In all cases the struct can own the relevant thing instead of having a
reference to it. This makes the code simpler, and in some cases removes
a struct lifetime.
Move `SanityCheck` and `MirPass`
They are currently in `rustc_middle`. This PR moves them to `rustc_mir_transform`, which makes more sense.
r? ``@cjgillot``
Because that's now the only crate that uses it.
Moving stuff out of `rustc_middle` is always welcome.
I chose to use `impl crate::MirPass`/`impl crate::MirLint` (with
explicit `crate::`) everywhere because that's the only mention of
`MirPass`/`MirLint` used in all of these files. (Prior to this change,
`MirPass` was mostly imported via `use rustc_middle::mir::*` items.)
By making it own the index maps, instead of holding references to them.
This requires moving the free function `find_candidate` into
`Candidate::reset_and_find`. It lets the `'alloc` lifetime be removed
everywhere that still has it.
Save liveness results for DestinationPropagation
`DestinationPropagation` needs to verify that merge candidates do not conflict with each other. This is done by verifying that a local is not live when its counterpart is written to.
To get the liveness information, the pass runs `MaybeLiveLocals` dataflow analysis repeatedly, once for each propagation round. This is quite costly, and the main driver for the perf impact on `ucd` and `diesel`. (See https://github.com/rust-lang/rust/pull/115105#issuecomment-1689205908)
In order to mitigate this cost, this PR proposes to save the result of the analysis into a `SparseIntervalMatrix`, and mirror merges of locals into that matrix: `liveness(destination) := liveness(destination) union liveness(source)`.
<details>
<summary>Proof</summary>
We denote by `'` all the quantities of the transformed program. Let $\varphi$ be a mapping of locals, which maps `source` to `destination`, and is identity otherwise. The exact liveness set after a statement is $out'(statement)$, and the proposed liveness set is $\varphi(out(statement))$.
Consider a statement. Suppose that the output state verifies $out' \subset phi(out)$. We want to prove that $in' \subset \varphi(in)$ where $in = (out - kill) \cup gen$, and conclude by induction.
We have 2 cases: either that statement is kept with locals renumbered by $\varphi$, or it is a tautological assignment and it removed.
1. If the statement is kept: the gen-set and the kill-set of $statement' = \varphi(statement)$ are $gen' = \varphi(gen)$ and $kill' = \varphi(kill)$ exactly.
From soundness requirement 3, $\varphi(in)$ is disjoint from $\varphi(kill)$.
This implies that $\varphi(out - kill)$ is disjoint from $\varphi(kill)$, and so $\varphi(out - kill) = \varphi(out) - \varphi(kill)$. Then $\varphi(in) = (\varphi(out) - \varphi(kill)) \cup \varphi(gen) = (\varphi(out) - kill') \cup gen'$.
We can conclude that $out' \subset \varphi(out) \implies in' \subset \varphi(in)$.
2. If the statement is removed. As $\varphi(statement)$ is a tautological assignment, we know that $\varphi(gen) = \varphi(kill) = \\{ destination \\}$, while $gen' = kill' = \emptyset$. So $\varphi(in) = \varphi(out) \cup \\{ destination \\}$. Then $in' = out' \subset out \subset \varphi(in)$.
By recursion, we can conclude by that $in' \subset \varphi(in)$ everywhere.
</details>
This approximate liveness results is only suboptimal if there are locals that fully disappear from the CFG due to an assignment cycle. These cases are quite unlikely, so we do not bother with them.
This change allows to reduce the perf impact of DestinationPropagation by half on diesel and ucd (https://github.com/rust-lang/rust/pull/115105#issuecomment-1694701904).
cc ````@JakobDegen````
By using FxIndexMap instead of FxHashMap, so that the order of visiting
of locals is deterministic.
We also need to bless
copy_propagation_arg.foo.DestinationPropagation.panic*.diff. Do not
review the diff of the diff. Instead look at the diff file before and
after this commit. Both before and after this commit, 3 statements are
replaced with nop. It's just that due to change in ordering, different
statements are replaced. But the net result is the same.
Unify terminology used in unwind action and terminator, and reflect
the fact that a nounwind panic is triggered instead of an immediate
abort is triggered for this terminator.
Implement simple CopyPropagation based on SSA analysis
This PR extracts the "copy propagation" logic from https://github.com/rust-lang/rust/pull/106285.
MIR may produce chains of assignment between locals, like `_x = move? _y`.
This PR attempts to remove such chains by unifying locals.
The current implementation is a bit overzealous in turning moves into copies, and in removing storage statements.
This patch adds a `MirPass` that tracks the number of back-edges and
function calls in the CFG, adds a new MIR instruction to increment a
counter every time they are encountered during Const Eval, and emit a
warning if a configured limit is breached.
