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Remove an unnecessary restriction in dest_prop

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This commit is contained in:
Jakob Degen 2022-02-24 09:47:13 -05:00
parent 9ecd75b831
commit 57c4163294
3 changed files with 20 additions and 33 deletions
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33
compiler/rustc_mir_transform/src/dest_prop.rs
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@ -38,12 +38,6 @@
//! It must also not contain any indexing projections, since those take an arbitrary `Local` as
//! the index, and that local might only be initialized shortly before `dest` is used.
//!
//! Subtle case: If `dest` is a, or projects through a union, then we have to make sure that there
//! remains an assignment to it, since that sets the "active field" of the union. But if `src` is
//! a ZST, it might not be initialized, so there might not be any use of it before the assignment,
//! and performing the optimization would simply delete the assignment, leaving `dest`
//! uninitialized.
//!
//! * `src` must be a bare `Local` without any indirections or field projections (FIXME: Is this a
//! fundamental restriction or just current impl state?). It can be copied or moved by the
//! assignment.
@ -103,7 +97,6 @@ use rustc_index::{
bit_set::{BitMatrix, BitSet},
vec::IndexVec,
};
use rustc_middle::mir::tcx::PlaceTy;
use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
use rustc_middle::mir::{dump_mir, PassWhere};
use rustc_middle::mir::{
@ -135,7 +128,7 @@ impl<'tcx> MirPass<'tcx> for DestinationPropagation {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let def_id = body.source.def_id();
let candidates = find_candidates(tcx, body);
let candidates = find_candidates(body);
if candidates.is_empty() {
debug!("{:?}: no dest prop candidates, done", def_id);
return;
@ -803,9 +796,8 @@ struct CandidateAssignment<'tcx> {
/// comment) and also throw out assignments that involve a local that has its address taken or is
/// otherwise ineligible (eg. locals used as array indices are ignored because we cannot propagate
/// arbitrary places into array indices).
fn find_candidates<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> Vec<CandidateAssignment<'tcx>> {
fn find_candidates<'tcx>(body: &Body<'tcx>) -> Vec<CandidateAssignment<'tcx>> {
let mut visitor = FindAssignments {
tcx,
body,
candidates: Vec::new(),
ever_borrowed_locals: ever_borrowed_locals(body),
@ -816,7 +808,6 @@ fn find_candidates<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> Vec<CandidateA
}
struct FindAssignments<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
body: &'a Body<'tcx>,
candidates: Vec<CandidateAssignment<'tcx>>,
ever_borrowed_locals: BitSet<Local>,
@ -845,10 +836,11 @@ impl<'tcx> Visitor<'tcx> for FindAssignments<'_, 'tcx> {
return;
}
// Can't optimize if both locals ever have their address taken (can introduce
// aliasing).
// FIXME: This can be smarter and take `StorageDead` into account (which
// invalidates borrows).
// Can't optimize if either local ever has their address taken. This optimization does
// liveness analysis only based on assignments, and a local can be live even if its
// never assigned to again, because a reference to it might be live.
// FIXME: This can be smarter and take `StorageDead` into account (which invalidates
// borrows).
if self.ever_borrowed_locals.contains(dest.local)
|| self.ever_borrowed_locals.contains(src.local)
{
@ -862,22 +854,11 @@ impl<'tcx> Visitor<'tcx> for FindAssignments<'_, 'tcx> {
return;
}
// Handle the "subtle case" described above by rejecting any `dest` that is or
// projects through a union.
let mut place_ty = PlaceTy::from_ty(self.body.local_decls[dest.local].ty);
if place_ty.ty.is_union() {
return;
}
for elem in dest.projection {
if let PlaceElem::Index(_) = elem {
// `dest` contains an indexing projection.
return;
}
place_ty = place_ty.projection_ty(self.tcx, elem);
if place_ty.ty.is_union() {
return;
}
}
self.candidates.push(CandidateAssignment {