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, ConstantKind, LocalDecls, Operand, Place, ProjectionElem, Rvalue,
   7     SourceInfo, Statement, StatementKind, Terminator, TerminatorKind, UnOp,
   8 };
   9 use rustc_middle::ty::{self, TyCtxt};
  10
  11 pub struct InstCombine;
  12
  13 impl<'tcx> MirPass<'tcx> for InstCombine {
  14     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
  15         sess.mir_opt_level() > 0
  16     }
  17
  18     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  19         let ctx = InstCombineContext { tcx, local_decls: &body.local_decls };
  20         for block in body.basic_blocks.as_mut() {
  21             for statement in block.statements.iter_mut() {
  22                 match statement.kind {
  23                     StatementKind::Assign(box (_place, ref mut rvalue)) => {
  24                         ctx.combine_bool_cmp(&statement.source_info, rvalue);
  25                         ctx.combine_ref_deref(&statement.source_info, rvalue);
  26                         ctx.combine_len(&statement.source_info, rvalue);
  27                     }
  28                     _ => {}
  29                 }
  30             }
  31
  32             ctx.combine_primitive_clone(
  33                 &mut block.terminator.as_mut().unwrap(),
  34                 &mut block.statements,
  35             );
  36         }
  37     }
  38 }
  39
  40 struct InstCombineContext<'tcx, 'a> {
  41     tcx: TyCtxt<'tcx>,
  42     local_decls: &'a LocalDecls<'tcx>,
  43 }
  44
  45 impl<'tcx> InstCombineContext<'tcx, '_> {
  46     fn should_combine(&self, source_info: &SourceInfo, rvalue: &Rvalue<'tcx>) -> bool {
  47         self.tcx.consider_optimizing(|| {
  48             format!("InstCombine - Rvalue: {:?} SourceInfo: {:?}", rvalue, source_info)
  49         })
  50     }
  51
  52     /// Transform boolean comparisons into logical operations.
  53     fn combine_bool_cmp(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
  54         match rvalue {
  55             Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), box (a, b)) => {
  56                 let new = match (op, self.try_eval_bool(a), self.try_eval_bool(b)) {
  57                     // Transform "Eq(a, true)" ==> "a"
  58                     (BinOp::Eq, _, Some(true)) => Some(Rvalue::Use(a.clone())),
  59
  60                     // Transform "Ne(a, false)" ==> "a"
  61                     (BinOp::Ne, _, Some(false)) => Some(Rvalue::Use(a.clone())),
  62
  63                     // Transform "Eq(true, b)" ==> "b"
  64                     (BinOp::Eq, Some(true), _) => Some(Rvalue::Use(b.clone())),
  65
  66                     // Transform "Ne(false, b)" ==> "b"
  67                     (BinOp::Ne, Some(false), _) => Some(Rvalue::Use(b.clone())),
  68
  69                     // Transform "Eq(false, b)" ==> "Not(b)"
  70                     (BinOp::Eq, Some(false), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),
  71
  72                     // Transform "Ne(true, b)" ==> "Not(b)"
  73                     (BinOp::Ne, Some(true), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),
  74
  75                     // Transform "Eq(a, false)" ==> "Not(a)"
  76                     (BinOp::Eq, _, Some(false)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),
  77
  78                     // Transform "Ne(a, true)" ==> "Not(a)"
  79                     (BinOp::Ne, _, Some(true)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),
  80
  81                     _ => None,
  82                 };
  83
  84                 if let Some(new) = new && self.should_combine(source_info, rvalue) {
  85                     *rvalue = new;
  86                 }
  87             }
  88
  89             _ => {}
  90         }
  91     }
  92
  93     fn try_eval_bool(&self, a: &Operand<'_>) -> Option<bool> {
  94         let a = a.constant()?;
  95         if a.literal.ty().is_bool() { a.literal.try_to_bool() } else { None }
  96     }
  97
  98     /// Transform "&(*a)" ==> "a".
  99     fn combine_ref_deref(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
 100         if let Rvalue::Ref(_, _, place) = rvalue {
 101             if let Some((base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
 102                 if let ty::Ref(_, _, Mutability::Not) =
 103                     base.ty(self.local_decls, self.tcx).ty.kind()
 104                 {
 105                     // The dereferenced place must have type `&_`, so that we don't copy `&mut _`.
 106                 } else {
 107                     return;
 108                 }
 109
 110                 if !self.should_combine(source_info, rvalue) {
 111                     return;
 112                 }
 113
 114                 *rvalue = Rvalue::Use(Operand::Copy(Place {
 115                     local: base.local,
 116                     projection: self.tcx.intern_place_elems(base.projection),
 117                 }));
 118             }
 119         }
 120     }
 121
 122     /// Transform "Len([_; N])" ==> "N".
 123     fn combine_len(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
 124         if let Rvalue::Len(ref place) = *rvalue {
 125             let place_ty = place.ty(self.local_decls, self.tcx).ty;
 126             if let ty::Array(_, len) = *place_ty.kind() {
 127                 if !self.should_combine(source_info, rvalue) {
 128                     return;
 129                 }
 130
 131                 let literal = ConstantKind::from_const(len, self.tcx);
 132                 let constant = Constant { span: source_info.span, literal, user_ty: None };
 133                 *rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
 134             }
 135         }
 136     }
 137
 138     fn combine_primitive_clone(
 139         &self,
 140         terminator: &mut Terminator<'tcx>,
 141         statements: &mut Vec<Statement<'tcx>>,
 142     ) {
 143         let TerminatorKind::Call { func, args, destination, target, .. } = &mut terminator.kind
 144         else { return };
 145
 146         // It's definitely not a clone if there are multiple arguments
 147         if args.len() != 1 {
 148             return;
 149         }
 150
 151         let Some(destination_block) = *target
 152         else { return };
 153
 154         // Only bother looking more if it's easy to know what we're calling
 155         let Some((fn_def_id, fn_substs)) = func.const_fn_def()
 156         else { return };
 157
 158         // Clone needs one subst, so we can cheaply rule out other stuff
 159         if fn_substs.len() != 1 {
 160             return;
 161         }
 162
 163         // These types are easily available from locals, so check that before
 164         // doing DefId lookups to figure out what we're actually calling.
 165         let arg_ty = args[0].ty(self.local_decls, self.tcx);
 166
 167         let ty::Ref(_region, inner_ty, Mutability::Not) = *arg_ty.kind()
 168         else { return };
 169
 170         if !inner_ty.is_trivially_pure_clone_copy() {
 171             return;
 172         }
 173
 174         let trait_def_id = self.tcx.trait_of_item(fn_def_id);
 175         if trait_def_id.is_none() || trait_def_id != self.tcx.lang_items().clone_trait() {
 176             return;
 177         }
 178
 179         if !self.tcx.consider_optimizing(|| {
 180             format!(
 181                 "InstCombine - Call: {:?} SourceInfo: {:?}",
 182                 (fn_def_id, fn_substs),
 183                 terminator.source_info
 184             )
 185         }) {
 186             return;
 187         }
 188
 189         let Some(arg_place) = args.pop().unwrap().place()
 190         else { return };
 191
 192         statements.push(Statement {
 193             source_info: terminator.source_info,
 194             kind: StatementKind::Assign(Box::new((
 195                 *destination,
 196                 Rvalue::Use(Operand::Copy(
 197                     arg_place.project_deeper(&[ProjectionElem::Deref], self.tcx),
 198                 )),
 199             ))),
 200         });
 201         terminator.kind = TerminatorKind::Goto { target: destination_block };
 202     }
 203 }