop[local] {
566 trace!("storing {:?} to {:?}", value, local);
567 assert!(self.places[local].is_none());
568 self.places[local] = Some(value);
574 self.super_statement(block, statement, location);
577 fn visit_terminator_kind(
580 kind: &TerminatorKind<'tcx>,
583 self.super_terminator_kind(block, kind, location);
584 let source_info = *self.mir.source_info(location);
585 if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
586 if let Some(value) = self.eval_operand(cond, source_info) {
587 trace!("assertion on {:?} should be {:?}", value, expected);
588 let expected = ScalarMaybeUndef::from(Scalar::from_bool(*expected));
589 if expected != self.ecx.read_scalar(value.0).unwrap() {
590 // poison all places this operand references so that further code
591 // doesn't use the invalid value
593 Operand::Move(ref place) | Operand::Copy(ref place) => {
594 let mut place = place;
595 while let Place::Projection(ref proj) = *place {
598 if let Place::Local(local) = *place {
599 self.places[local] = None;
602 Operand::Constant(_) => {}
604 let span = self.mir[block]
613 .as_local_node_id(self.source.def_id)
614 .expect("some part of a failing const eval must be local");
615 use rustc::mir::interpret::EvalErrorKind::*;
616 let msg = match msg {
620 RemainderByZero => msg.description().to_owned(),
621 BoundsCheck { ref len, ref index } => {
623 .eval_operand(len, source_info)
624 .expect("len must be const");
625 let len = match self.ecx.read_scalar(len.0) {
626 Ok(ScalarMaybeUndef::Scalar(Scalar::Bits {
629 other => bug!("const len not primitive: {:?}", other),
632 .eval_operand(index, source_info)
633 .expect("index must be const");
634 let index = match self.ecx.read_scalar(index.0) {
635 Ok(ScalarMaybeUndef::Scalar(Scalar::Bits {
638 other => bug!("const index not primitive: {:?}", other),
641 "index out of bounds: \
642 the len is {} but the index is {}",
647 // Need proper const propagator for these
651 ::rustc::lint::builtin::CONST_ERR,