diff options
Diffstat (limited to 'compiler')
| -rw-r--r-- | compiler/rustc_mir_transform/src/dest_prop.rs | 33 |
1 files changed, 7 insertions, 26 deletions
diff --git a/compiler/rustc_mir_transform/src/dest_prop.rs b/compiler/rustc_mir_transform/src/dest_prop.rs index 237ead591a5..7878d6eaab1 100644 --- a/compiler/rustc_mir_transform/src/dest_prop.rs +++ b/compiler/rustc_mir_transform/src/dest_prop.rs @@ -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 { |