Jakub,
this patch fixes a gcc_assert in expand_omp_for_static_chunk.
The assert follows a loop with composite loop condition:
...
vec<edge_var_map> *head = redirect_edge_var_map_vector (re);
ene = single_succ_edge (entry_bb);
psi = gsi_start_phis (fin_bb);
for (i = 0; !gsi_end_p (psi) && head->iterate (i, &vm);
gsi_next (&psi), ++i)
...
AFAIU, the intention of the assert is that it tries to check that both:
- all phis have been handled (gsi_end_p (psi)), and
- all elements of head have been used (head->length () == i).
In other words, that we have stopped iterating because both loop conditions are
false.
The current assert checks that *not* all phis have been handled:
...
gcc_assert (!gsi_end_p (psi) && i == head->length ());
...
Looking back in the history, it seems we started out with the 'all phis handled'
semantics, but I suspect that that got lost due to a typo:
...
79acaae1 2007-09-07
gcc_assert (!phi && !args);
75a70cf95 2008-07-28
gcc_assert (!gsi_end_p (psi) && i == VEC_length (edge_var_map, head));
f1f41a6c 2012-11-18
gcc_assert (!gsi_end_p (psi) && i == head->length ());
...
Now, if the current assert is incorrect, why didn't it trigger?
The assert is in ssa-handling code in expand_omp_for_static_chunk. Ssa-handling
code in omp-low.c is only triggered by pass_parallelize_loops, and that pass
doesn't specify a chunk size on the GIMPLE_OMP_FOR it constructs, so that will
only call expand_omp_for_static_nochunk.
I managed to trigger this assert in my oacc kernels directive patch set (on top
of the gomp-4_0-branch), which constructs an oacc for loop in
pass_parallelize_loops, and then this code in gomp-4_0-branch has the effect
that we trigger expand_omp_for_static_chunk:
...
//TODO
/* For OpenACC loops, force a chunk size of one, as this avoids the default
scheduling where several subsequent iterations are being executed by the
same thread. */
if (gimple_omp_for_kind (for_stmt) == GF_OMP_FOR_KIND_OACC_LOOP)
{
gcc_assert (fd->chunk_size == NULL_TREE);
fd->chunk_size = build_int_cst (TREE_TYPE (fd->loop.v), 1);
}
...
So, AFAIU, this assert (and associated ssa-handling code in
expand_omp_for_static_chunk) is dead on trunk, but I'm excercising the code
currently in my patch series, so I'd prefer to fix it rather than remove it.
Bootstrapped and reg-tested on x86_64, on top of trunk, gomp-4_0-branch and
internal oacc dev branch.
OK for trunk?