src/librustc_mir/transform/instcombine.rs

   1 //! Performs various peephole optimizations.
   2
   3 use rustc::mir::{Constant, Location, Place, PlaceBase, Mir, Operand, ProjectionElem, Rvalue, Local};
   4 use rustc::mir::visit::{MutVisitor, Visitor};
   5 use rustc::ty::{self, TyCtxt};
   6 use rustc::util::nodemap::{FxHashMap, FxHashSet};
   7 use rustc_data_structures::indexed_vec::Idx;
   8 use std::mem;
   9 use crate::transform::{MirPass, MirSource};
  10
  11 pub struct InstCombine;
  12
  13 impl MirPass for InstCombine {
  14     fn run_pass<'a, 'tcx>(&self,
  15                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  16                           _: MirSource<'tcx>,
  17                           mir: &mut Mir<'tcx>) {
  18         // We only run when optimizing MIR (at any level).
  19         if tcx.sess.opts.debugging_opts.mir_opt_level == 0 {
  20             return
  21         }
  22
  23         // First, find optimization opportunities. This is done in a pre-pass to keep the MIR
  24         // read-only so that we can do global analyses on the MIR in the process (e.g.
  25         // `Place::ty()`).
  26         let optimizations = {
  27             let mut optimization_finder = OptimizationFinder::new(mir, tcx);
  28             optimization_finder.visit_mir(mir);
  29             optimization_finder.optimizations
  30         };
  31
  32         // Then carry out those optimizations.
  33         MutVisitor::visit_mir(&mut InstCombineVisitor { optimizations }, mir);
  34     }
  35 }
  36
  37 pub struct InstCombineVisitor<'tcx> {
  38     optimizations: OptimizationList<'tcx>,
  39 }
  40
  41 impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
  42     fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
  43         if self.optimizations.and_stars.remove(&location) {
  44             debug!("Replacing `&*`: {:?}", rvalue);
  45             let new_place = match *rvalue {
  46                 Rvalue::Ref(_, _, Place::Projection(ref mut projection)) => {
  47                     // Replace with dummy
  48                     mem::replace(&mut projection.base, Place::Base(PlaceBase::Local(Local::new(0))))
  49                 }
  50                 _ => bug!("Detected `&*` but didn't find `&*`!"),
  51             };
  52             *rvalue = Rvalue::Use(Operand::Copy(new_place))
  53         }
  54
  55         if let Some(constant) = self.optimizations.arrays_lengths.remove(&location) {
  56             debug!("Replacing `Len([_; N])`: {:?}", rvalue);
  57             *rvalue = Rvalue::Use(Operand::Constant(box constant));
  58         }
  59
  60         self.super_rvalue(rvalue, location)
  61     }
  62 }
  63
  64 /// Finds optimization opportunities on the MIR.
  65 struct OptimizationFinder<'b, 'a, 'tcx:'a+'b> {
  66     mir: &'b Mir<'tcx>,
  67     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  68     optimizations: OptimizationList<'tcx>,
  69 }
  70
  71 impl<'b, 'a, 'tcx:'b> OptimizationFinder<'b, 'a, 'tcx> {
  72     fn new(mir: &'b Mir<'tcx>, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> OptimizationFinder<'b, 'a, 'tcx> {
  73         OptimizationFinder {
  74             mir,
  75             tcx,
  76             optimizations: OptimizationList::default(),
  77         }
  78     }
  79 }
  80
  81 impl<'b, 'a, 'tcx> Visitor<'tcx> for OptimizationFinder<'b, 'a, 'tcx> {
  82     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
  83         if let Rvalue::Ref(_, _, Place::Projection(ref projection)) = *rvalue {
  84             if let ProjectionElem::Deref = projection.elem {
  85                 if projection.base.ty(self.mir, self.tcx).ty.is_region_ptr() {
  86                     self.optimizations.and_stars.insert(location);
  87                 }
  88             }
  89         }
  90
  91         if let Rvalue::Len(ref place) = *rvalue {
  92             let place_ty = place.ty(&self.mir.local_decls, self.tcx).ty;
  93             if let ty::Array(_, len) = place_ty.sty {
  94                 let span = self.mir.source_info(location).span;
  95                 let ty = self.tcx.types.usize;
  96                 let constant = Constant { span, ty, literal: len, user_ty: None };
  97                 self.optimizations.arrays_lengths.insert(location, constant);
  98             }
  99         }
 100
 101         self.super_rvalue(rvalue, location)
 102     }
 103 }
 104
 105 #[derive(Default)]
 106 struct OptimizationList<'tcx> {
 107     and_stars: FxHashSet<Location>,
 108     arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
 109 }