The following test was triggering a runtime crash **on the host before
launching the kernel**:
```fortran
program test_omp_target_map_bug_v5
implicit none
type nested_type
real, allocatable :: alloc_field(:)
end type nested_type
type nesting_type
integer :: int_field
type(nested_type) :: derived_field
end type nesting_type
type(nesting_type) :: config
allocate(config%derived_field%alloc_field(1))
!$OMP TARGET ENTER DATA MAP(TO:config, config%derived_field%alloc_field)
!$OMP TARGET
config%derived_field%alloc_field(1) = 1.0
!$OMP END TARGET
deallocate(config%derived_field%alloc_field)
end program test_omp_target_map_bug_v5
```
In particular, the runtime was producing a segmentation fault when the
test is compiled with any optimization level > 0; if you compile with
-O0 the sample ran fine.
After debugging the runtime, it turned out the crash was happening at
the point where the runtime calls the default mapper emitted by the
compiler for `nesting_type; in particular at this point in the runtime:
c62cd2877c/offload/libomptarget/omptarget.cpp (L307).
Bisecting the optimization pipeline using `-mllvm -opt-bisect-limit=N`,
the first pass that triggered the issue on `O1` was the `instcombine`
pass. Debugging this further, the issue narrows down to canonicalizing
`getelementptr` instructions from using struct types (in this case the
`nesting_type` in the sample above) to using addressing bytes (`i8`). In
particular, in `O0`, you would see something like this:
```llvm
define internal void @.omp_mapper._QQFnesting_type_omp_default_mapper(ptr noundef %0, ptr noundef %1, ptr noundef %2, i64 noundef %3, i64 noundef %4, ptr noundef %5) #6 {
entry:
%6 = udiv exact i64 %3, 56
%7 = getelementptr %_QFTnesting_type, ptr %2, i64 %6
....
}
```
```llvm
define internal void @.omp_mapper._QQFnesting_type_omp_default_mapper(ptr noundef %0, ptr noundef %1, ptr noundef %2, i64 noundef %3, i64 noundef %4, ptr noundef %5) #6 {
entry:
%6 = getelementptr i8, ptr %2, i64 %3
....
}
```
The `udiv exact` instruction emitted by the OMP IR Builder (see:
c62cd2877c/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp (L9154))
allows `instcombine` to assume that `%3` is divisible by the struct size
(here `56`) and, therefore, replaces the result of the division with
direct GEP on `i8` rather than the struct type.
However, the runtime was calling
`@.omp_mapper._QQFnesting_type_omp_default_mapper` not with `56` (the
proper struct size) but with `48`!
Debugging this further, I found that the size of `omp.map.info`
operation to which the default mapper is attached computes the value of
`48` because we set the map to partial (see:
c62cd2877c/flang/lib/Optimizer/OpenMP/MapInfoFinalization.cpp (L1146)
and
c62cd2877c/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp (L4501-L4512)).
However, I think this is incorrect since the emitted mapper (and
user-defined mappers in general) are defined on the whole struct type
and should never be marked as partial. Hence, the fix in this PR.
e82399dac2