Re: [PATCH PR43513, 1/3] Replace vla with array - Implementation.

[Richard Guenther <richard dot guenther at gmail dot com>]

On Sat, Sep 24, 2011 at 5:29 PM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> This is an updated version of the patch. I have 2 new patches and an
>> updated testcase which I will sent out individually.
>>
>> Patch set was bootstrapped and reg-tested on x86_64.
>>
>> Ok for trunk?
>>
>> Thanks,
>> - Tom
>>
>> 2011-07-30 ?Tom de Vries ?<tom@codesourcery.com>
>>
>> ? ? ? PR middle-end/43513
>> ? ? ? * Makefile.in (tree-ssa-ccp.o): Add $(PARAMS_H) to rule.
>> ? ? ? * tree-ssa-ccp.c (params.h): Include.
>> ? ? ? (fold_builtin_alloca_for_var): New function.
>> ? ? ? (ccp_fold_stmt): Use fold_builtin_alloca_for_var.
>
> We have detected another fallout on some Ada code: the transformation replaces
> a call to __builtin_alloca with &var, i.e. it introduces an aliased variable,
> which invalidates the points-to information of some subsequent call, fooling
> DSE into thinking that it can eliminate a live store.

Ugh, yeah.  I suppose PTA assigned a HEAP var as pointed-to object for the
original pointer, even if the transformed stmt

 orig_ptr_1 = &a;

has the points-to information preserved for orig_ptr_1 further propagation of
&a will make accesses through orig_ptr_1 have different alias properties.

What should work in this special case of a singleton points-to set of orig_ptr_1
(might want to check that) is, do

  SET_DECL_PT_UID (decl-of-a, DECL_UID (pointed-to orig_ptr_1));

The brute force approach is not acceptable (it'll wreck IPA points-to info).

A helper like pt_solution_singleton_p (struct pt_solution *pt, unsigned *uid)
whould be nice to have for this.

Note that we don't have points-to information computed during the first
CCP pass, so the above should be conditional on SSA_NAME_PTR_INFO
being present and not ! ->anything (but then assert that we actually do have
a singleton, or fail the folding).

Richard.

> The brute force approach
>
> Index: tree-ssa-ccp.c
> ===================================================================
> --- tree-ssa-ccp.c ? ? ?(revision 179038)
> +++ tree-ssa-ccp.c ? ? ?(working copy)
> @@ -2014,7 +2014,10 @@ do_ssa_ccp (void)
> ? ccp_initialize ();
> ? ssa_propagate (ccp_visit_stmt, ccp_visit_phi_node);
> ? if (ccp_finalize ())
> - ? ?return (TODO_cleanup_cfg | TODO_update_ssa | TODO_remove_unused_locals);
> + ? ?return (TODO_cleanup_cfg
> + ? ? ? ? ? | TODO_update_ssa
> + ? ? ? ? ? | TODO_rebuild_alias
> + ? ? ? ? ? | TODO_remove_unused_locals);
> ? else
> ? ? return 0;
> ?}
>
> works, but we might want to be move clever. ?Thoughts?
>
> --
> Eric Botcazou
>