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separately intern the outermost alloc from the rest

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This commit is contained in:
Oli Scherer 2024-01-24 11:46:57 +00:00
parent a73c44889a
commit a57a00ebf6
1 changed files with 39 additions and 41 deletions
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80
compiler/rustc_const_eval/src/interpret/intern.rs
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@ -125,10 +125,11 @@ pub fn intern_const_alloc_recursive<
// Intern the base allocation, and initialize todo list for recursive interning.
let base_alloc_id = ret.ptr().provenance.unwrap().alloc_id();
let mut todo = vec![(base_alloc_id, base_mutability)];
let mut todo: Vec<_> =
intern_shallow(ecx, base_alloc_id, base_mutability).unwrap().map(|prov| prov).collect();
// We need to distinguish "has just been interned" from "was already in `tcx`",
// so we track this in a separate set.
let mut just_interned = FxHashSet::default();
let mut just_interned: FxHashSet<_> = std::iter::once(base_alloc_id).collect();
// Whether we encountered a bad mutable pointer.
// We want to first report "dangling" and then "mutable", so we need to delay reporting these
// errors.
@ -142,52 +143,49 @@ pub fn intern_const_alloc_recursive<
// raw pointers, so we cannot rely on validation to catch them -- and since interning runs
// before validation, and interning doesn't know the type of anything, this means we can't show
// better errors. Maybe we should consider doing validation before interning in the future.
while let Some((alloc_id, mutability)) = todo.pop() {
while let Some(prov) = todo.pop() {
let alloc_id = prov.alloc_id();
if intern_kind != InternKind::Promoted
&& inner_mutability == Mutability::Not
&& !prov.immutable()
{
if ecx.tcx.try_get_global_alloc(alloc_id).is_some()
&& !just_interned.contains(&alloc_id)
{
// This is a pointer to some memory from another constant. We encounter mutable
// pointers to such memory since we do not always track immutability through
// these "global" pointers. Allowing them is harmless; the point of these checks
// during interning is to justify why we intern the *new* allocations immutably,
// so we can completely ignore existing allocations. We also don't need to add
// this to the todo list, since after all it is already interned.
continue;
}
// Found a mutable pointer inside a const where inner allocations should be
// immutable. We exclude promoteds from this, since things like `&mut []` and
// `&None::<Cell<i32>>` lead to promotion that can produce mutable pointers. We rely
// on the promotion analysis not screwing up to ensure that it is sound to intern
// promoteds as immutable.
found_bad_mutable_pointer = true;
}
if ecx.tcx.try_get_global_alloc(alloc_id).is_some() {
// Already interned.
debug_assert!(!ecx.memory.alloc_map.contains_key(&alloc_id));
continue;
}
just_interned.insert(alloc_id);
let provs = intern_shallow(ecx, alloc_id, mutability).map_err(|()| {
// We always intern with `inner_mutability`, and furthermore we ensured above that if
// that is "immutable", then there are *no* mutable pointers anywhere in the newly
// interned memory -- justifying that we can indeed intern immutably. However this also
// means we can *not* easily intern immutably here if `prov.immutable()` is true and
// `inner_mutability` is `Mut`: there might be other pointers to that allocation, and
// we'd have to somehow check that they are *all* immutable before deciding that this
// allocation can be made immutable. In the future we could consider analyzing all
// pointers before deciding which allocations can be made immutable; but for now we are
// okay with losing some potential for immutability here. This can anyway only affect
// `static mut`.
todo.extend(intern_shallow(ecx, alloc_id, inner_mutability).map_err(|()| {
ecx.tcx.dcx().emit_err(DanglingPtrInFinal { span: ecx.tcx.span, kind: intern_kind })
})?;
for prov in provs {
let alloc_id = prov.alloc_id();
if intern_kind != InternKind::Promoted
&& inner_mutability == Mutability::Not
&& !prov.immutable()
{
if ecx.tcx.try_get_global_alloc(alloc_id).is_some()
&& !just_interned.contains(&alloc_id)
{
// This is a pointer to some memory from another constant. We encounter mutable
// pointers to such memory since we do not always track immutability through
// these "global" pointers. Allowing them is harmless; the point of these checks
// during interning is to justify why we intern the *new* allocations immutably,
// so we can completely ignore existing allocations. We also don't need to add
// this to the todo list, since after all it is already interned.
continue;
}
// Found a mutable pointer inside a const where inner allocations should be
// immutable. We exclude promoteds from this, since things like `&mut []` and
// `&None::<Cell<i32>>` lead to promotion that can produce mutable pointers. We rely
// on the promotion analysis not screwing up to ensure that it is sound to intern
// promoteds as immutable.
found_bad_mutable_pointer = true;
}
// We always intern with `inner_mutability`, and furthermore we ensured above that if
// that is "immutable", then there are *no* mutable pointers anywhere in the newly
// interned memory -- justifying that we can indeed intern immutably. However this also
// means we can *not* easily intern immutably here if `prov.immutable()` is true and
// `inner_mutability` is `Mut`: there might be other pointers to that allocation, and
// we'd have to somehow check that they are *all* immutable before deciding that this
// allocation can be made immutable. In the future we could consider analyzing all
// pointers before deciding which allocations can be made immutable; but for now we are
// okay with losing some potential for immutability here. This can anyway only affect
// `static mut`.
todo.push((alloc_id, inner_mutability));
}
})?);
}
if found_bad_mutable_pointer {
return Err(ecx