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This patch adds support for selecting which functions are lowered to LLVM IR
from MLIR depending on declare target information and whether host or device
code is being generated.
The approach proposed by this patch is to perform the filtering in two stages:
- An MLIR transformation pass, which is added to the Flang translation flow
after the `OMPEarlyOutliningPass`. The functions that are kept are those
that match the OpenMP processor (host or device) the compiler invocation
is targeting, according to the presence of the `-fopenmp-is-target-device`
compiler option and declare target information. All functions contaning an
`omp.target` are also kept, regardless of the declare target information of
the function, due to the need for keeping target regions visible for both
host and device compilation.
- A filtering step during translation to LLVM IR, which is peformed for those
functions that were kept because of the presence of a target region inside.
If the targeted OpenMP processor does not match the declare target
information of the function, then it is removed from the LLVM IR after its
contents have been processed and translated. Since they should only contain
an omp.target operation which, in turn, should have been outlined into
another LLVM IR function, the wrapper can be deleted at that point.
Depends on D150328 and D150329.
Differential Revision: https://reviews.llvm.org/D147641