[patch] Fix PR rtl-optimization/54870

[Eric Botcazou]

> Hmm.  I think this points to an issue with update_alias_info_with_stack_vars
> instead.  That is, this function should have already cared for handling
> this case where two decls have their stack slot shared.

The problem here is that mark_addressable is called _after_ the function is
run.  IOW, by the time update_alias_info_with_stack_vars is run, there are no 
aliased variables in the function.

> static bool
> can_escape (tree expr)
> {
>   tree base;
>   if (!expr)
>     return true;
>   base = get_base_address (expr);
>   if (DECL_P (base)
>       && !may_be_aliased (base))
>     return false;
>   return true;
> 
> it needs to check decls_to_pointers[base] and then check
> if any of the pointed-to decls may be aliased.

That's essentially what the patch does though (except that it does it more 
efficiently), since update_alias_info_with_stack_vars correctly computes
cfun->gimple_df->escaped.vars for partitioned decls.

> Now, that's not that easy because we don't have a
> mapping from DECL UID to DECL (and the decl
> isn't in the escaped solution if it is just used by
> memcpy), but we could compute a bitmap of
> all address-taken decls in update_alias_info_with_stack_vars
> or simply treat all check decls_to_pointers[base] != NULL
> bases as possibly having their address taken.

OK, we can populate another bitmap in update_alias_info_with_stack_vars and 
update it in mark_addressable by means of decls_to_pointers and pi->pt.vars.
That seems a bit redundant with cfun->gimple_df->escaped.vars, but why not.

-- 
Eric Botcazou