On Mon, 17 May 2021 21:14:26 +0800 Chung-Lin Tang <clt...@codesourcery.com> wrote:
> On 2021/5/11 4:57 PM, Julian Brown wrote: > > This work-in-progress patch tries to get > > GOMP_MAP_ATTACH_ZERO_LENGTH_ARRAY_SECTION to behave more like > > GOMP_MAP_ATTACH_DETACH -- in that the mapping is made to form groups > > to be processed by build_struct_group/build_struct_comp_map. I > > think that's important to integrate with how groups of mappings for > > array sections are handled in other cases. > > > > This patch isn't sufficient by itself to fix a couple of broken > > test cases at present (libgomp.c++/target-lambda-1.C, > > libgomp.c++/target-this-4.C), though. > > No, GOMP_MAP_ATTACH_ZERO_LENGTH_ARRAY_SECTION is supposed to be just > a slightly different behavior version of GOMP_MAP_ATTACH; it > tolerates an unmapped pointer-target and assigns NULL on the device, > instead of just gomp_fatal(). (see its handling in libgomp/target.c) > > In case OpenACC can have the same such zero-length array section > behavior, we can just share one GOMP_MAP_ATTACH map. For now it is > treated as separate cases. OK, understood. But, I'm a bit concerned that we're ignoring some "hidden rules" with regards to OMP pointer clause ordering/grouping that certain code (at least the bit that creates GOMP_MAP_STRUCT node groups, and parts of omp-low.c) relies on. I believe those rules are as follows: - an array slice is mapped using two or three pointers -- two for a normal (non-reference) base pointer, and three if we have a reference to a pointer (i.e. in C++) or an array descriptor (i.e. in Fortran). So we can have e.g. GOMP_MAP_TO GOMP_MAP_ALWAYS_POINTER GOMP_MAP_TO GOMP_MAP_.*_POINTER GOMP_MAP_ALWAYS_POINTER GOMP_MAP_TO GOMP_MAP_TO_PSET GOMP_MAP_ALWAYS_POINTER - for OpenACC, we extend this to allow (up to and including gimplify.c) the GOMP_MAP_ATTACH_DETACH mapping. So we can have (for component refs): GOMP_MAP_TO GOMP_MAP_ATTACH_DETACH GOMP_MAP_TO GOMP_MAP_TO_PSET GOMP_MAP_ATTACH_DETACH GOMP_MAP_TO GOMP_MAP_.*_POINTER GOMP_MAP_ATTACH_DETACH For the scanning in insert_struct_comp_map (as it is at present) to work right, these groups must stay intact. I think the current behaviour of omp_target_reorder_clauses on the og10 branch can break those groups apart though! (The "prev_list_p" stuff in the loop in question in gimplify.c just keeps track of the first node in these groups.) For OpenACC, the GOMP_MAP_ATTACH_DETACH code does *not* depend on the previous clause when lowering in omp-low.c. But GOMP_MAP_ALWAYS_POINTER does! And in one case ("update" directive), GOMP_MAP_ATTACH_DETACH is rewritten to GOMP_MAP_ALWAYS_POINTER, so for that case at least, the dependency on the preceding mapping node must stay intact. OpenACC also allows "bare" GOMP_MAP_ATTACH and GOMP_MAP_DETACH nodes (corresponding to the "attach" and "detach" clauses). Those are handled a bit differently to GOMP_MAP_ATTACH_DETACH in gimplify.c -- but GOMP_MAP_ATTACH_Z_L_A_S doesn't quite behave like that either, I don't think? Anyway: I've not entirely understood what omp_target_reorder_clauses is doing, but I think it may need to try harder to keep the groups mentioned above together. What do you think? Thanks, Julian