`PRIVATE | ATTACH` maps can be used to represent firstprivate pointers
that should be initialized by doing doing the pointee's device address,
if its lookup succeeds, or retain the original host pointee's address
otherwise.
With this, for a test like the following:
```f90
integer, pointer :: p(:)
!$omp target map(p(1))
... print*, p(1)
!$omp end target
```
The codegen can look like:
```llvm
; maps for p:
; &p(1), &p(1), sizeof(p(1)), TO|FROM //(1)
; &ref_ptr(p), &p(1), sizeof(ref_ptr(p)), ATTACH //(2)
; &ref_ptr(p), &p(1), sizeof(ref_ptr(p)), PRIVATE|ATTACH|PARAM //(3)
call... @__omp_outlined...(ptr %ref_ptr_of_p)
```
* `(1)` maps the pointee `p(1)`.
* `(2)` attaches it to the (previously) mapped `ref_ptr(p)`, if present.
It can be controlled via OpenMP 6.1's `attach(auto/always/never)`
map-type modifiers.
* `(3)` privatizes and initializes the local `ref_ptr(p)`, which gets
passed
in as the kernel argument `%ref_ptr_of_p`. Can be skipped if p is not
referenced directly within the region.
While similar mapping can be used for C/C++, it's more important/useful
for Fortran as we can avoid creating another argument for passing the
descriptor, and use that to initialize the private copy in the body of
the kernel.
This change adds support for saving full contents of attached Fortran
descriptors, and not just their pointee address, in the shadow-pointer
table.
With this, we now support:
* comparing full contents of descriptors to check whether a previous
shadow-pointer entry is stale;
* restoring the full contents of descriptors
And with that, we can now use ATTACH map-types (added in #149036) for
mapping Fortran pointer/allocatable arrays, and array-sections on them.
e.g.:
```f90
integer, allocatable :: x(:)
!$omp target enter data map(to: x(:))
```
as:
```
void* addr_of_pointee = allocated(x) ? &x(1) : nullptr;
int64_t sizeof_pointee = allocated(x) ? sizeof(x(:)) : 0
addr_of_pointee, addr_of_pointee, sizeof_pointee, TO
addr_of_descriptor, addr_of_pointee, size_of_descriptor, ATTACH
```
This patch introduces libomptarget support for the ATTACH map-type,
which can be used to implement OpenMP conditional compliant pointer
attachment, based on whether the pointer/pointee is newly mapped on a
given construct.
For example, for the following:
```c
int *p;
#pragma omp target enter data map(p[1:10])
```
The following maps can be emitted by clang:
```
(A)
&p[0], &p[1], 10 * sizeof(p[1]), TO | FROM
&p, &p[1], sizeof(p), ATTACH
```
Without this map-type, these two possible maps could be emitted by
clang:
```
(B)
&p[0], &p[1], 10 * sizeof(p[1]), TO | FROM
(C)
&p, &p[1], 10 * sizeof(p[1]), TO | FROM | PTR_AND_OBJ
````
(B) does not perform any pointer attachment, while (C) also maps the
pointer p, which are both incorrect.
In terms of implementation, maps with the ATTACH map-type are handled
after all other maps have been processed, as it requires knowledge of
which new allocations happened as part of the construct. As per OpenMP
5.0, an attachment should happen only when either the pointer or the
pointee was newly mapped while handling the construct.
Maps with ATTACH map-type-bit do not increase/decrease the ref-count.
With OpenMP 6.1, `attach(always/never)` can be used to force/prevent
attachment. For `attach(always)`, the compiler will insert the ALWAYS
map-type, which would let libomptarget bypass the check about one of the
pointer/pointee being new. With `attach(never)`, the ATTACH map will not
be emitted at all.
The size argument of the ATTACH map-type can specify values greater than
`sizeof(void*)` which can be used to support pointer attachment on
Fortran descriptors. Note that this also requires shadow-pointer
tracking to also support them. That has not been implemented in this
patch.
This was worked upon in coordination with Ravi Narayanaswamy, who has
since retired. Happy retirement, Ravi!
---------
Co-authored-by: Alex Duran <alejandro.duran@intel.com>
