------- Comment #6 from ebotcazou at gcc dot gnu dot org 2010-02-26 17:46
-------
> We might be able to save the day with the help of TYPE_CANONICAL in this case
> since the size is fixed.
TYPE_CANONICAL is too strong, it will cause useless_type_conversion_p to return
true for conversions between type and subtypes and, even with fixed size, these
conversions aren't useless. So we need this for Ada:
Index: tree-ssa-alias.c
===================================================================
--- tree-ssa-alias.c (revision 156989)
+++ tree-ssa-alias.c (working copy)
@@ -544,13 +544,15 @@ same_type_for_tbaa (tree type1, tree typ
&& TREE_CODE (type2) == ARRAY_TYPE)
return -1;
- /* In Ada, an lvalue of unconstrained type can be used to access an object
- of one of its constrained subtypes, for example when a function with an
- unconstrained parameter passed by reference is called on a constrained
- object and inlined. In this case, the types have the same alias set. */
- if (TYPE_SIZE (type1) && TYPE_SIZE (type2)
- && TREE_CONSTANT (TYPE_SIZE (type1)) != TREE_CONSTANT (TYPE_SIZE
(type2))
- && get_alias_set (type1) == get_alias_set (type2))
+ /* ??? In Ada, an lvalue of an unconstrained type can be used to access an
+ object of one of its constrained subtypes, e.g. when a function with an
+ unconstrained parameter passed by reference is called on an object and
+ inlined. But, even in the case of a fixed size, type and subtypes are
+ not equivalent enough as to share the same TYPE_CANONICAL, since this
+ would mean that conversions between them are useless, whereas they are
+ not (e.g. type and subtypes can have different modes). So, in the end,
+ they are only guaranteed to have the same alias set. */
+ if (get_alias_set (type1) == get_alias_set (type2))
return -1;
/* The types are known to be not equal. */
The change doesn't introduce any regressions in the C/C++ testsuite on x86 so
it might not be a big pessimization.
Richard, what do you think?
--
ebotcazou at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu dot
| |org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43096
