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optimize redundant borrows

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This commit is contained in:
Niko Matsakis 2018-08-07 18:01:29 -04:00
parent 3b7989d1df
commit 1bae4f5aec
3 changed files with 22 additions and 8 deletions
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2
src/librustc_mir/borrow_check/borrow_set.rs
View file

@ -159,7 +159,7 @@ impl<'a, 'gcx, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'gcx, 'tcx> {
location: mir::Location,
) {
if let mir::Rvalue::Ref(region, kind, ref borrowed_place) = *rvalue {
if borrowed_place.is_unsafe_place(self.tcx, self.mir) {
if borrowed_place.ignore_borrow(self.tcx, self.mir) {
return;
}

26
src/librustc_mir/borrow_check/place_ext.rs
View file

@ -15,8 +15,11 @@ use rustc::ty::{self, TyCtxt};
/// Extension methods for the `Place` type.
crate trait PlaceExt<'tcx> {
/// True if this is a deref of a raw pointer.
fn is_unsafe_place(&self, tcx: TyCtxt<'_, '_, 'tcx>, mir: &Mir<'tcx>) -> bool;
/// Returns true if we can safely ignore borrows of this place.
/// This is true whenever there is no action that the user can do
/// to the place `self` that would invalidate the borrow. This is true
/// for borrows of raw pointer dereferents as well as shared references.
fn ignore_borrow(&self, tcx: TyCtxt<'_, '_, 'tcx>, mir: &Mir<'tcx>) -> bool;
/// If this is a place like `x.f.g`, returns the local
/// `x`. Returns `None` if this is based in a static.
@ -24,7 +27,7 @@ crate trait PlaceExt<'tcx> {
}
impl<'tcx> PlaceExt<'tcx> for Place<'tcx> {
fn is_unsafe_place(&self, tcx: TyCtxt<'_, '_, 'tcx>, mir: &Mir<'tcx>) -> bool {
fn ignore_borrow(&self, tcx: TyCtxt<'_, '_, 'tcx>, mir: &Mir<'tcx>) -> bool {
match self {
Place::Promoted(_) |
Place::Local(_) => false,
@ -36,12 +39,23 @@ impl<'tcx> PlaceExt<'tcx> for Place<'tcx> {
| ProjectionElem::Downcast(..)
| ProjectionElem::Subslice { .. }
| ProjectionElem::ConstantIndex { .. }
| ProjectionElem::Index(_) => proj.base.is_unsafe_place(tcx, mir),
| ProjectionElem::Index(_) => proj.base.ignore_borrow(tcx, mir),
ProjectionElem::Deref => {
let ty = proj.base.ty(mir, tcx).to_ty(tcx);
match ty.sty {
ty::TyRawPtr(..) => true,
_ => proj.base.is_unsafe_place(tcx, mir),
// For both derefs of raw pointers and `&T`
// references, the original path is `Copy` and
// therefore not significant. In particular,
// there is nothing the user can do to the
// original path that would invalidate the
// newly created reference -- and if there
// were, then the user could have copied the
// original path into a new variable and
// borrowed *that* one, leaving the original
// path unborrowed.
ty::TyRawPtr(..) | ty::TyRef(_, _, hir::MutImmutable) => true,
_ => proj.base.ignore_borrow(tcx, mir),
}
}
},

2
src/librustc_mir/dataflow/impls/borrows.rs
View file

@ -233,7 +233,7 @@ impl<'a, 'gcx, 'tcx> BitDenotation for Borrows<'a, 'gcx, 'tcx> {
// propagate_call_return method.
if let mir::Rvalue::Ref(region, _, ref place) = *rhs {
if place.is_unsafe_place(self.tcx, self.mir) { return; }
if place.ignore_borrow(self.tcx, self.mir) { return; }
let index = self.borrow_set.location_map.get(&location).unwrap_or_else(|| {
panic!("could not find BorrowIndex for location {:?}", location);
});