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librustc_mir: Remove `&*x` when `x` has a reference type.

This introduces a new `InstCombine` pass for us to place such peephole
optimizations.
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pcwalton committed Sep 16, 2016
1 parent 8394685 commit e8a44d29b633412b4173be8f1bfb6dae7ac3c45e
Showing with 115 additions and 0 deletions.
  1. +3 −0 src/librustc_driver/driver.rs
  2. +110 −0 src/librustc_mir/transform/instcombine.rs
  3. +2 −0 src/librustc_mir/transform/mod.rs
@@ -1017,13 +1017,16 @@ 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("no-landing-pads"));

// From here on out, regions are gone.
passes.push_pass(box mir::transform::erase_regions::EraseRegions);

passes.push_pass(box mir::transform::add_call_guards::AddCallGuards);
passes.push_pass(box borrowck::ElaborateDrops);
passes.push_pass(box mir::transform::no_landing_pads::NoLandingPads);
passes.push_pass(box mir::transform::simplify_cfg::SimplifyCfg::new("elaborate-drops"));

// No lifetime analysis based on borrowing can be done from here on out.
passes.push_pass(box mir::transform::instcombine::InstCombine::new());
passes.push_pass(box mir::transform::deaggregator::Deaggregator);

passes.push_pass(box mir::transform::add_call_guards::AddCallGuards);
@@ -0,0 +1,110 @@
// 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.

//! Performs various peephole optimizations.

use rustc::mir::repr::{Location, Lvalue, Mir, Operand, ProjectionElem, Rvalue};
use rustc::mir::transform::{MirPass, MirSource, Pass};
use rustc::mir::visit::{MutVisitor, Visitor};
use rustc::ty::TyCtxt;
use rustc::util::nodemap::FnvHashSet;
use std::mem;

pub struct InstCombine {
optimizations: OptimizationList,
}

impl InstCombine {
pub fn new() -> InstCombine {
InstCombine {
optimizations: OptimizationList::default(),
}
}
}

impl Pass for InstCombine {}

impl<'tcx> MirPass<'tcx> for InstCombine {
fn run_pass<'a>(&mut self,
tcx: TyCtxt<'a, 'tcx, 'tcx>,
_: MirSource,
mir: &mut Mir<'tcx>) {
// We only run when optimizing MIR (at any level).
if tcx.sess.opts.debugging_opts.mir_opt_level == Some(0) {
return
}

// First, find optimization opportunities. This is done in a pre-pass to keep the MIR
// read-only so that we can do global analyses on the MIR in the process (e.g.
// `Lvalue::ty()`).
{
let mut optimization_finder = OptimizationFinder::new(mir, tcx);
optimization_finder.visit_mir(mir);
self.optimizations = optimization_finder.optimizations
}

// Then carry out those optimizations.
MutVisitor::visit_mir(&mut *self, mir);
}
}

impl<'tcx> MutVisitor<'tcx> for InstCombine {
fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
if self.optimizations.and_stars.remove(&location) {
debug!("Replacing `&*`: {:?}", rvalue);
let new_lvalue = match *rvalue {
Rvalue::Ref(_, _, Lvalue::Projection(ref mut projection)) => {
mem::replace(&mut projection.base, Lvalue::ReturnPointer)
}
_ => bug!("Detected `&*` but didn't find `&*`!"),
};
*rvalue = Rvalue::Use(Operand::Consume(new_lvalue))
}

self.super_rvalue(rvalue, location)
}
}

/// Finds optimization opportunities on the MIR.
struct OptimizationFinder<'b, 'a, 'tcx:'a+'b> {
mir: &'b Mir<'tcx>,
tcx: TyCtxt<'a, 'tcx, 'tcx>,
optimizations: OptimizationList,
}

impl<'b, 'a, 'tcx:'b> OptimizationFinder<'b, 'a, 'tcx> {
fn new(mir: &'b Mir<'tcx>, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> OptimizationFinder<'b, 'a, 'tcx> {
OptimizationFinder {
mir: mir,
tcx: tcx,
optimizations: OptimizationList::default(),
}
}
}

impl<'b, 'a, 'tcx> Visitor<'tcx> for OptimizationFinder<'b, 'a, 'tcx> {
fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
if let Rvalue::Ref(_, _, Lvalue::Projection(ref projection)) = *rvalue {
if let ProjectionElem::Deref = projection.elem {
if projection.base.ty(self.mir, self.tcx).to_ty(self.tcx).is_region_ptr() {
self.optimizations.and_stars.insert(location);
}
}
}

self.super_rvalue(rvalue, location)
}
}

#[derive(Default)]
struct OptimizationList {
and_stars: FnvHashSet<Location>,
}

@@ -18,3 +18,5 @@ pub mod promote_consts;
pub mod qualify_consts;
pub mod dump_mir;
pub mod deaggregator;
pub mod instcombine;

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