Remove dereferencing of Box from codegen #95576
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,184 @@ | ||
| //! This pass transforms derefs of Box into a deref of the pointer inside Box. | ||
| //! | ||
| //! Box is not actually a pointer so it is incorrect to dereference it directly. | ||
| use crate::MirPass; | ||
| use rustc_hir::def_id::DefId; | ||
| use rustc_index::vec::Idx; | ||
| use rustc_middle::mir::patch::MirPatch; | ||
| use rustc_middle::mir::visit::MutVisitor; | ||
| use rustc_middle::mir::*; | ||
| use rustc_middle::ty::subst::Subst; | ||
| use rustc_middle::ty::{Ty, TyCtxt}; | ||
|
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||
| /// Constructs the types used when accessing a Box's pointer | ||
| pub fn build_ptr_tys<'tcx>( | ||
| tcx: TyCtxt<'tcx>, | ||
| pointee: Ty<'tcx>, | ||
| unique_did: DefId, | ||
| nonnull_did: DefId, | ||
| ) -> (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>) { | ||
| let substs = tcx.intern_substs(&[pointee.into()]); | ||
| let unique_ty = tcx.bound_type_of(unique_did).subst(tcx, substs); | ||
| let nonnull_ty = tcx.bound_type_of(nonnull_did).subst(tcx, substs); | ||
| let ptr_ty = tcx.mk_imm_ptr(pointee); | ||
|
|
||
| (unique_ty, nonnull_ty, ptr_ty) | ||
| } | ||
|
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||
| // Constructs the projection needed to access a Box's pointer | ||
| pub fn build_projection<'tcx>( | ||
| unique_ty: Ty<'tcx>, | ||
| nonnull_ty: Ty<'tcx>, | ||
| ptr_ty: Ty<'tcx>, | ||
| ) -> [PlaceElem<'tcx>; 3] { | ||
| [ | ||
| PlaceElem::Field(Field::new(0), unique_ty), | ||
| PlaceElem::Field(Field::new(0), nonnull_ty), | ||
| PlaceElem::Field(Field::new(0), ptr_ty), | ||
| ] | ||
| } | ||
|
|
||
| struct ElaborateBoxDerefVisitor<'tcx, 'a> { | ||
| tcx: TyCtxt<'tcx>, | ||
| unique_did: DefId, | ||
| nonnull_did: DefId, | ||
| local_decls: &'a mut LocalDecls<'tcx>, | ||
| patch: MirPatch<'tcx>, | ||
| } | ||
|
|
||
| impl<'tcx, 'a> MutVisitor<'tcx> for ElaborateBoxDerefVisitor<'tcx, 'a> { | ||
| fn tcx(&self) -> TyCtxt<'tcx> { | ||
| self.tcx | ||
| } | ||
|
|
||
| fn visit_place( | ||
| &mut self, | ||
| place: &mut Place<'tcx>, | ||
| context: visit::PlaceContext, | ||
| location: Location, | ||
| ) { | ||
| let tcx = self.tcx; | ||
|
|
||
| let base_ty = self.local_decls[place.local].ty; | ||
|
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| // Derefer ensures that derefs are always the first projection | ||
| if place.projection.first() == Some(&PlaceElem::Deref) && base_ty.is_box() { | ||
| let source_info = self.local_decls[place.local].source_info; | ||
|
|
||
| let (unique_ty, nonnull_ty, ptr_ty) = | ||
| build_ptr_tys(tcx, base_ty.boxed_ty(), self.unique_did, self.nonnull_did); | ||
|
|
||
| let ptr_local = self.patch.new_temp(ptr_ty, source_info.span); | ||
| self.local_decls.push(LocalDecl::new(ptr_ty, source_info.span)); | ||
|
|
||
| self.patch.add_statement(location, StatementKind::StorageLive(ptr_local)); | ||
|
|
||
| self.patch.add_assign( | ||
| location, | ||
| Place::from(ptr_local), | ||
| Rvalue::Use(Operand::Copy( | ||
| Place::from(place.local) | ||
| .project_deeper(&build_projection(unique_ty, nonnull_ty, ptr_ty), tcx), | ||
| )), | ||
| ); | ||
|
|
||
| place.local = ptr_local; | ||
|
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| self.patch.add_statement( | ||
| Location { block: location.block, statement_index: location.statement_index + 1 }, | ||
| StatementKind::StorageDead(ptr_local), | ||
| ); | ||
| } | ||
|
|
||
| self.super_place(place, context, location); | ||
| } | ||
| } | ||
|
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| pub struct ElaborateBoxDerefs; | ||
|
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||
| impl<'tcx> MirPass<'tcx> for ElaborateBoxDerefs { | ||
| fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { | ||
| if let Some(def_id) = tcx.lang_items().owned_box() { | ||
|
There was a problem hiding this comment. not new in this PR, but we should probably start caching information like this by computing it when we create |
||
| let unique_did = tcx.adt_def(def_id).non_enum_variant().fields[0].did; | ||
|
|
||
| let Some(nonnull_def) = tcx.type_of(unique_did).ty_adt_def() else { | ||
| span_bug!(tcx.def_span(unique_did), "expected Box to contain Unique") | ||
|
There was a problem hiding this comment. This should probably be using a lang item for Reported as an ICE by #98372; rustc_codegen_gcc's minicore defines |
||
| }; | ||
|
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| let nonnull_did = nonnull_def.non_enum_variant().fields[0].did; | ||
|
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| let patch = MirPatch::new(body); | ||
|
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| let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut(); | ||
|
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| let mut visitor = | ||
| ElaborateBoxDerefVisitor { tcx, unique_did, nonnull_did, local_decls, patch }; | ||
|
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||
| for (block, BasicBlockData { statements, terminator, .. }) in | ||
| basic_blocks.iter_enumerated_mut() | ||
| { | ||
| let mut index = 0; | ||
| for statement in statements { | ||
| let location = Location { block, statement_index: index }; | ||
| visitor.visit_statement(statement, location); | ||
| index += 1; | ||
| } | ||
|
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||
| if let Some(terminator) = terminator | ||
| && !matches!(terminator.kind, TerminatorKind::Yield{..}) | ||
| { | ||
| let location = Location { block, statement_index: index }; | ||
| visitor.visit_terminator(terminator, location); | ||
| } | ||
|
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| let location = Location { block, statement_index: index }; | ||
| match terminator { | ||
| // yielding into a box is handled when lowering generators | ||
| Some(Terminator { kind: TerminatorKind::Yield { value, .. }, .. }) => { | ||
| visitor.visit_operand(value, location); | ||
| } | ||
| Some(terminator) => { | ||
| visitor.visit_terminator(terminator, location); | ||
| } | ||
| None => {} | ||
| } | ||
| } | ||
|
|
||
| visitor.patch.apply(body); | ||
|
There was a problem hiding this comment. One thing that came up in the derefer PR is that derefs also happen in VarDebugInfo, but we don't have a good way to avoid multiple derefs there yet, but for your PR it may be useful to also unroll box derefs into multiple derefs There was a problem hiding this comment. What code actually generates debug info that dereferences box? There was a problem hiding this comment. I pushed some code to do this, but I have no idea if its actually correct because I don't know how to test it. There was a problem hiding this comment. Not sure either. You could just panic the def id to find a function that exhibits it and then try to make a mir opt out of it? There was a problem hiding this comment. Ooh, maybe it works via box patterns? |
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| for debug_info in body.var_debug_info.iter_mut() { | ||
| if let VarDebugInfoContents::Place(place) = &mut debug_info.value { | ||
| let mut new_projections: Option<Vec<_>> = None; | ||
| let mut last_deref = 0; | ||
|
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||
| for (i, (base, elem)) in place.iter_projections().enumerate() { | ||
| let base_ty = base.ty(&body.local_decls, tcx).ty; | ||
|
|
||
| if elem == PlaceElem::Deref && base_ty.is_box() { | ||
| let new_projections = new_projections.get_or_insert_default(); | ||
|
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| let (unique_ty, nonnull_ty, ptr_ty) = | ||
| build_ptr_tys(tcx, base_ty.boxed_ty(), unique_did, nonnull_did); | ||
|
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||
| new_projections.extend_from_slice(&base.projection[last_deref..]); | ||
| new_projections.extend_from_slice(&build_projection( | ||
| unique_ty, nonnull_ty, ptr_ty, | ||
| )); | ||
| new_projections.push(PlaceElem::Deref); | ||
|
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| last_deref = i; | ||
| } | ||
| } | ||
|
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||
| if let Some(mut new_projections) = new_projections { | ||
| new_projections.extend_from_slice(&place.projection[last_deref..]); | ||
| place.projection = tcx.intern_place_elems(&new_projections); | ||
| } | ||
| } | ||
| } | ||
| } else { | ||
| // box is not present, this pass doesn't need to do anything | ||
| } | ||
| } | ||
| } | ||
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Could you add a similar assertion to Miri?
deref_operandwould probably be a good spot.There was a problem hiding this comment.
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Also, does this mean you can remove the special cases I listed at #95453 (comment) ?
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The first 3 special cases have been removed. The debuginfo one probably shouldn't be changed because it could make debugging more annoying for no benefit. I'm currently investigating to see if the last one can be changed safely without breaking things.
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Also please remember to add a similar assertion to Miri. :)
Sorry for raising two points in the same thread. Sadly GitHub does not let one open multiple threads at the same line of code.