Re: [committed, gomp4] Fix release_dangling_ssa_names

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

On 05/08/15 11:30, Richard Biener wrote:
> On Wed, 5 Aug 2015, Tom de Vries wrote:
>
>> On 05/08/15 09:29, Richard Biener wrote:
>>>> This patch fixes that by making sure we reset the def stmt to NULL. This
>>>> means
>>>>> we can simplify release_dangling_ssa_names to just test for NULL def
>>>> stmts.
>>> Not sure if I understand the problem correctly but why are you not simply
>>> releasing the SSA name when you remove its definition?
>>
>> In move_sese_region_to_fn we move a region of blocks from one function to
>> another, bit by bit.
>>
>> When we encounter an ssa_name as def or use in the region, we:
>> - generate a new ssa_name,
>> - set the def stmt of the old name as def stmt of the new name, and
>> - add a mapping from the old to the new name.
>> The next time we encounter the same ssa_name in another statement, we find it
>> in the map.
>>
>> If we release the old ssa name, we effectively create statements with operands
>> in the free-list. The first point where that cause breakage, is in
>> walk_gimple_op, which expects the TREE_TYPE of the lhs of an assign to be
>> defined, which is not the case if it's in the free-list:
>> ...
>> case GIMPLE_ASSIGN:
>>    /* Walk the RHS operands.  If the LHS is of a non-renamable type or
>>       is a register variable, we may use a COMPONENT_REF on the RHS.*/
>>    if (wi)
>>      {
>>        tree lhs = gimple_assign_lhs (stmt);
>>        wi->val_only
>>          = (is_gimple_reg_type (TREE_TYPE (lhs)) && !is_gimple_reg (lhs))
>>             || gimple_assign_rhs_class (stmt) != GIMPLE_SINGLE_RHS;
>>      }
>> ...
>
> Hmm, ok, probably because the stmt moving doesn't happen in DOM
> order (move defs before uses).  But
>

There seems to be similar code for the rhs, so I don't think changing 
the order would fix anything.

> +
> +      if (!SSA_NAME_IS_DEFAULT_DEF (name))
> +       /* The statement has been moved to the child function.  It no
> longer
> +          defines name in the original function.  Mark the def stmt NULL,
> and
> +          let release_dangling_ssa_names deal with it.  */
> +       SSA_NAME_DEF_STMT (name) = NULL;
>
> applies also to uses - I don't see why it couldn't happen that you
> move a use but not its def (the def would be a parameter to the
> split-out function).  You'd wreck the IL of the source function this way.
>

If you first move a use, you create a mapping. When you encounter the 
def, you use the mapping. Indeed, if the def is a default def, we don't 
encounter the def. Which is why we create a nop as defining def for 
those cases. The default def in the source function still has a defining 
nop, and has no uses anymore. I don't understand what is broken here.

> I think that the whole dance of actually moving things instead of
> just copying it isn't worth the extra maintainance (well, if we already
> have a machinery duplicating a SESE region to another function - I
> suppose gimple_duplicate_sese_region could be trivially changed to
> support that).
>

I'll mention that as todo. For now, I think the fastest way to get a 
working version is to fix move_sese_region_to_fn.

>Trunk doesn't have release_dangling_ssa_names it seems

Yep, I only ran into this trouble for the kernels region handling. But I 
don't exclude the possibility it could happen for trunk as well.

> but I think
> it belongs to move_sese_region_to_fn and not to omp-low.c

Makes sense indeed.

> and it
> could also just walk the d->vars_map replace_ssa_name fills to
> iterate over the removal candidates

Agreed, I suppose in general that's a win over iterating over all the 
ssa names.

> (and if the situation of
> moving uses but not defs cannot happen you don't need any
> SSA_NAME_DEF_STMT dance either).

I'd prefer to keep the SSA_NAME_DEF_STMT () = NULL bit. It makes sure a 
stmt is the defining stmt of only one ssa-name at all times.

I'll prepare a patch for trunk then.

Thanks,
- Tom