In the common case, we cannot have redundant @owned and @guaranteed phi
args. However, if the incoming values had none ownership and were passed
as an @owned or an @guaranteed phi arg, they can be considered as redundant.
For such redundant @owned phi args, create a copy value, and use the
copy to replace all uses of the redundant phi arg.
For such redundant @guaranteed phi args, we turn off the optimization
currently. We cannot just create a borrow scope and use the new borrowed
value for replacement. We will also have to re-write the consuming uses of
the @guaranteed values such that the newly created borrow scope is
within the scope of its operand.
If only a single field of a struct phi-argument is used, replace the argument by the field value.
br bb(%str)
bb(%phi):
%f = struct_extract %phi, #Field // the only use of %phi
use %f
is replaced with
%f = struct_extract %str, #Field
br bb(%f)
bb(%phi):
use %phi
This also works if the phi-argument is in a def-use cycle.
The new PhiExpansionPass is in the same file as the RedundantPhiEliminationPass. Therefore I renamed the source file to PhiArgumentOptimizations.cpp
