In C++20, the compiler will synthesize a version of the operator
with its arguments reversed to ease commutativity. This reversed
version is ambiguous with the hand-written operator when the
argument is const but `this` isn't.
Although by analogy with def instructions as barrier instructions one
could understand how a block where the def appears as a phi could be
regarded as a barrier block, the analogy is nonobvious.
Reachability knows the difference between an initial block and a barrier
block. Although most current clients don't care about this distinction,
one does. Here, Reachability calls back with visitInitialBlock for the
former and visitBarrierBlock for the latter.
Most clients are updated to have the same implementation in both
visitBarrierBlock and visitInitialBlock. The findBarriersBackward
client is updated to retain the distinction and pass it on to its
clients. Its one client, CanonicalizeOSSALifetime is updated to have a
simpler handling for barrier edges and to ignore the initial blocks.
llvm::SmallSetVector changed semantics
(https://reviews.llvm.org/D152497) resulting in build failures in Swift.
The old semantics allowed usage of types that did not have an
`operator==` because `SmallDenseSet` uses `DenseSetInfo<T>::isEqual` to
determine equality. The new implementation switched to using
`std::find`, which internally uses `operator==`. This type is used
pretty frequently with `swift::Type`, which intentionally deletes
`operator==` as it is not the canonical type and therefore cannot be
compared in normal circumstances.
This patch adds a new type-alias to the Swift namespace that provides
the old semantic behavior for `SmallSetVector`. I've also gone through
and replaced usages of `llvm::SmallSetVector` with the
`Swift::SmallSetVector` in places where we're storing a type that
doesn't implement or explicitly deletes `operator==`. The changes to
`llvm::SmallSetVector` should improve compile-time performance, so I
left the `llvm::SmallSetVector` where possible.
IterableBackwardReachability just requires an iterable list of gens.
ShrinkBorrowScope, LexicalDestroyHoisting, and SSADestroyHoisting all
need to be able to check whether a given instruction is scope ending
quickly. Use a SmallSetVector rather than a SmallVector for the gens in
all three.
The new optimistic, iterative backward reachability is optimized to do
as little work as possible. Only blocks not all of whose successors are
kills participate in the worklist at all. The blocks within that
discovered set are visited via a worklist which tracks blocks which have
been found to be unreachable at begin and whose
unreachable-at-begin-ness must be propagated into
unreachable-at-end-ness of its predecessors.
rdar://92545900
Previously, Reachability assumed that phis were not barriers. Here,
handling for barrier phis is added. To that end, a new delegate
callback `checkReachableBarrier(PhiValue)` is added. Before marking the
beginning of a block as reached (or any of its predecessors), check
whether each argument that is a phi is a barrier. If any is, then
reachability is done.
Implemented the new method in SSADestroyHoisting by splitting apart the
classification of an instruction and the work to do in response to
visiting an instruction. Then, when visiting a PhiValue, just check
whether any of the predecessors terminators are classified as barriers.
That way, seeing that they're classified that way doesn't result in
noting down that those terminators had been reached (which indeed they
will not have been if any of the terminators from which the values are
flowing into the phi are barriers).
Pessimistic, non-iterative data flow for analyzing backward reachability
from a set of last uses to their dominating def or nearest barrier.
Meet: ReachableEnd(predecessor) = intersection(ReachableBegin, successors)
Intended for frequently called utilities where minimizing the cost of
data flow is more important than analyzing reachability across
loops. Expected to visit very few blocks because barriers often occur
close to a last use.
Note: this does not require initializing bitsets for all blocks in the
function for each SSA value being analyzed.
