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|
//! Performs various peephole optimizations.
use crate::transform::{MirPass, MirSource};
use rustc_data_structures::fx::FxHashMap;
use rustc_index::vec::Idx;
use rustc_middle::mir::visit::{MutVisitor, Visitor};
use rustc_middle::mir::{
Body, Constant, Local, Location, Mutability, Operand, Place, PlaceRef, ProjectionElem, Rvalue,
};
use rustc_middle::ty::{self, TyCtxt};
use std::mem;
pub struct InstCombine;
impl<'tcx> MirPass<'tcx> for InstCombine {
fn run_pass(&self, tcx: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut Body<'tcx>) {
// 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.
// `Place::ty()`).
let optimizations = {
let mut optimization_finder = OptimizationFinder::new(body, tcx);
optimization_finder.visit_body(body);
optimization_finder.optimizations
};
// Then carry out those optimizations.
MutVisitor::visit_body(&mut InstCombineVisitor { optimizations, tcx }, body);
}
}
pub struct InstCombineVisitor<'tcx> {
optimizations: OptimizationList<'tcx>,
tcx: TyCtxt<'tcx>,
}
impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
fn tcx(&self) -> TyCtxt<'tcx> {
self.tcx
}
fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
if let Some(mtbl) = self.optimizations.and_stars.remove(&location) {
debug!("replacing `&*`: {:?}", rvalue);
let new_place = match rvalue {
Rvalue::Ref(_, _, place) => {
if let &[ref proj_l @ .., proj_r] = place.projection.as_ref() {
place.projection = self.tcx().intern_place_elems(&[proj_r]);
Place {
// Replace with dummy
local: mem::replace(&mut place.local, Local::new(0)),
projection: self.tcx().intern_place_elems(proj_l),
}
} else {
unreachable!();
}
}
_ => bug!("Detected `&*` but didn't find `&*`!"),
};
*rvalue = Rvalue::Use(match mtbl {
Mutability::Mut => Operand::Move(new_place),
Mutability::Not => Operand::Copy(new_place),
});
}
if let Some(constant) = self.optimizations.arrays_lengths.remove(&location) {
debug!("replacing `Len([_; N])`: {:?}", rvalue);
*rvalue = Rvalue::Use(Operand::Constant(box constant));
}
self.super_rvalue(rvalue, location)
}
}
/// Finds optimization opportunities on the MIR.
struct OptimizationFinder<'b, 'tcx> {
body: &'b Body<'tcx>,
tcx: TyCtxt<'tcx>,
optimizations: OptimizationList<'tcx>,
}
impl OptimizationFinder<'b, 'tcx> {
fn new(body: &'b Body<'tcx>, tcx: TyCtxt<'tcx>) -> OptimizationFinder<'b, 'tcx> {
OptimizationFinder { body, tcx, optimizations: OptimizationList::default() }
}
}
impl Visitor<'tcx> for OptimizationFinder<'b, 'tcx> {
fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
if let Rvalue::Ref(_, _, place) = rvalue {
if let PlaceRef { local, projection: &[ref proj_base @ .., ProjectionElem::Deref] } =
place.as_ref()
{
// The dereferenced place must have type `&_`.
let ty = Place::ty_from(local, proj_base, self.body, self.tcx).ty;
if let ty::Ref(_, _, mtbl) = ty.kind {
self.optimizations.and_stars.insert(location, mtbl);
}
}
}
if let Rvalue::Len(ref place) = *rvalue {
let place_ty = place.ty(&self.body.local_decls, self.tcx).ty;
if let ty::Array(_, len) = place_ty.kind {
let span = self.body.source_info(location).span;
let constant = Constant { span, literal: len, user_ty: None };
self.optimizations.arrays_lengths.insert(location, constant);
}
}
self.super_rvalue(rvalue, location)
}
}
#[derive(Default)]
struct OptimizationList<'tcx> {
and_stars: FxHashMap<Location, Mutability>,
arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
}
|
