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
