src/librustc_mir/transform/deaggregator.rs

   1 // Copyright 2016 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use rustc::hir;
  12 use rustc::ty::TyCtxt;
  13 use rustc::mir::*;
  14 use rustc_data_structures::indexed_vec::Idx;
  15 use transform::{MirPass, MirSource};
  16
  17 pub struct Deaggregator;
  18
  19 impl MirPass for Deaggregator {
  20     fn run_pass<'a, 'tcx>(&self,
  21                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
  22                           source: MirSource,
  23                           mir: &mut Mir<'tcx>) {
  24         let node_path = tcx.item_path_str(source.def_id);
  25         debug!("running on: {:?}", node_path);
  26         // we only run when mir_opt_level > 2
  27         if tcx.sess.opts.debugging_opts.mir_opt_level <= 2 {
  28             return;
  29         }
  30
  31         // Don't run on constant MIR, because trans might not be able to
  32         // evaluate the modified MIR.
  33         // FIXME(eddyb) Remove check after miri is merged.
  34         let id = tcx.hir.as_local_node_id(source.def_id).unwrap();
  35         match (tcx.hir.body_owner_kind(id), source.promoted) {
  36             (hir::BodyOwnerKind::Fn, None) => {},
  37             _ => return
  38         }
  39         // In fact, we might not want to trigger in other cases.
  40         // Ex: when we could use SROA.  See issue #35259
  41
  42         for bb in mir.basic_blocks_mut() {
  43             let mut curr: usize = 0;
  44             while let Some(idx) = get_aggregate_statement_index(curr, &bb.statements) {
  45                 // do the replacement
  46                 debug!("removing statement {:?}", idx);
  47                 let src_info = bb.statements[idx].source_info;
  48                 let suffix_stmts = bb.statements.split_off(idx+1);
  49                 let orig_stmt = bb.statements.pop().unwrap();
  50                 let (lhs, rhs) = match orig_stmt.kind {
  51                     StatementKind::Assign(ref lhs, ref rhs) => (lhs, rhs),
  52                     _ => span_bug!(src_info.span, "expected assign, not {:?}", orig_stmt),
  53                 };
  54                 let (agg_kind, operands) = match rhs {
  55                     &Rvalue::Aggregate(ref agg_kind, ref operands) => (agg_kind, operands),
  56                     _ => span_bug!(src_info.span, "expected aggregate, not {:?}", rhs),
  57                 };
  58                 let (adt_def, variant, substs) = match **agg_kind {
  59                     AggregateKind::Adt(adt_def, variant, substs, None)
  60                         => (adt_def, variant, substs),
  61                     _ => span_bug!(src_info.span, "expected struct, not {:?}", rhs),
  62                 };
  63                 let n = bb.statements.len();
  64                 bb.statements.reserve(n + operands.len() + suffix_stmts.len());
  65                 for (i, op) in operands.iter().enumerate() {
  66                     let ref variant_def = adt_def.variants[variant];
  67                     let ty = variant_def.fields[i].ty(tcx, substs);
  68                     let rhs = Rvalue::Use(op.clone());
  69
  70                     let lhs_cast = if adt_def.variants.len() > 1 {
  71                         Lvalue::Projection(Box::new(LvalueProjection {
  72                             base: lhs.clone(),
  73                             elem: ProjectionElem::Downcast(adt_def, variant),
  74                         }))
  75                     } else {
  76                         lhs.clone()
  77                     };
  78
  79                     let lhs_proj = Lvalue::Projection(Box::new(LvalueProjection {
  80                         base: lhs_cast,
  81                         elem: ProjectionElem::Field(Field::new(i), ty),
  82                     }));
  83                     let new_statement = Statement {
  84                         source_info: src_info,
  85                         kind: StatementKind::Assign(lhs_proj, rhs),
  86                     };
  87                     debug!("inserting: {:?} @ {:?}", new_statement, idx + i);
  88                     bb.statements.push(new_statement);
  89                 }
  90
  91                 // if the aggregate was an enum, we need to set the discriminant
  92                 if adt_def.variants.len() > 1 {
  93                     let set_discriminant = Statement {
  94                         kind: StatementKind::SetDiscriminant {
  95                             lvalue: lhs.clone(),
  96                             variant_index: variant,
  97                         },
  98                         source_info: src_info,
  99                     };
 100                     bb.statements.push(set_discriminant);
 101                 };
 102
 103                 curr = bb.statements.len();
 104                 bb.statements.extend(suffix_stmts);
 105             }
 106         }
 107     }
 108 }
 109
 110 fn get_aggregate_statement_index<'a, 'tcx, 'b>(start: usize,
 111                                          statements: &Vec<Statement<'tcx>>)
 112                                          -> Option<usize> {
 113     for i in start..statements.len() {
 114         let ref statement = statements[i];
 115         let rhs = match statement.kind {
 116             StatementKind::Assign(_, ref rhs) => rhs,
 117             _ => continue,
 118         };
 119         let (kind, operands) = match rhs {
 120             &Rvalue::Aggregate(ref kind, ref operands) => (kind, operands),
 121             _ => continue,
 122         };
 123         let (adt_def, variant) = match **kind {
 124             AggregateKind::Adt(adt_def, variant, _, None) => (adt_def, variant),
 125             _ => continue,
 126         };
 127         if operands.len() == 0 {
 128             // don't deaggregate ()
 129             continue;
 130         }
 131         debug!("getting variant {:?}", variant);
 132         debug!("for adt_def {:?}", adt_def);
 133         return Some(i);
 134     };
 135     None
 136 }