src/librustc_mir/transform/instcombine.rs

   1 //! Performs various peephole optimizations.
   2
   3 use crate::transform::{MirPass, MirSource};
   4 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
   5 use rustc_index::vec::Idx;
   6 use rustc_middle::mir::visit::{MutVisitor, Visitor};
   7 use rustc_middle::mir::{
   8     read_only, Body, BodyAndCache, Constant, Local, Location, Operand, Place, PlaceRef,
   9     ProjectionElem, Rvalue,
  10 };
  11 use rustc_middle::ty::{self, TyCtxt};
  12 use std::mem;
  13
  14 pub struct InstCombine;
  15
  16 impl<'tcx> MirPass<'tcx> for InstCombine {
  17     fn run_pass(&self, tcx: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut BodyAndCache<'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 read_only_cache = read_only!(body);
  28             let mut optimization_finder = OptimizationFinder::new(body, tcx);
  29             optimization_finder.visit_body(&read_only_cache);
  30             optimization_finder.optimizations
  31         };
  32
  33         // Then carry out those optimizations.
  34         MutVisitor::visit_body(&mut InstCombineVisitor { optimizations, tcx }, body);
  35     }
  36 }
  37
  38 pub struct InstCombineVisitor<'tcx> {
  39     optimizations: OptimizationList<'tcx>,
  40     tcx: TyCtxt<'tcx>,
  41 }
  42
  43 impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
  44     fn tcx(&self) -> TyCtxt<'tcx> {
  45         self.tcx
  46     }
  47
  48     fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
  49         if self.optimizations.and_stars.remove(&location) {
  50             debug!("replacing `&*`: {:?}", rvalue);
  51             let new_place = match rvalue {
  52                 Rvalue::Ref(_, _, place) => {
  53                     if let &[ref proj_l @ .., proj_r] = place.projection.as_ref() {
  54                         place.projection = self.tcx().intern_place_elems(&[proj_r]);
  55
  56                         Place {
  57                             // Replace with dummy
  58                             local: mem::replace(&mut place.local, Local::new(0)),
  59                             projection: self.tcx().intern_place_elems(proj_l),
  60                         }
  61                     } else {
  62                         unreachable!();
  63                     }
  64                 }
  65                 _ => bug!("Detected `&*` but didn't find `&*`!"),
  66             };
  67             *rvalue = Rvalue::Use(Operand::Copy(new_place))
  68         }
  69
  70         if let Some(constant) = self.optimizations.arrays_lengths.remove(&location) {
  71             debug!("replacing `Len([_; N])`: {:?}", rvalue);
  72             *rvalue = Rvalue::Use(Operand::Constant(box constant));
  73         }
  74
  75         self.super_rvalue(rvalue, location)
  76     }
  77 }
  78
  79 /// Finds optimization opportunities on the MIR.
  80 struct OptimizationFinder<'b, 'tcx> {
  81     body: &'b Body<'tcx>,
  82     tcx: TyCtxt<'tcx>,
  83     optimizations: OptimizationList<'tcx>,
  84 }
  85
  86 impl OptimizationFinder<'b, 'tcx> {
  87     fn new(body: &'b Body<'tcx>, tcx: TyCtxt<'tcx>) -> OptimizationFinder<'b, 'tcx> {
  88         OptimizationFinder { body, tcx, optimizations: OptimizationList::default() }
  89     }
  90 }
  91
  92 impl Visitor<'tcx> for OptimizationFinder<'b, 'tcx> {
  93     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
  94         if let Rvalue::Ref(_, _, place) = rvalue {
  95             if let PlaceRef { local, projection: &[ref proj_base @ .., ProjectionElem::Deref] } =
  96                 place.as_ref()
  97             {
  98                 if Place::ty_from(local, proj_base, self.body, self.tcx).ty.is_region_ptr() {
  99                     self.optimizations.and_stars.insert(location);
 100                 }
 101             }
 102         }
 103
 104         if let Rvalue::Len(ref place) = *rvalue {
 105             let place_ty = place.ty(&self.body.local_decls, self.tcx).ty;
 106             if let ty::Array(_, len) = place_ty.kind {
 107                 let span = self.body.source_info(location).span;
 108                 let constant = Constant { span, literal: len, user_ty: None };
 109                 self.optimizations.arrays_lengths.insert(location, constant);
 110             }
 111         }
 112
 113         self.super_rvalue(rvalue, location)
 114     }
 115 }
 116
 117 #[derive(Default)]
 118 struct OptimizationList<'tcx> {
 119     and_stars: FxHashSet<Location>,
 120     arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
 121 }