domtree. When finding a nearest common dominator, if neither A dominates
B nor B dominates A, we immediately resorted to a tree walk. The tree
walk here is *particularly* expensive because we have to build
a (potentially very large) set for one side's dominators and compare it
with the other side's.
If at any point we have DFS info, we don't need to do any of this. We
can just walk up one side's immediate dominators and return the first
one which dominates the other side. Because of the DFS info, the
dominates queries are trivially constant time.
This reduces the optimizers time in the test case on PR19499 by 70%. It
now optimizes in about 30 seconds for me. And there is still more to be
done for this case.
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can be used by both the new pass manager and the old.
This removes it from any of the virtual mess of the pass interfaces and
lets it derive cleanly from the DominatorTreeBase<> template. In turn,
tons of boilerplate interface can be nuked and it turns into a very
straightforward extension of the base DominatorTree interface.
The old analysis pass is now a simple wrapper. The names and style of
this split should match the split between CallGraph and
CallGraphWrapperPass. All of the users of DominatorTree have been
updated to match using many of the same tricks as with CallGraph. The
goal is that the common type remains the resulting DominatorTree rather
than the pass. This will make subsequent work toward the new pass
manager significantly easier.
Also in numerous places things became cleaner because I switched from
re-running the pass (!!! mid way through some other passes run!!!) to
directly recomputing the domtree.
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support notionally const queries even though they may trigger DFS
numbering updates.
The updating of DFS numbers and tracking of slow queries do not mutate
the observable state of the domtree. They should be const to
differentiate them from the APIs which mutate the tree directly to do
incremental updates.
This will make it possible in a world where the DominatorTree is not
a pass but merely the result of running a pass to derive DominatorTree
from the base class as it was originally designed, removing a huge
duplication of API in DominatorTree.
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trees into the Support library.
These are all expressed in terms of the generic GraphTraits and CFG,
with no reliance on any concrete IR types. Putting them in support
clarifies that and makes the fact that the static analyzer in Clang uses
them much more sane. When moving the Dominators.h file into the IR
library I claimed that this was the right home for it but not something
I planned to work on. Oops.
So why am I doing this? It happens to be one step toward breaking the
requirement that IR verification can only be performed from inside of
a pass context, which completely blocks the implementation of
verification for the new pass manager infrastructure. Fixing it will
also allow removing the concept of the "preverify" step (WTF???) and
allow the verifier to cleanly flag functions which fail verification in
a way that precludes even computing dominance information. Currently,
that results in a fatal error even when you ask the verifier to not
fatally error. It's awesome like that.
The yak shaving will continue...
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