https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127616
Bug ID: 127616
Summary: OpenMP map iterator wrong code
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: parras at gcc dot gnu.org
Target Milestone: ---
Created attachment 65687
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65687&action=edit
Runtime test
The following map-iterator shape produces wrong results:
iterator(ii=1:N, jj=1:M): a(1:2, ii), b(1:2, jj)
where a's update only depends on ii, b's only on jj.
This is because remove_unused_omp_iterator_vars (gimplify.cc) finds in
OMP_CLAUSE_DECL no literal reference to either ii or jj, which are concealed in
the OMP_ITERATOR_BLOCK (see -fdump-tree-original-details). The root cause is in
the Fortran FE: OMP_CLAUSE_DECL goes through gfc_evaluate_now (via
gfc_conv_expr_descriptor), which replaces the iterator variables with scalar
temporaries (parm.0, parm.1, etc.).
Fixing it by walking the whole OMP_ITERATOR_BLOCK in
remove_unused_omp_iterator_vars would be tricky and error-prone.
A cleaner solution would be for Fortran to skip remove_unused_omp_iterator_vars
entirely and do the pruning in the front end instead. Note: not pruning at all
would restore the correct behaviour but result in a blow-up in the iteration
space.
Concretely, after pruning, the initial code should have the shape:
iterator(ii=1:N): a(1:2, ii)
iterator(jj=1:M): b(1:2, jj)
