Dominator-order information is only needed for coverage graphs, and is easy
enough to collect by just traversing the graph again.
This avoids wasted work when computing graph dominators for any other purpose.
This version shaves off ca 2% of the cycles in my experiments
and makes the control flow easier to follow for me and hopefully
others, including the compiler.
Someone gave me a working profiler and by God I'm using it.
This patch has been extracted from #123720. It specifically enhances
`Sccs` to allow tracking arbitrary commutative properties of SCCs, including
- reachable values (max/min)
- SCC-internal values (max/min)
This helps with among other things universe computation: we can now identify
SCC universes as a straightforward "find max/min" operation during SCC construction.
It's also more or less zero-cost; don't use the new features, don't pay for them.
This commit also vastly extends the documentation of the SCCs module, which I had a very hard time following.
Does not necessarily change much, but we never overwrite it, so I see no reason
for it to be in the `Successors` trait. (+we already have a similar `is_cyclic`)
detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
Preprocess and cache dominator tree
Preprocessing dominators has a very strong effect for https://github.com/rust-lang/rust/pull/111344.
That pass checks that assignments dominate their uses repeatedly. Using the unprocessed dominator tree caused a quadratic runtime (number of bbs x depth of the dominator tree).
This PR also caches the dominator tree and the pre-processed dominators in the MIR cfg cache.
Rebase of https://github.com/rust-lang/rust/pull/107157
cc `@tmiasko`
Change the immediate_dominator return type to Option, and use None to
indicate that node has no immediate dominator.
Also fix the issue where the start node would be returned as its own
immediate dominator.
