https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111666
Bug ID: 111666
Summary: [OpenMP] Fortran FE creates too many temporaries for
clauses / 'omp target'+'omp team' #teams/#threads
optimize_target_teams optimization fails
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Keywords: missed-optimization, openmp
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: burnus at gcc dot gnu.org
CC: andrewjenner at gcc dot gnu.org, jakub at gcc dot gnu.org
Target Milestone: ---
I wonder whether trans-openmp.cc in general converts too much into temporaries.
Namely the following function. the 'gfc_evaluate_now' returns a constant as a
constant but otherwise creates a temporary:
static inline tree
gfc_convert_expr_to_tree (stmtblock_t *block, gfc_expr *expr)
{
gfc_se se;
tree result;
gfc_init_se (&se, NULL );
gfc_conv_expr (&se, expr);
gfc_add_block_to_block (block, &se.pre);
result = gfc_evaluate_now (se.expr, block);
gfc_add_block_to_block (block, &se.post);
return result;
}
* * *
This bites us for something similar to libgomp.fortran/examples-4/teams-2.f90
but modified:
function dotprod (..., num_teams, block_threads)
integer, VALUE :: num_teams, block_threads
!$omp target map(to: B, C, block_size) ...
!$omp teams num_teams(num_teams) thread_limit(block_threads) ...
The problem is that the gfc_evaluate_now produces code like:
D.4323 = num_teams;
D.4324 = block_threads;
#pragma omp teams ... num_teams(D.4323) thread_limit(D.4324)
And this is then too complex for gcc/gimplify.cc's optimize_target_teams
which no longer recognizes that num_teams and block_threads are (implicitly)
firstprivatize.
* * *
A similar issue shows up above without the VALUE attribute; this matches the
OpenMP Example document teams-2.f90 example - except that the values are passed
by reference.
(That's != libgomp.fortran/examples-4/teams-2.f90; the latter has 'map(tofrom:'
which prevents the optimize_target_teams optimization.)
In order to make that work, we would need to preserve 'num_teams(*num_teams)'
in the evaluation – and handle indirect-ref+DECL_P in optimize_target_teams.
