compiler/rustc_mir_transform/src/instcombine.rs

   1 //! Performs various peephole optimizations.
   2
   3 use crate::MirPass;
   4 use rustc_hir::Mutability;
   5 use rustc_middle::mir::{
   6     BinOp, Body, Constant, LocalDecls, Operand, Place, ProjectionElem, Rvalue, SourceInfo,
   7     StatementKind, UnOp,
   8 };
   9 use rustc_middle::ty::{self, TyCtxt};
  10
  11 pub struct InstCombine;
  12
  13 impl<'tcx> MirPass<'tcx> for InstCombine {
  14     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  15         let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
  16         let ctx = InstCombineContext { tcx, local_decls };
  17         for block in basic_blocks.iter_mut() {
  18             for statement in block.statements.iter_mut() {
  19                 match statement.kind {
  20                     StatementKind::Assign(box (_place, ref mut rvalue)) => {
  21                         ctx.combine_bool_cmp(&statement.source_info, rvalue);
  22                         ctx.combine_ref_deref(&statement.source_info, rvalue);
  23                         ctx.combine_len(&statement.source_info, rvalue);
  24                     }
  25                     _ => {}
  26                 }
  27             }
  28         }
  29     }
  30 }
  31
  32 struct InstCombineContext<'tcx, 'a> {
  33     tcx: TyCtxt<'tcx>,
  34     local_decls: &'a LocalDecls<'tcx>,
  35 }
  36
  37 impl<'tcx, 'a> InstCombineContext<'tcx, 'a> {
  38     fn should_combine(&self, source_info: &SourceInfo, rvalue: &Rvalue<'tcx>) -> bool {
  39         self.tcx.consider_optimizing(|| {
  40             format!("InstCombine - Rvalue: {:?} SourceInfo: {:?}", rvalue, source_info)
  41         })
  42     }
  43
  44     /// Transform boolean comparisons into logical operations.
  45     fn combine_bool_cmp(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
  46         match rvalue {
  47             Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), box (a, b)) => {
  48                 let new = match (op, self.try_eval_bool(a), self.try_eval_bool(b)) {
  49                     // Transform "Eq(a, true)" ==> "a"
  50                     (BinOp::Eq, _, Some(true)) => Some(Rvalue::Use(a.clone())),
  51
  52                     // Transform "Ne(a, false)" ==> "a"
  53                     (BinOp::Ne, _, Some(false)) => Some(Rvalue::Use(a.clone())),
  54
  55                     // Transform "Eq(true, b)" ==> "b"
  56                     (BinOp::Eq, Some(true), _) => Some(Rvalue::Use(b.clone())),
  57
  58                     // Transform "Ne(false, b)" ==> "b"
  59                     (BinOp::Ne, Some(false), _) => Some(Rvalue::Use(b.clone())),
  60
  61                     // Transform "Eq(false, b)" ==> "Not(b)"
  62                     (BinOp::Eq, Some(false), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),
  63
  64                     // Transform "Ne(true, b)" ==> "Not(b)"
  65                     (BinOp::Ne, Some(true), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),
  66
  67                     // Transform "Eq(a, false)" ==> "Not(a)"
  68                     (BinOp::Eq, _, Some(false)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),
  69
  70                     // Transform "Ne(a, true)" ==> "Not(a)"
  71                     (BinOp::Ne, _, Some(true)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),
  72
  73                     _ => None,
  74                 };
  75
  76                 if let Some(new) = new {
  77                     if self.should_combine(source_info, rvalue) {
  78                         *rvalue = new;
  79                     }
  80                 }
  81             }
  82
  83             _ => {}
  84         }
  85     }
  86
  87     fn try_eval_bool(&self, a: &Operand<'_>) -> Option<bool> {
  88         let a = a.constant()?;
  89         if a.literal.ty().is_bool() { a.literal.try_to_bool() } else { None }
  90     }
  91
  92     /// Transform "&(*a)" ==> "a".
  93     fn combine_ref_deref(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
  94         if let Rvalue::Ref(_, _, place) = rvalue {
  95             if let Some((base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
  96                 if let ty::Ref(_, _, Mutability::Not) =
  97                     base.ty(self.local_decls, self.tcx).ty.kind()
  98                 {
  99                     // The dereferenced place must have type `&_`, so that we don't copy `&mut _`.
 100                 } else {
 101                     return;
 102                 }
 103
 104                 if !self.should_combine(source_info, rvalue) {
 105                     return;
 106                 }
 107
 108                 *rvalue = Rvalue::Use(Operand::Copy(Place {
 109                     local: base.local,
 110                     projection: self.tcx.intern_place_elems(base.projection),
 111                 }));
 112             }
 113         }
 114     }
 115
 116     /// Transform "Len([_; N])" ==> "N".
 117     fn combine_len(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
 118         if let Rvalue::Len(ref place) = *rvalue {
 119             let place_ty = place.ty(self.local_decls, self.tcx).ty;
 120             if let ty::Array(_, len) = *place_ty.kind() {
 121                 if !self.should_combine(source_info, rvalue) {
 122                     return;
 123                 }
 124
 125                 let constant =
 126                     Constant { span: source_info.span, literal: len.into(), user_ty: None };
 127                 *rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
 128             }
 129         }
 130     }
 131 }