src/librustc_mir/transform/instcombine.rs

   1 // Copyright 2016 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.
   4 //
   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.
  10
  11 //! Performs various peephole optimizations.
  12
  13 use rustc::mir::{Constant, Location, Place, Mir, Operand, ProjectionElem, Rvalue, Local};
  14 use rustc::mir::visit::{MutVisitor, Visitor};
  15 use rustc::ty::{TyCtxt, TypeVariants};
  16 use rustc::util::nodemap::{FxHashMap, FxHashSet};
  17 use rustc_data_structures::indexed_vec::Idx;
  18 use std::mem;
  19 use transform::{MirPass, MirSource};
  20
  21 pub struct InstCombine;
  22
  23 impl MirPass for InstCombine {
  24     fn run_pass<'a, 'tcx>(&self,
  25                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  26                           _: MirSource,
  27                           mir: &mut Mir<'tcx>) {
  28         // We only run when optimizing MIR (at any level).
  29         if tcx.sess.opts.debugging_opts.mir_opt_level == 0 {
  30             return
  31         }
  32
  33         // First, find optimization opportunities. This is done in a pre-pass to keep the MIR
  34         // read-only so that we can do global analyses on the MIR in the process (e.g.
  35         // `Place::ty()`).
  36         let optimizations = {
  37             let mut optimization_finder = OptimizationFinder::new(mir, tcx);
  38             optimization_finder.visit_mir(mir);
  39             optimization_finder.optimizations
  40         };
  41
  42         // Then carry out those optimizations.
  43         MutVisitor::visit_mir(&mut InstCombineVisitor { optimizations }, mir);
  44     }
  45 }
  46
  47 pub struct InstCombineVisitor<'tcx> {
  48     optimizations: OptimizationList<'tcx>,
  49 }
  50
  51 impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
  52     fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
  53         if self.optimizations.and_stars.remove(&location) {
  54             debug!("Replacing `&*`: {:?}", rvalue);
  55             let new_place = match *rvalue {
  56                 Rvalue::Ref(_, _, Place::Projection(ref mut projection)) => {
  57                     // Replace with dummy
  58                     mem::replace(&mut projection.base, Place::Local(Local::new(0)))
  59                 }
  60                 _ => bug!("Detected `&*` but didn't find `&*`!"),
  61             };
  62             *rvalue = Rvalue::Use(Operand::Copy(new_place))
  63         }
  64
  65         if let Some(constant) = self.optimizations.arrays_lengths.remove(&location) {
  66             debug!("Replacing `Len([_; N])`: {:?}", rvalue);
  67             *rvalue = Rvalue::Use(Operand::Constant(box constant));
  68         }
  69
  70         self.super_rvalue(rvalue, location)
  71     }
  72 }
  73
  74 /// Finds optimization opportunities on the MIR.
  75 struct OptimizationFinder<'b, 'a, 'tcx:'a+'b> {
  76     mir: &'b Mir<'tcx>,
  77     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  78     optimizations: OptimizationList<'tcx>,
  79 }
  80
  81 impl<'b, 'a, 'tcx:'b> OptimizationFinder<'b, 'a, 'tcx> {
  82     fn new(mir: &'b Mir<'tcx>, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> OptimizationFinder<'b, 'a, 'tcx> {
  83         OptimizationFinder {
  84             mir,
  85             tcx,
  86             optimizations: OptimizationList::default(),
  87         }
  88     }
  89 }
  90
  91 impl<'b, 'a, 'tcx> Visitor<'tcx> for OptimizationFinder<'b, 'a, 'tcx> {
  92     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
  93         if let Rvalue::Ref(_, _, Place::Projection(ref projection)) = *rvalue {
  94             if let ProjectionElem::Deref = projection.elem {
  95                 if projection.base.ty(self.mir, self.tcx).to_ty(self.tcx).is_region_ptr() {
  96                     self.optimizations.and_stars.insert(location);
  97                 }
  98             }
  99         }
 100
 101         if let Rvalue::Len(ref place) = *rvalue {
 102             let place_ty = place.ty(&self.mir.local_decls, self.tcx).to_ty(self.tcx);
 103             if let TypeVariants::TyArray(_, len) = place_ty.sty {
 104                 let span = self.mir.source_info(location).span;
 105                 let ty = self.tcx.types.usize;
 106                 let constant = Constant { span, ty, literal: len };
 107                 self.optimizations.arrays_lengths.insert(location, constant);
 108             }
 109         }
 110
 111         self.super_rvalue(rvalue, location)
 112     }
 113 }
 114
 115 #[derive(Default)]
 116 struct OptimizationList<'tcx> {
 117     and_stars: FxHashSet<Location>,
 118     arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
 119 }