Re: [PATCH, PR66846] Mark inner loop for fixup in parloops

[Tom de Vries <Tom_deVries@mentor.com>]

On 16/07/15 10:44, Richard Biener wrote:
> On Wed, Jul 15, 2015 at 9:36 PM, Tom de Vries <Tom_deVries@mentor.com> wrote:
>> Hi,
>>
>> I.
>>
>> In openmp expansion of loops, we do some effort to try to create matching
>> loops in the loop state of the child function, f.i.in
>> expand_omp_for_generic:
>> ...
>>        struct loop *outer_loop;
>>        if (seq_loop)
>>          outer_loop = l0_bb->loop_father;
>>        else
>>          {
>>            outer_loop = alloc_loop ();
>>            outer_loop->header = l0_bb;
>>            outer_loop->latch = l2_bb;
>>            add_loop (outer_loop, l0_bb->loop_father);
>>          }
>>
>>        if (!gimple_omp_for_combined_p (fd->for_stmt))
>>          {
>>            struct loop *loop = alloc_loop ();
>>            loop->header = l1_bb;
>>            /* The loop may have multiple latches.  */
>>            add_loop (loop, outer_loop);
>>          }
>> ...
>>
>> And if that doesn't work out, we try to mark the loop state for fixup, in
>> expand_omp_taskreg and expand_omp_target:
>> ...
>>        /* When the OMP expansion process cannot guarantee an up-to-date
>>           loop tree arrange for the child function to fixup loops.  */
>>        if (loops_state_satisfies_p (LOOPS_NEED_FIXUP))
>>          child_cfun->x_current_loops->state |= LOOPS_NEED_FIXUP;
>> ...
>>
>> and expand_omp_for:
>> ...
>>    else
>>      /* If there isn't a continue then this is a degerate case where
>>         the introduction of abnormal edges during lowering will prevent
>>         original loops from being detected.  Fix that up.  */
>>      loops_state_set (LOOPS_NEED_FIXUP);
>> ...
>>
>> However, loops are fixed up anyway, because the first pass we execute with
>> the new child function is pass_fixup_cfg.
>>
>> The new child function contains a function call to
>> __builtin_omp_get_num_threads, which is marked with ECF_CONST, so
>> execute_fixup_cfg marks the function for TODO_cleanup_cfg, and subsequently
>> the loops with LOOPS_NEED_FIXUP.
>>
>>
>> II.
>>
>> This patch adds a verification that at the end of the omp-expand processing
>> of the child function, either the loop structure is ok, or marked for fixup.
>>
>> This verfication triggered a failure in parloops. When an outer loop is
>> being parallelized, both the outer and inner loop are cancelled. Then during
>> omp-expansion, we create a loop in the loop state for the outer loop (the
>> one that is transformed), but not for the inner, which causes the
>> verification failure:
>> ...
>> outer-1.c:11:3: error: loop with header 5 not in loop tree
>> ...
>>
>> [ I ran into this verification failure with an openacc kernels testcase on
>> the gomp-4_0-branch, where parloops is called additionally from a different
>> location, and pass_fixup_cfg is not the first pass that the child function
>> is processed by. ]
>>
>> The patch contains a bit that makes sure that the loop state of the child
>> function is marked for fixup in parloops. The bit is non-trival since it
>> create a loop state and sets the fixup flag on the loop state, but postpones
>> the init_loops_structure call till move_sese_region_to_fn, where it can
>> succeed.
>>
>>
>> III.
>>
>> Bootstrapped and reg-tested on x86_64.
>>
>> OK for trunk?
>
> +  struct loops *loops;
> +  int loop_state_flags = 0;
> +  if (dest_cfun->x_current_loops == NULL)
> +    {
> +      /* Initialize an empty loop tree.  */
> +      loops = ggc_cleared_alloc<struct loops> ();
> +      set_loops_for_fn (dest_cfun, loops);
> +    }
> +  else
> +    {
> +      loops = dest_cfun->x_current_loops;
> +      loop_state_flags = loops->state;
> +    }
> +
> +  if (loops->tree_root == NULL)
> +    {
> +      init_loops_structure (dest_cfun, loops, 1);
> +      loops->state |= loop_state_flags;
> +    }
> +
> +  loops->state |= LOOPS_MAY_HAVE_MULTIPLE_LATCHES;
>
> this looks twisted just because you do a half-way init of the loop tree here:
>
> +  if (loop->inner)
> +    {
> +      struct function *child_cfun = DECL_STRUCT_FUNCTION (child_fn);
> +      struct loops *loops = ggc_cleared_alloc<struct loops> ();
> +      set_loops_for_fn (child_cfun, loops);
> +      child_cfun->x_current_loops->state = LOOPS_NEED_FIXUP;
> +    }
>
> why not unconditionally initialize the loop tree properly in autopar?
>

Because init_loops_structure accesses f.i. n_basic_blocks_for_fn 
(child_cfun), in other words child_cfun->cfg->x_n_basic_blocks. At this 
point in parloops, there's no child_cfun->cfg yet, that field is set by 
the following pass_expand_omp_ssa.

Normally, the solution is to do loops_state_set (LOOPS_NEED_FIXUP) for 
the parent function, which will get propagated to the child. But I'm 
trying to be more precise than that, by only setting LOOPS_NEED_FIXUP 
for the child, not the parent.

Thanks,
- Tom