Re: Aliasing violation generated by fold_builtin_memcmp?

[Olivier Hainque <hainque at adacore dot com>]

Richard Henderson wrote:
> Try
> 
>   cst_uchar_ptr_node
>     = build_pointer_type_for_mode (cst_uchar_node, ptr_mode, true);
> 
> which is apparently in use by the Ada front end, but only if a
> certain pragma is given.  Dunno how reliably that's likely to work.

 We are seeing regressions in our local testsuite on cases exercising
 that pragma.

 Passing 'true' as CAN_ALIAS_ALL sets TYPE_REF_CAN_ALIAS_ALL (t), but
 this apparently has no influence on what tree-ssa-alias computes.

 Out of a preliminary look into this code (new to me), a possible place
 to address that appears to be 'get_tmt_for', which presumably should assign
 a zero alias set to tags for pointer types with that bit set.

 The current code doesn't do that:

   tree tag_type = TREE_TYPE (TREE_TYPE (ptr));
   HOST_WIDE_INT tag_set = get_alias_set (tag_type);


 I'd be happy to work-on and submit a patch to deal with this the proper
 way. I'd welcome hints or directions on what the proper way should be, as
 I don't yet have a global view of the complete alias analysis circuitry.

 The 'tag alias set should be zero if ...' idea above seems logical to me.
 I still may well not yet see a number of other options.