https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66812
--- Comment #9 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Notes to self:
For the given stmt, fre1's call to:
val = vn_reference_lookup (gimple_assign_lhs (stmt),
gimple_vuse (stmt), VN_WALK, NULL);
is returning NULL for "val", whereas libgccjit is returning the const_int 0
node,
hence the latter regards stmt as as redundant assignment and eliminates it.
Within the call to vn_reference_lookup:
2232 vr1.set = get_alias_set (op);
For cc1:
(gdb) p vr1.set
$24 = 0
(gdb) p wvnresult
$26 = (vn_reference_t) 0x0
and it bails out of vn_reference_lookup here:
2262 return NULL_TREE;
Whereas for libgccjit, the call to vn_reference_lookup does this:
(gdb) p vr1.set
$6 = 2
and walk_non_aliased_vuses returns non-NULL:
(gdb) p wvnresult
$7 = (vn_reference_t) 0x677dd0
libgccjit:
(gdb) call get_alias_set (op)
$11 = 2
cc1:
(gdb) call get_alias_set(op)
$27 = 0
and this is because of a langhook, implemented for C/C++/ObjC, but not (yet)
for libgccjit:
837 alias_set_type
838 get_alias_set (tree t)
839 {
(snip)
849
850 /* We can be passed either an expression or a type. This and the
851 language-specific routine may make mutually-recursive calls to
each other
852 to figure out what to do. At each juncture, we see if this is a
tree
853 that the language may need to handle specially. First handle
things that
854 aren't types. */
855 if (! TYPE_P (t))
856 {
857 /* Give the language a chance to do something with this tree
858 before we look at it. */
859 STRIP_NOPS (t);
860 set = lang_hooks.get_alias_set (t);
861 if (set != -1)
862 return set;
c/c-objc-common.h:45:#undef LANG_HOOKS_GET_ALIAS_SET
c/c-objc-common.h:46:#define LANG_HOOKS_GET_ALIAS_SET c_common_get_alias_set
c-family/c-common.c:
5157 /* Return the typed-based alias set for T, which may be an expression
5158 or a type. Return -1 if we don't do anything special. */
5159
5160 alias_set_type
5161 c_common_get_alias_set (tree t)
5162 {
5163 tree u;
(snip)
5173
5174 /* Permit type-punning when accessing a union, provided the access
5175 is directly through the union. For example, this code does not
5176 permit taking the address of a union member and then storing
5177 through it. Even the type-punning allowed here is a GCC
5178 extension, albeit a common and useful one; the C standard says
5179 that such accesses have implementation-defined behavior. */
5180 for (u = t;
5181 TREE_CODE (u) == COMPONENT_REF || TREE_CODE (u) == ARRAY_REF;
5182 u = TREE_OPERAND (u, 0))
5183 if (TREE_CODE (u) == COMPONENT_REF
5184 && TREE_CODE (TREE_TYPE (TREE_OPERAND (u, 0))) == UNION_TYPE)
5185 return 0;
For C, it's exiting at line 5185 for the given statement's LHS.
