src/librustc_mir/transform/deaggregator.rs

// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use rustc::ty::TyCtxt;
use rustc::mir::repr::*;
use rustc::mir::transform::{MirPass, MirSource, Pass};
use rustc_data_structures::indexed_vec::Idx;
use rustc::ty::VariantKind;

pub struct Deaggregator;

impl Pass for Deaggregator {}

impl<'tcx> MirPass<'tcx> for Deaggregator {
    fn run_pass<'a>(&mut self, tcx: TyCtxt<'a, 'tcx, 'tcx>,
                    source: MirSource, mir: &mut Mir<'tcx>) {
        let node_id = source.item_id();
        let node_path = tcx.item_path_str(tcx.map.local_def_id(node_id));
        debug!("running on: {:?}", node_path);
        // we only run when mir_opt_level > 1
        match tcx.sess.opts.debugging_opts.mir_opt_level {
            Some(0) |
            Some(1) |
            None => { return; },
            _ => {}
        };

        // Do not trigger on constants.  Could be revised in future
        if let MirSource::Fn(_) = source {} else { return; }
        // In fact, we might not want to trigger in other cases.
        // Ex: when we could use SROA.  See issue #35259

        let mut curr: usize = 0;
        for bb in mir.basic_blocks_mut() {
            let idx = match get_aggregate_statement(curr, &bb.statements) {
                Some(idx) => idx,
                None => continue,
            };
            // do the replacement
            debug!("removing statement {:?}", idx);
            let src_info = bb.statements[idx].source_info;
            let suffix_stmts = bb.statements.split_off(idx+1);
            let orig_stmt = bb.statements.pop().unwrap();
            let StatementKind::Assign(ref lhs, ref rhs) = orig_stmt.kind;
            let (agg_kind, operands) = match rhs {
                &Rvalue::Aggregate(ref agg_kind, ref operands) => (agg_kind, operands),
                _ => span_bug!(src_info.span, "expected aggregate, not {:?}", rhs),
            };
            let (adt_def, variant, substs) = match agg_kind {
                &AggregateKind::Adt(adt_def, variant, substs) => (adt_def, variant, substs),
                _ => span_bug!(src_info.span, "expected struct, not {:?}", rhs),
            };
            let n = bb.statements.len();
            bb.statements.reserve(n + operands.len() + suffix_stmts.len());
            for (i, op) in operands.iter().enumerate() {
                let ref variant_def = adt_def.variants[variant];
                let ty = variant_def.fields[i].ty(tcx, substs);
                let rhs = Rvalue::Use(op.clone());

                // since we don't handle enums, we don't need a cast
                let lhs_cast = lhs.clone();

                // FIXME we cannot deaggregate enums issue: #35186

                let lhs_proj = Lvalue::Projection(Box::new(LvalueProjection {
                    base: lhs_cast,
                    elem: ProjectionElem::Field(Field::new(i), ty),
                }));
                let new_statement = Statement {
                    source_info: src_info,
                    kind: StatementKind::Assign(lhs_proj, rhs),
                };
                debug!("inserting: {:?} @ {:?}", new_statement, idx + i);
                bb.statements.push(new_statement);
            }
            curr = bb.statements.len();
            bb.statements.extend(suffix_stmts);
        }
    }
}

fn get_aggregate_statement<'a, 'tcx, 'b>(curr: usize,
                                         statements: &Vec<Statement<'tcx>>)
                                         -> Option<usize> {
    for i in curr..statements.len() {
        let ref statement = statements[i];
        let StatementKind::Assign(_, ref rhs) = statement.kind;
        let (kind, operands) = match rhs {
            &Rvalue::Aggregate(ref kind, ref operands) => (kind, operands),
            _ => continue,
        };
        let (adt_def, variant) = match kind {
            &AggregateKind::Adt(adt_def, variant, _) => (adt_def, variant),
            _ => continue,
        };
        if operands.len() == 0 || adt_def.variants.len() > 1 {
            // don't deaggregate ()
            // don't deaggregate enums ... for now
            continue;
        }
        debug!("getting variant {:?}", variant);
        debug!("for adt_def {:?}", adt_def);
        let variant_def = &adt_def.variants[variant];
        if variant_def.kind == VariantKind::Struct {
            return Some(i);
        }
    };
    None
}