[MIR] Deaggregate structs to enable further optimizations

src/librustc_driver/driver.rs

@@ -994,6 +994,8 @@ pub fn phase_4_translate_to_llvm<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         passes.push_pass(box mir::transform::no_landing_pads::NoLandingPads);
         passes.push_pass(box mir::transform::simplify_cfg::SimplifyCfg::new("elaborate-drops"));
 
+        passes.push_pass(box mir::transform::deaggregator::Deaggregator);
+
         passes.push_pass(box mir::transform::add_call_guards::AddCallGuards);
         passes.push_pass(box mir::transform::dump_mir::Marker("PreTrans"));
 

src/librustc_mir/transform/deaggregator.rs

@@ -0,0 +1,111 @@
+// 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; },
+            _ => {}
+        };
+        if let MirSource::Fn(_) = source {} else { return; }
+
+        let mut curr: usize = 0;
+        for bb in mir.basic_blocks_mut() {
+            while let Some(idx) = get_aggregate_statement(curr, &bb.statements) {
+                // do the replacement
+                debug!("removing statement {:?}", idx);
+                let src_info = bb.statements[idx].source_info;
+                let mut suffix_stmts = bb.statements.split_off(idx);
+                let orig_stmt = suffix_stmts.remove(0);
+                let StatementKind::Assign(ref lhs, ref rhs) = orig_stmt.kind;
+                if let &Rvalue::Aggregate(ref agg_kind, ref operands) = rhs {
+                    if let &AggregateKind::Adt(adt_def, variant, substs) = agg_kind {
+                        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[variant].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();
+
+                            // if we handled enums:
+                            // let lhs_cast = if adt_def.variants.len() > 1 {
+                            //     Lvalue::Projection(Box::new(LvalueProjection {
+                            //         base: ai.lhs.clone(),
+                            //         elem: ProjectionElem::Downcast(ai.adt_def, ai.variant),
+                            //     }))
+                            // } else {
+                            //     lhs_cast
+                            // };
+
+                            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;
+        if let &Rvalue::Aggregate(ref kind, ref operands) = rhs {
+            if let &AggregateKind::Adt(adt_def, variant, _) = kind {
+                if operands.len() > 0 { // don't deaggregate ()
+                    if adt_def.variants.len() > 1 {
+                        // only deaggrate structs 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
+}

src/librustc_mir/transform/mod.rs

@@ -17,3 +17,4 @@ pub mod add_call_guards;
 pub mod promote_consts;
 pub mod qualify_consts;
 pub mod dump_mir;
+pub mod deaggregator;