src/librustc_mir/transform/instcombine.rs

   1 //! Performs various peephole optimizations.
   2
   3 use rustc::mir::{Constant, Location, Place, PlaceBase, Body, Operand, ProjectionElem, Rvalue,
   4     Local};
   5 use rustc::mir::visit::{MutVisitor, Visitor};
   6 use rustc::ty::{self, TyCtxt};
   7 use rustc::util::nodemap::{FxHashMap, FxHashSet};
   8 use rustc_data_structures::indexed_vec::Idx;
   9 use std::mem;
  10 use crate::transform::{MirPass, MirSource};
  11
  12 pub struct InstCombine;
  13
  14 impl MirPass for InstCombine {
  15     fn run_pass<'a, 'tcx>(&self,
  16                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  17                           _: MirSource<'tcx>,
  18                           body: &mut Body<'tcx>) {
  19         // We only run when optimizing MIR (at any level).
  20         if tcx.sess.opts.debugging_opts.mir_opt_level == 0 {
  21             return
  22         }
  23
  24         // First, find optimization opportunities. This is done in a pre-pass to keep the MIR
  25         // read-only so that we can do global analyses on the MIR in the process (e.g.
  26         // `Place::ty()`).
  27         let optimizations = {
  28             let mut optimization_finder = OptimizationFinder::new(body, tcx);
  29             optimization_finder.visit_body(body);
  30             optimization_finder.optimizations
  31         };
  32
  33         // Then carry out those optimizations.
  34         MutVisitor::visit_body(&mut InstCombineVisitor { optimizations }, body);
  35     }
  36 }
  37
  38 pub struct InstCombineVisitor<'tcx> {
  39     optimizations: OptimizationList<'tcx>,
  40 }
  41
  42 impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
  43     fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
  44         if self.optimizations.and_stars.remove(&location) {
  45             debug!("Replacing `&*`: {:?}", rvalue);
  46             let new_place = match *rvalue {
  47                 Rvalue::Ref(_, _, Place::Projection(ref mut projection)) => {
  48                     // Replace with dummy
  49                     mem::replace(&mut projection.base, Place::Base(PlaceBase::Local(Local::new(0))))
  50                 }
  51                 _ => bug!("Detected `&*` but didn't find `&*`!"),
  52             };
  53             *rvalue = Rvalue::Use(Operand::Copy(new_place))
  54         }
  55
  56         if let Some(constant) = self.optimizations.arrays_lengths.remove(&location) {
  57             debug!("Replacing `Len([_; N])`: {:?}", rvalue);
  58             *rvalue = Rvalue::Use(Operand::Constant(box constant));
  59         }
  60
  61         self.super_rvalue(rvalue, location)
  62     }
  63 }
  64
  65 /// Finds optimization opportunities on the MIR.
  66 struct OptimizationFinder<'b, 'a, 'tcx:'a+'b> {
  67     body: &'b Body<'tcx>,
  68     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  69     optimizations: OptimizationList<'tcx>,
  70 }
  71
  72 impl<'b, 'a, 'tcx:'b> OptimizationFinder<'b, 'a, 'tcx> {
  73     fn new(body: &'b Body<'tcx>, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> OptimizationFinder<'b, 'a, 'tcx> {
  74         OptimizationFinder {
  75             body,
  76             tcx,
  77             optimizations: OptimizationList::default(),
  78         }
  79     }
  80 }
  81
  82 impl<'b, 'a, 'tcx> Visitor<'tcx> for OptimizationFinder<'b, 'a, 'tcx> {
  83     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
  84         if let Rvalue::Ref(_, _, Place::Projection(ref projection)) = *rvalue {
  85             if let ProjectionElem::Deref = projection.elem {
  86                 if projection.base.ty(self.body, self.tcx).ty.is_region_ptr() {
  87                     self.optimizations.and_stars.insert(location);
  88                 }
  89             }
  90         }
  91
  92         if let Rvalue::Len(ref place) = *rvalue {
  93             let place_ty = place.ty(&self.body.local_decls, self.tcx).ty;
  94             if let ty::Array(_, len) = place_ty.sty {
  95                 let span = self.body.source_info(location).span;
  96                 let ty = self.tcx.types.usize;
  97                 let constant = Constant { span, ty, literal: len, user_ty: None };
  98                 self.optimizations.arrays_lengths.insert(location, constant);
  99             }
 100         }
 101
 102         self.super_rvalue(rvalue, location)
 103     }
 104 }
 105
 106 #[derive(Default)]
 107 struct OptimizationList<'tcx> {
 108     and_stars: FxHashSet<Location>,
 109     arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
 110 }