The `ConstProp` can cause many locals to be initialized to a constant
value and then never read from. `ConstProp` can also evaluate ZSTs into
constant values. Previously, many of these would be removed by other
parts of the MIR optimization pipeline. However, evaluating ZSTs
(especially `()`) into constant values defeated those parts of the
optimizer and so in a2e3ed5c05, I added a
hack to `ConstProp` that skips evaluating ZSTs to avoid that regression.
This commit changes `SimplifyLocals` so that it doesn't consider writes
of const values to a local to be a use of that local. In doing so,
`SimplifyLocals` is able to remove otherwise unused locals left behind
by other optimization passes (`ConstProp` in particular).
The commit should have changed comments as well.
At the time of writting, it passes the tidy and check tool.
Revisions asked by eddyb :
- Renamed of all the occurences of {visit/super}_mir
- Renamed test structures `CachedMir` to `Cached`
Fixing the missing import on `AggregateKind`
This commit refactors `PlaceContext` to split it into four different
smaller enums based on if the context represents a mutating use,
non-mutating use, maybe-mutating use or a non-use (this is based on the
recommendation from @oli-obk on Zulip[1]).
This commit then introduces a `PlaceContext::AscribeUserTy` variant.
`StatementKind::AscribeUserTy` is now correctly mapped to
`PlaceContext::AscribeUserTy` instead of `PlaceContext::Validate`.
`PlaceContext::AscribeUserTy` can also now be correctly categorized as a
non-use which fixes an issue with constant promotion in statics after a
cast introduces a `AscribeUserTy` statement.
[1]: https://rust-lang.zulipchat.com/#narrow/stream/122657-wg-nll/subject/.2355288.20cast.20fails.20to.20promote.20to.20'static/near/136536949
Currently we have two files implementing bitsets (and 2D bit matrices).
This commit combines them into one, taking the best features from each.
This involves renaming a lot of things. The high level changes are as
follows.
- bitvec.rs --> bit_set.rs
- indexed_set.rs --> (removed)
- BitArray + IdxSet --> BitSet (merged, see below)
- BitVector --> GrowableBitSet
- {,Sparse,Hybrid}IdxSet --> {,Sparse,Hybrid}BitSet
- BitMatrix --> BitMatrix
- SparseBitMatrix --> SparseBitMatrix
The changes within the bitset types themselves are as follows.
```
OLD OLD NEW
BitArray<C> IdxSet<T> BitSet<T>
-------- ------ ------
grow - grow
new - (remove)
new_empty new_empty new_empty
new_filled new_filled new_filled
- to_hybrid to_hybrid
clear clear clear
set_up_to set_up_to set_up_to
clear_above - clear_above
count - count
contains(T) contains(&T) contains(T)
contains_all - superset
is_empty - is_empty
insert(T) add(&T) insert(T)
insert_all - insert_all()
remove(T) remove(&T) remove(T)
words words words
words_mut words_mut words_mut
- overwrite overwrite
merge union union
- subtract subtract
- intersect intersect
iter iter iter
```
In general, when choosing names I went with:
- names that are more obvious (e.g. `BitSet` over `IdxSet`).
- names that are more like the Rust libraries (e.g. `T` over `C`,
`insert` over `add`);
- names that are more set-like (e.g. `union` over `merge`, `superset`
over `contains_all`, `domain_size` over `num_bits`).
Also, using `T` for index arguments seems more sensible than `&T` --
even though the latter is standard in Rust collection types -- because
indices are always copyable. It also results in fewer `&` and `*`
sigils in practice.
This simplifies analysis and borrow-checking because liveness at the
resume point can always be simply propagated.
Later on, the "dead" Resumes are removed.
Removing nops can allow more basic blocks to be merged, but merging
basic blocks can't allow for more nops to be removed, so we should
remove nops first.
This doesn't matter *that* much, because normally we run SimplifyCfg
several times, but there's no reason not to do it.
Overall goal: reduce the amount of context a mir pass needs so that it
resembles a query.
- The hooks are no longer "threaded down" to the pass, but rather run
automatically from the top-level (we also thread down the current pass
number, so that the files are sorted better).
- The hook now receives a *single* callback, rather than a callback per-MIR.
- The traits are no longer lifetime parameters, which moved to the
methods -- given that we required
`for<'tcx>` objecs, there wasn't much point to that.
- Several passes now store a `String` instead of a `&'l str` (again, no
point).
