Before this commit, `forceEmptyMetadata` did not return a bool to notify
when it changed something.
This in turn caused the pass to return the wrong value when the only
change applied was to force the empty metadata.
This commit fixes the issue, letting `forceEmptyMetadata` return a bool,
and propagating that until where it matters.
We now reduce the number of cases in which we restart the collection of
the `DivergenceDescriptor`s after a IDS modification on the `ScopeGraph`
is performed.
We previously restarted the collection after each IDS transformation.
Now, restart is only performed after IDS is applied on a `Conditional`
node whose all successors are divergent exits which are `goto` exits.
After such `Conditional` is processed, it may happen that a new `goto`
exit becomes divergent for another `Conditional` upwards in the
`ScopeGraph`.
Therefore, if we do not restart, it may happen that a non-`goto` exit is
processed before a `goto` exit, which may lead to suboptimal situations
in the emitted number of `goto`s, due to how IDS interacts with
`MaterializeTrivialGotos`.
When performing the IDS procedure for a divergence which is entered
through multiple `Successor`s of the `Conditional`, we insert a `Head`
header which collects the entry to the scope, in order to maintain
decidedness if there is a common post-dominator to all the `Successor`s
(similar to what weaving did).
Perform the IDS transformation, by taking into account possible multiple
divergent scopes for a certain conditional node at the same time.
This reduces the run times, since it reduces the number of time IDS is
called on a conditional, but mostly proceduce a more _compact_
`ScopeGraph`, where all the divergent scopes for a certain conditional
node rejoing the path toward the one true exit in a single point,
instead of having a chain of IDS resulting regions.
Make IDS unit tests compliant with the new multiple divergence
implementation.
This commit adds handling for a corner case of
MergePointeesOfPointerUnion, that wasn't handled before.
Said corner case happens when one of the scalars that gets merged as
part of the DLAStep operations is also itself a pointer to another
scalar that gets merged.
This wasn't accounted for before this commit, but it's actually a valid
scenario that can happen on some inputs.
Andrea Gussoni