1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Propagates constants for early reporting of statically known
12 //! assertion failures
15 use rustc::hir::def::Def;
16 use rustc::mir::{Constant, Location, Place, Mir, Operand, Rvalue, Local};
17 use rustc::mir::{NullOp, StatementKind, Statement, BasicBlock, LocalKind};
18 use rustc::mir::{TerminatorKind, ClearCrossCrate, SourceInfo, BinOp, ProjectionElem};
19 use rustc::mir::visit::{Visitor, PlaceContext};
20 use rustc::mir::interpret::{ConstEvalErr, EvalErrorKind};
21 use rustc::ty::{TyCtxt, self, Instance};
22 use rustc::mir::interpret::{Value, Scalar, GlobalId, EvalResult};
23 use interpret::EvalContext;
24 use interpret::CompileTimeEvaluator;
25 use interpret::{eval_promoted, mk_borrowck_eval_cx, ValTy};
26 use transform::{MirPass, MirSource};
27 use syntax::codemap::{Span, DUMMY_SP};
28 use rustc::ty::subst::Substs;
29 use rustc_data_structures::indexed_vec::IndexVec;
30 use rustc::ty::ParamEnv;
31 use rustc::ty::layout::{
32 LayoutOf, TyLayout, LayoutError,
33 HasTyCtxt, TargetDataLayout, HasDataLayout,
38 impl MirPass for ConstProp {
39 fn run_pass<'a, 'tcx>(&self,
40 tcx: TyCtxt<'a, 'tcx, 'tcx>,
42 mir: &mut Mir<'tcx>) {
43 // will be evaluated by miri and produce its errors there
44 if source.promoted.is_some() {
47 match tcx.describe_def(source.def_id) {
48 // skip statics/consts because they'll be evaluated by miri anyway
49 Some(Def::Const(..)) |
50 Some(Def::Static(..)) => return,
51 // we still run on associated constants, because they might not get evaluated
52 // within the current crate
55 trace!("ConstProp starting for {:?}", source.def_id);
57 // FIXME(oli-obk, eddyb) Optimize locals (or even local paths) to hold
58 // constants, instead of just checking for const-folding succeeding.
59 // That would require an uniform one-def no-mutation analysis
60 // and RPO (or recursing when needing the value of a local).
61 let mut optimization_finder = ConstPropagator::new(mir, tcx, source);
62 optimization_finder.visit_mir(mir);
64 trace!("ConstProp done for {:?}", source.def_id);
68 type Const<'tcx> = (Value, ty::Ty<'tcx>, Span);
70 /// Finds optimization opportunities on the MIR.
71 struct ConstPropagator<'b, 'a, 'tcx:'a+'b> {
72 ecx: EvalContext<'a, 'b, 'tcx, CompileTimeEvaluator>,
74 tcx: TyCtxt<'a, 'tcx, 'tcx>,
76 places: IndexVec<Local, Option<Const<'tcx>>>,
77 can_const_prop: IndexVec<Local, bool>,
78 param_env: ParamEnv<'tcx>,
81 impl<'a, 'b, 'tcx> LayoutOf for &'a ConstPropagator<'a, 'b, 'tcx> {
82 type Ty = ty::Ty<'tcx>;
83 type TyLayout = Result<TyLayout<'tcx>, LayoutError<'tcx>>;
85 fn layout_of(self, ty: ty::Ty<'tcx>) -> Self::TyLayout {
86 self.tcx.layout_of(self.param_env.and(ty))
90 impl<'a, 'b, 'tcx> HasDataLayout for &'a ConstPropagator<'a, 'b, 'tcx> {
92 fn data_layout(&self) -> &TargetDataLayout {
97 impl<'a, 'b, 'tcx> HasTyCtxt<'tcx> for &'a ConstPropagator<'a, 'b, 'tcx> {
99 fn tcx<'c>(&'c self) -> TyCtxt<'c, 'tcx, 'tcx> {
104 impl<'b, 'a, 'tcx:'b> ConstPropagator<'b, 'a, 'tcx> {
107 tcx: TyCtxt<'a, 'tcx, 'tcx>,
109 ) -> ConstPropagator<'b, 'a, 'tcx> {
110 let param_env = tcx.param_env(source.def_id);
111 let substs = Substs::identity_for_item(tcx, source.def_id);
112 let instance = Instance::new(source.def_id, substs);
113 let ecx = mk_borrowck_eval_cx(tcx, instance, mir, DUMMY_SP).unwrap();
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 use rustc_data_structures::indexed_vec::Idx;
137 //FIXME(#51314): remove this check
138 if source_info.scope.index() >= ivs.len() {
141 ivs[source_info.scope].lint_root
143 ClearCrossCrate::Clear => return None,
145 let r = match f(self) {
146 Ok(val) => Some(val),
149 // don't report these, they make no sense in a const prop context
150 EvalErrorKind::MachineError(_) => {},
152 let (frames, span) = self.ecx.generate_stacktrace(None);
153 let err = ConstEvalErr {
160 "this expression will panic at runtime",
168 self.ecx.tcx.span = DUMMY_SP;
175 source_info: SourceInfo,
176 ) -> Option<Const<'tcx>> {
177 self.ecx.tcx.span = source_info.span;
178 match self.ecx.const_to_value(c.literal.val) {
179 Ok(val) => Some((val, c.literal.ty, c.span)),
181 let (stacktrace, span) = self.ecx.generate_stacktrace(None);
182 let err = ConstEvalErr {
188 self.tcx.at(source_info.span),
189 "could not evaluate constant",
196 fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
198 Place::Local(loc) => self.places[loc].clone(),
199 Place::Projection(ref proj) => match proj.elem {
200 ProjectionElem::Field(field, _) => {
201 trace!("field proj on {:?}", proj.base);
202 let (base, ty, span) = self.eval_place(&proj.base, source_info)?;
203 let valty = self.use_ecx(source_info, |this| {
204 this.ecx.read_field(base, None, field, ty)
206 Some((valty.value, valty.ty, span))
210 Place::Promoted(ref promoted) => {
211 let generics = self.tcx.generics_of(self.source.def_id);
212 if generics.requires_monomorphization(self.tcx) {
213 // FIXME: can't handle code with generics
216 let substs = Substs::identity_for_item(self.tcx, self.source.def_id);
217 let instance = Instance::new(self.source.def_id, substs);
220 promoted: Some(promoted.0),
222 // cannot use `const_eval` here, because that would require having the MIR
223 // for the current function available, but we're producing said MIR right now
224 let (value, _, ty) = self.use_ecx(source_info, |this| {
225 eval_promoted(&mut this.ecx, cid, this.mir, this.param_env)
227 let val = (value, ty, source_info.span);
228 trace!("evaluated promoted {:?} to {:?}", promoted, val);
235 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
237 Operand::Constant(ref c) => self.eval_constant(c, source_info),
238 | Operand::Move(ref place)
239 | Operand::Copy(ref place) => self.eval_place(place, source_info),
245 rvalue: &Rvalue<'tcx>,
246 place_ty: ty::Ty<'tcx>,
247 source_info: SourceInfo,
248 ) -> Option<Const<'tcx>> {
249 let span = source_info.span;
251 // This branch exists for the sanity type check
252 Rvalue::Use(Operand::Constant(ref c)) => {
253 assert_eq!(c.ty, place_ty);
254 self.eval_constant(c, source_info)
256 Rvalue::Use(ref op) => {
257 self.eval_operand(op, source_info)
261 Rvalue::Aggregate(..) |
262 Rvalue::NullaryOp(NullOp::Box, _) |
263 Rvalue::Discriminant(..) => None,
265 Rvalue::Cast(kind, ref operand, _) => {
266 let (value, ty, span) = self.eval_operand(operand, source_info)?;
267 self.use_ecx(source_info, |this| {
268 let dest_ptr = this.ecx.alloc_ptr(place_ty)?;
269 let place_align = this.ecx.layout_of(place_ty)?.align;
270 let dest = ::interpret::Place::from_ptr(dest_ptr, place_align);
271 this.ecx.cast(ValTy { value, ty }, kind, place_ty, dest)?;
273 Value::ByRef(dest_ptr.into(), place_align),
280 // FIXME(oli-obk): evaluate static/constant slice lengths
281 Rvalue::Len(_) => None,
282 Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
283 let param_env = self.tcx.param_env(self.source.def_id);
284 type_size_of(self.tcx, param_env, ty).map(|n| (
285 Value::Scalar(Scalar::Bits {
287 defined: self.tcx.data_layout.pointer_size.bits() as u8,
289 self.tcx.types.usize,
293 Rvalue::UnaryOp(op, ref arg) => {
294 let def_id = if self.tcx.is_closure(self.source.def_id) {
295 self.tcx.closure_base_def_id(self.source.def_id)
299 let generics = self.tcx.generics_of(def_id);
300 if generics.requires_monomorphization(self.tcx) {
301 // FIXME: can't handle code with generics
305 let val = self.eval_operand(arg, source_info)?;
306 let prim = self.use_ecx(source_info, |this| {
307 this.ecx.value_to_scalar(ValTy { value: val.0, ty: val.1 })
309 let val = self.use_ecx(source_info, |this| this.ecx.unary_op(op, prim, val.1))?;
310 Some((Value::Scalar(val), place_ty, span))
312 Rvalue::CheckedBinaryOp(op, ref left, ref right) |
313 Rvalue::BinaryOp(op, ref left, ref right) => {
314 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
315 let right = self.eval_operand(right, source_info)?;
316 let def_id = if self.tcx.is_closure(self.source.def_id) {
317 self.tcx.closure_base_def_id(self.source.def_id)
321 let generics = self.tcx.generics_of(def_id);
322 if generics.requires_monomorphization(self.tcx) {
323 // FIXME: can't handle code with generics
327 let r = self.use_ecx(source_info, |this| {
328 this.ecx.value_to_scalar(ValTy { value: right.0, ty: right.1 })
330 if op == BinOp::Shr || op == BinOp::Shl {
331 let left_ty = left.ty(self.mir, self.tcx);
334 .layout_of(self.param_env.and(left_ty))
338 let right_size = self.tcx.layout_of(self.param_env.and(right.1)).unwrap().size;
339 if r.to_bits(right_size).ok().map_or(false, |b| b >= left_bits as u128) {
340 let source_scope_local_data = match self.mir.source_scope_local_data {
341 ClearCrossCrate::Set(ref data) => data,
342 ClearCrossCrate::Clear => return None,
344 let dir = if op == BinOp::Shr {
349 let node_id = source_scope_local_data[source_info.scope].lint_root;
351 ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
354 &format!("attempt to shift {} with overflow", dir));
358 let left = self.eval_operand(left, source_info)?;
359 let l = self.use_ecx(source_info, |this| {
360 this.ecx.value_to_scalar(ValTy { value: left.0, ty: left.1 })
362 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
363 let (val, overflow) = self.use_ecx(source_info, |this| {
364 this.ecx.binary_op(op, l, left.1, r, right.1)
366 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
369 Scalar::from_bool(overflow),
373 let err = EvalErrorKind::Overflow(op).into();
374 let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
379 Some((val, place_ty, span))
385 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
386 param_env: ty::ParamEnv<'tcx>,
387 ty: ty::Ty<'tcx>) -> Option<u64> {
388 tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
391 struct CanConstProp {
392 can_const_prop: IndexVec<Local, bool>,
393 // false at the beginning, once set, there are not allowed to be any more assignments
394 found_assignment: IndexVec<Local, bool>,
398 /// returns true if `local` can be propagated
399 fn check(mir: &Mir) -> IndexVec<Local, bool> {
400 let mut cpv = CanConstProp {
401 can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
402 found_assignment: IndexVec::from_elem(false, &mir.local_decls),
404 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
405 // cannot use args at all
406 // cannot use locals because if x < y { y - x } else { x - y } would
408 // FIXME(oli-obk): lint variables until they are used in a condition
409 // FIXME(oli-obk): lint if return value is constant
410 *val = mir.local_kind(local) == LocalKind::Temp;
417 impl<'tcx> Visitor<'tcx> for CanConstProp {
421 context: PlaceContext<'tcx>,
424 use rustc::mir::visit::PlaceContext::*;
426 // Constants must have at most one write
427 // FIXME(oli-obk): we could be more powerful here, if the multiple writes
428 // only occur in independent execution paths
429 Store => if self.found_assignment[local] {
430 self.can_const_prop[local] = false;
432 self.found_assignment[local] = true
434 // Reading constants is allowed an arbitrary number of times
436 StorageDead | StorageLive |
440 _ => self.can_const_prop[local] = false,
445 impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
448 constant: &Constant<'tcx>,
451 trace!("visit_constant: {:?}", constant);
452 self.super_constant(constant, location);
453 let source_info = *self.mir.source_info(location);
454 self.eval_constant(constant, source_info);
460 statement: &Statement<'tcx>,
463 trace!("visit_statement: {:?}", statement);
464 if let StatementKind::Assign(ref place, ref rval) = statement.kind {
466 .ty(&self.mir.local_decls, self.tcx)
468 if let Some(value) = self.const_prop(rval, place_ty, statement.source_info) {
469 if let Place::Local(local) = *place {
470 trace!("checking whether {:?} can be stored to {:?}", value, local);
471 if self.can_const_prop[local] {
472 trace!("storing {:?} to {:?}", value, local);
473 assert!(self.places[local].is_none());
474 self.places[local] = Some(value);
479 self.super_statement(block, statement, location);
482 fn visit_terminator_kind(
485 kind: &TerminatorKind<'tcx>,
488 self.super_terminator_kind(block, kind, location);
489 let source_info = *self.mir.source_info(location);
490 if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
491 if let Some(value) = self.eval_operand(cond, source_info) {
492 trace!("assertion on {:?} should be {:?}", value, expected);
493 if Value::Scalar(Scalar::from_bool(*expected)) != value.0 {
494 // poison all places this operand references so that further code
495 // doesn't use the invalid value
497 Operand::Move(ref place) | Operand::Copy(ref place) => {
498 let mut place = place;
499 while let Place::Projection(ref proj) = *place {
502 if let Place::Local(local) = *place {
503 self.places[local] = None;
506 Operand::Constant(_) => {}
508 let span = self.mir[block]
517 .as_local_node_id(self.source.def_id)
518 .expect("some part of a failing const eval must be local");
519 use rustc::mir::interpret::EvalErrorKind::*;
520 let msg = match msg {
524 RemainderByZero => msg.description().to_owned(),
525 BoundsCheck { ref len, ref index } => {
527 .eval_operand(len, source_info)
528 .expect("len must be const");
529 let len = match len.0 {
530 Value::Scalar(Scalar::Bits { bits, ..}) => bits,
531 _ => bug!("const len not primitive: {:?}", len),
534 .eval_operand(index, source_info)
535 .expect("index must be const");
536 let index = match index.0 {
537 Value::Scalar(Scalar::Bits { bits, .. }) => bits,
538 _ => bug!("const index not primitive: {:?}", index),
541 "index out of bounds: \
542 the len is {} but the index is {}",
547 // Need proper const propagator for these
551 ::rustc::lint::builtin::CONST_ERR,