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, Literal, 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>> {
178 Literal::Value { value } => {
179 self.ecx.tcx.span = source_info.span;
180 match self.ecx.const_to_value(value.val) {
181 Ok(val) => Some((val, value.ty, c.span)),
183 let (stacktrace, span) = self.ecx.generate_stacktrace(None);
184 let err = ConstEvalErr {
190 self.tcx.at(source_info.span),
191 "could not evaluate constant",
197 // evaluate the promoted and replace the constant with the evaluated result
198 Literal::Promoted { index } => {
199 let generics = self.tcx.generics_of(self.source.def_id);
200 if generics.requires_monomorphization(self.tcx) {
201 // FIXME: can't handle code with generics
204 let substs = Substs::identity_for_item(self.tcx, self.source.def_id);
205 let instance = Instance::new(self.source.def_id, substs);
208 promoted: Some(index),
210 // cannot use `const_eval` here, because that would require having the MIR
211 // for the current function available, but we're producing said MIR right now
212 let (value, _, ty) = self.use_ecx(source_info, |this| {
213 eval_promoted(&mut this.ecx, cid, this.mir, this.param_env)
215 let val = (value, ty, c.span);
216 trace!("evaluated {:?} to {:?}", c, val);
222 fn eval_place(&mut self, place: &Place<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
224 Place::Local(loc) => self.places[loc].clone(),
225 Place::Projection(ref proj) => match proj.elem {
226 ProjectionElem::Field(field, _) => {
227 trace!("field proj on {:?}", proj.base);
228 let (base, ty, span) = self.eval_place(&proj.base, source_info)?;
229 let valty = self.use_ecx(source_info, |this| {
230 this.ecx.read_field(base, None, field, ty)
232 Some((valty.value, valty.ty, span))
240 fn eval_operand(&mut self, op: &Operand<'tcx>, source_info: SourceInfo) -> Option<Const<'tcx>> {
242 Operand::Constant(ref c) => self.eval_constant(c, source_info),
243 | Operand::Move(ref place)
244 | Operand::Copy(ref place) => self.eval_place(place, source_info),
250 rvalue: &Rvalue<'tcx>,
251 place_ty: ty::Ty<'tcx>,
252 source_info: SourceInfo,
253 ) -> Option<Const<'tcx>> {
254 let span = source_info.span;
256 // This branch exists for the sanity type check
257 Rvalue::Use(Operand::Constant(ref c)) => {
258 assert_eq!(c.ty, place_ty);
259 self.eval_constant(c, source_info)
261 Rvalue::Use(ref op) => {
262 self.eval_operand(op, source_info)
266 Rvalue::Aggregate(..) |
267 Rvalue::NullaryOp(NullOp::Box, _) |
268 Rvalue::Discriminant(..) => None,
270 Rvalue::Cast(kind, ref operand, _) => {
271 let (value, ty, span) = self.eval_operand(operand, source_info)?;
272 self.use_ecx(source_info, |this| {
273 let dest_ptr = this.ecx.alloc_ptr(place_ty)?;
274 let place_align = this.ecx.layout_of(place_ty)?.align;
275 let dest = ::interpret::Place::from_ptr(dest_ptr, place_align);
276 this.ecx.cast(ValTy { value, ty }, kind, place_ty, dest)?;
278 Value::ByRef(dest_ptr.into(), place_align),
285 // FIXME(oli-obk): evaluate static/constant slice lengths
286 Rvalue::Len(_) => None,
287 Rvalue::NullaryOp(NullOp::SizeOf, ty) => {
288 let param_env = self.tcx.param_env(self.source.def_id);
289 type_size_of(self.tcx, param_env, ty).map(|n| (
290 Value::Scalar(Scalar::Bits {
292 defined: self.tcx.data_layout.pointer_size.bits() as u8,
294 self.tcx.types.usize,
298 Rvalue::UnaryOp(op, ref arg) => {
299 let def_id = if self.tcx.is_closure(self.source.def_id) {
300 self.tcx.closure_base_def_id(self.source.def_id)
304 let generics = self.tcx.generics_of(def_id);
305 if generics.requires_monomorphization(self.tcx) {
306 // FIXME: can't handle code with generics
310 let val = self.eval_operand(arg, source_info)?;
311 let prim = self.use_ecx(source_info, |this| {
312 this.ecx.value_to_scalar(ValTy { value: val.0, ty: val.1 })
314 let val = self.use_ecx(source_info, |this| this.ecx.unary_op(op, prim, val.1))?;
315 Some((Value::Scalar(val), place_ty, span))
317 Rvalue::CheckedBinaryOp(op, ref left, ref right) |
318 Rvalue::BinaryOp(op, ref left, ref right) => {
319 trace!("rvalue binop {:?} for {:?} and {:?}", op, left, right);
320 let right = self.eval_operand(right, source_info)?;
321 let def_id = if self.tcx.is_closure(self.source.def_id) {
322 self.tcx.closure_base_def_id(self.source.def_id)
326 let generics = self.tcx.generics_of(def_id);
327 if generics.requires_monomorphization(self.tcx) {
328 // FIXME: can't handle code with generics
332 let r = self.use_ecx(source_info, |this| {
333 this.ecx.value_to_scalar(ValTy { value: right.0, ty: right.1 })
335 if op == BinOp::Shr || op == BinOp::Shl {
336 let left_ty = left.ty(self.mir, self.tcx);
339 .layout_of(self.param_env.and(left_ty))
343 let right_size = self.tcx.layout_of(self.param_env.and(right.1)).unwrap().size;
344 if r.to_bits(right_size).ok().map_or(false, |b| b >= left_bits as u128) {
345 let source_scope_local_data = match self.mir.source_scope_local_data {
346 ClearCrossCrate::Set(ref data) => data,
347 ClearCrossCrate::Clear => return None,
349 let dir = if op == BinOp::Shr {
354 let node_id = source_scope_local_data[source_info.scope].lint_root;
356 ::rustc::lint::builtin::EXCEEDING_BITSHIFTS,
359 &format!("attempt to shift {} with overflow", dir));
363 let left = self.eval_operand(left, source_info)?;
364 let l = self.use_ecx(source_info, |this| {
365 this.ecx.value_to_scalar(ValTy { value: left.0, ty: left.1 })
367 trace!("const evaluating {:?} for {:?} and {:?}", op, left, right);
368 let (val, overflow) = self.use_ecx(source_info, |this| {
369 this.ecx.binary_op(op, l, left.1, r, right.1)
371 let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
374 Scalar::from_bool(overflow),
378 let err = EvalErrorKind::Overflow(op).into();
379 let _: Option<()> = self.use_ecx(source_info, |_| Err(err));
384 Some((val, place_ty, span))
390 fn type_size_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
391 param_env: ty::ParamEnv<'tcx>,
392 ty: ty::Ty<'tcx>) -> Option<u64> {
393 tcx.layout_of(param_env.and(ty)).ok().map(|layout| layout.size.bytes())
396 struct CanConstProp {
397 can_const_prop: IndexVec<Local, bool>,
398 // false at the beginning, once set, there are not allowed to be any more assignments
399 found_assignment: IndexVec<Local, bool>,
403 /// returns true if `local` can be propagated
404 fn check(mir: &Mir) -> IndexVec<Local, bool> {
405 let mut cpv = CanConstProp {
406 can_const_prop: IndexVec::from_elem(true, &mir.local_decls),
407 found_assignment: IndexVec::from_elem(false, &mir.local_decls),
409 for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
410 // cannot use args at all
411 // cannot use locals because if x < y { y - x } else { x - y } would
413 // FIXME(oli-obk): lint variables until they are used in a condition
414 // FIXME(oli-obk): lint if return value is constant
415 *val = mir.local_kind(local) == LocalKind::Temp;
422 impl<'tcx> Visitor<'tcx> for CanConstProp {
426 context: PlaceContext<'tcx>,
429 use rustc::mir::visit::PlaceContext::*;
431 // Constants must have at most one write
432 // FIXME(oli-obk): we could be more powerful here, if the multiple writes
433 // only occur in independent execution paths
434 Store => if self.found_assignment[local] {
435 self.can_const_prop[local] = false;
437 self.found_assignment[local] = true
439 // Reading constants is allowed an arbitrary number of times
441 StorageDead | StorageLive |
445 _ => self.can_const_prop[local] = false,
450 impl<'b, 'a, 'tcx> Visitor<'tcx> for ConstPropagator<'b, 'a, 'tcx> {
453 constant: &Constant<'tcx>,
456 trace!("visit_constant: {:?}", constant);
457 self.super_constant(constant, location);
458 let source_info = *self.mir.source_info(location);
459 self.eval_constant(constant, source_info);
465 statement: &Statement<'tcx>,
468 trace!("visit_statement: {:?}", statement);
469 if let StatementKind::Assign(ref place, ref rval) = statement.kind {
471 .ty(&self.mir.local_decls, self.tcx)
473 if let Some(value) = self.const_prop(rval, place_ty, statement.source_info) {
474 if let Place::Local(local) = *place {
475 trace!("checking whether {:?} can be stored to {:?}", value, local);
476 if self.can_const_prop[local] {
477 trace!("storing {:?} to {:?}", value, local);
478 assert!(self.places[local].is_none());
479 self.places[local] = Some(value);
484 self.super_statement(block, statement, location);
487 fn visit_terminator_kind(
490 kind: &TerminatorKind<'tcx>,
493 self.super_terminator_kind(block, kind, location);
494 let source_info = *self.mir.source_info(location);
495 if let TerminatorKind::Assert { expected, msg, cond, .. } = kind {
496 if let Some(value) = self.eval_operand(cond, source_info) {
497 trace!("assertion on {:?} should be {:?}", value, expected);
498 if Value::Scalar(Scalar::from_bool(*expected)) != value.0 {
499 // poison all places this operand references so that further code
500 // doesn't use the invalid value
502 Operand::Move(ref place) | Operand::Copy(ref place) => {
503 let mut place = place;
504 while let Place::Projection(ref proj) = *place {
507 if let Place::Local(local) = *place {
508 self.places[local] = None;
511 Operand::Constant(_) => {}
513 let span = self.mir[block]
522 .as_local_node_id(self.source.def_id)
523 .expect("some part of a failing const eval must be local");
524 use rustc::mir::interpret::EvalErrorKind::*;
525 let msg = match msg {
529 RemainderByZero => msg.description().to_owned(),
530 BoundsCheck { ref len, ref index } => {
532 .eval_operand(len, source_info)
533 .expect("len must be const");
534 let len = match len.0 {
535 Value::Scalar(Scalar::Bits { bits, ..}) => bits,
536 _ => bug!("const len not primitive: {:?}", len),
539 .eval_operand(index, source_info)
540 .expect("index must be const");
541 let index = match index.0 {
542 Value::Scalar(Scalar::Bits { bits, .. }) => bits,
543 _ => bug!("const index not primitive: {:?}", index),
546 "index out of bounds: \
547 the len is {} but the index is {}",
552 // Need proper const propagator for these
556 ::rustc::lint::builtin::CONST_ERR,