[Bug jit/66812] jit code-generation example that unexpectedly required -fno-strict-aliasing to work

[dmalcolm at gcc dot gnu.org]

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.