Eric, the current way component_uses_parent_alias_set is called from
get_alias_set causes the reference tree chain to be walked O(n^2)
in case there is any DECL_NONADDRESSABLE_P or TYPE_NONALIASED_COMPONENT
reference in the tree chain. Also the comment above
component_uses_parent_alias_set doesn't seem to match behavior
in get_alias_set. get_alias_set ends up using exactly the type of
the parent
of the DECL_NONADDRESSABLE_P / TYPE_NONALIASED_COMPONENT reference
instead of "the alias set provided by the object at the heart of T"
I also fail to see why component_uses_parent_alias_set invokes
get_alias_set (is that to make 'a.c' in struct { char c; } a;
use the alias set of 'a' instead of the alias set of 'char'?
Or is it for correctness reasons in case there is a ref-all
indirect ref at the base? For the former I believe it doesn't
work because it only checks the non-outermost type).
So, the following patch removes the quadraticness by returning
the parent of the innermost non-addressable (or alias-set zero)
reference component instead of just a flag. That changes behavior
for the alias-set zero case but still doesn't match the overall
function comment.
The only other use besides from get_alias_set is to set
MEM_KEEP_ALIAS_SET_P - whatever that is exactly for, I can
see a single non-obvious use in store_constructor_field
(didn't bother to lookup how callers specify the alias_set argument).
In
if (MEM_P (to_rtx) && !MEM_KEEP_ALIAS_SET_P (to_rtx)
&& DECL_NONADDRESSABLE_P (field))
{
to_rtx = copy_rtx (to_rtx);
MEM_KEEP_ALIAS_SET_P (to_rtx) = 1;
}
we can just drop the MEM_KEEP_ALIAS_SET_P check (well, in case
MEM_KEEP_ALIAS_SET_P is dropped). And for the following call
store_constructor_field (to_rtx, bitsize, bitpos, mode,
value, cleared,
get_alias_set (TREE_TYPE (field)));
pass MEM_ALIAS_SET instead of get_alias_set (TREE_TYPE (field)) for
DECL_NONADDRESSABLE_P (field), similar for the array handling in
the other use.
Btw, checks like
if (!MEM_KEEP_ALIAS_SET_P (to_rtx) && MEM_ALIAS_SET (to_rtx) != 0)
set_mem_alias_set (to_rtx, alias_set);
seem to miss MEM_ALIAS_SET (to_rtx) being ALIAS_SET_MEMORY_BARRIER?
Or alias_set being zero / ALIAS_SET_MEMORY_BARRIER?
Anyway, the patch nearly preserves behavior for the emit-rtl.c use.
Comments?
Thanks,
Richard.
Index: gcc/alias.c
===================================================================
--- gcc/alias.c (revision 202865)
+++ gcc/alias.c (working copy)
@@ -510,41 +510,49 @@ objects_must_conflict_p (tree t1, tree t
alias set used by the component, but we don't have per-FIELD_DECL
assignable alias sets. */
-bool
+tree
component_uses_parent_alias_set (const_tree t)
{
- while (1)
+ const_tree non_addressable = NULL_TREE;
+ while (handled_component_p (t))
{
- /* If we're at the end, it vacuously uses its own alias set. */
- if (!handled_component_p (t))
- return false;
-
switch (TREE_CODE (t))
{
case COMPONENT_REF:
if (DECL_NONADDRESSABLE_P (TREE_OPERAND (t, 1)))
- return true;
+ non_addressable = t;
break;
case ARRAY_REF:
case ARRAY_RANGE_REF:
if (TYPE_NONALIASED_COMPONENT (TREE_TYPE (TREE_OPERAND (t, 0))))
- return true;
+ non_addressable = t;
break;
case REALPART_EXPR:
case IMAGPART_EXPR:
break;
- default:
+ case BIT_FIELD_REF:
+ case VIEW_CONVERT_EXPR:
/* Bitfields and casts are never addressable. */
- return true;
+ non_addressable = t;
+ break;
+
+ default:
+ gcc_unreachable ();
}
- t = TREE_OPERAND (t, 0);
if (get_alias_set (TREE_TYPE (t)) == 0)
- return true;
+ non_addressable = t;
+
+ t = TREE_OPERAND (t, 0);
}
+
+ if (non_addressable)
+ return TREE_OPERAND (non_addressable, 0);
+
+ return NULL_TREE;
}
@@ -645,14 +653,11 @@ reference_alias_ptr_type_1 (tree *t)
(TREE_TYPE (TREE_TYPE (TREE_OPERAND (inner, 1))))))
return TREE_TYPE (TREE_OPERAND (inner, 1));
- /* Otherwise, pick up the outermost object that we could have a pointer
- to, processing conversions as above. */
- /* ??? Ick, this is worse than quadratic! */
- while (component_uses_parent_alias_set (*t))
- {
- *t = TREE_OPERAND (*t, 0);
- STRIP_NOPS (*t);
- }
+ /* Otherwise, pick up the outermost object that we could have
+ a pointer to. */
+ tree tem = component_uses_parent_alias_set (*t);
+ if (tem)
+ *t = tem;
return NULL_TREE;
}
Index: gcc/alias.h
===================================================================
--- gcc/alias.h (revision 202865)
+++ gcc/alias.h (working copy)
@@ -33,7 +33,7 @@ extern alias_set_type get_alias_set (tre
extern alias_set_type get_deref_alias_set (tree);
extern alias_set_type get_varargs_alias_set (void);
extern alias_set_type get_frame_alias_set (void);
-extern bool component_uses_parent_alias_set (const_tree);
+extern tree component_uses_parent_alias_set (const_tree);
extern bool alias_set_subset_of (alias_set_type, alias_set_type);
extern void record_alias_subset (alias_set_type, alias_set_type);
extern void record_component_aliases (tree);
