Re: [RFC] propagate malloc attribute in ipa-pure-const pass

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

On 17 August 2017 at 18:02, Prathamesh Kulkarni
<prathamesh.kulkarni@linaro.org> wrote:
> On 8 August 2017 at 09:50, Prathamesh Kulkarni
> <prathamesh.kulkarni@linaro.org> wrote:
>> On 31 July 2017 at 23:53, Prathamesh Kulkarni
>> <prathamesh.kulkarni@linaro.org> wrote:
>>> On 23 May 2017 at 19:10, Prathamesh Kulkarni
>>> <prathamesh.kulkarni@linaro.org> wrote:
>>>> On 19 May 2017 at 19:02, Jan Hubicka <hubicka@ucw.cz> wrote:
>>>>>>
>>>>>> * LTO and memory management
>>>>>> This is a general question about LTO and memory management.
>>>>>> IIUC the following sequence takes place during normal LTO:
>>>>>> LGEN: generate_summary, write_summary
>>>>>> WPA: read_summary, execute ipa passes, write_opt_summary
>>>>>>
>>>>>> So I assumed it was OK in LGEN to allocate return_callees_map in
>>>>>> generate_summary and free it in write_summary and during WPA, allocate
>>>>>> return_callees_map in read_summary and free it after execute (since
>>>>>> write_opt_summary does not require return_callees_map).
>>>>>>
>>>>>> However with fat LTO, it seems the sequence changes for LGEN with
>>>>>> execute phase takes place after write_summary. However since
>>>>>> return_callees_map is freed in pure_const_write_summary and
>>>>>> propagate_malloc() accesses it in execute stage, it results in
>>>>>> segmentation fault.
>>>>>>
>>>>>> To work around this, I am using the following hack in pure_const_write_summary:
>>>>>> // FIXME: Do not free if -ffat-lto-objects is enabled.
>>>>>> if (!global_options.x_flag_fat_lto_objects)
>>>>>>   free_return_callees_map ();
>>>>>> Is there a better approach for handling this ?
>>>>>
>>>>> I think most passes just do not free summaries with -flto.  We probably want
>>>>> to fix it to make it possible to compile multiple units i.e. from plugin by
>>>>> adding release_summaries method...
>>>>> So I would say it is OK to do the same as others do and leak it with -flto.
>>>>>> diff --git a/gcc/ipa-pure-const.c b/gcc/ipa-pure-const.c
>>>>>> index e457166ea39..724c26e03f6 100644
>>>>>> --- a/gcc/ipa-pure-const.c
>>>>>> +++ b/gcc/ipa-pure-const.c
>>>>>> @@ -56,6 +56,7 @@ along with GCC; see the file COPYING3.  If not see
>>>>>>  #include "tree-scalar-evolution.h"
>>>>>>  #include "intl.h"
>>>>>>  #include "opts.h"
>>>>>> +#include "ssa.h"
>>>>>>
>>>>>>  /* Lattice values for const and pure functions.  Everything starts out
>>>>>>     being const, then may drop to pure and then neither depending on
>>>>>> @@ -69,6 +70,15 @@ enum pure_const_state_e
>>>>>>
>>>>>>  const char *pure_const_names[3] = {"const", "pure", "neither"};
>>>>>>
>>>>>> +enum malloc_state_e
>>>>>> +{
>>>>>> +  PURE_CONST_MALLOC_TOP,
>>>>>> +  PURE_CONST_MALLOC,
>>>>>> +  PURE_CONST_MALLOC_BOTTOM
>>>>>> +};
>>>>>
>>>>> It took me a while to work out what PURE_CONST means here :)
>>>>> I would just call it something like STATE_MALLOC_TOP... or so.
>>>>> ipa_pure_const is outdated name from the time pass was doing only
>>>>> those two.
>>>>>> @@ -109,6 +121,10 @@ typedef struct funct_state_d * funct_state;
>>>>>>
>>>>>>  static vec<funct_state> funct_state_vec;
>>>>>>
>>>>>> +/* A map from node to subset of callees. The subset contains those callees
>>>>>> + * whose return-value is returned by the node. */
>>>>>> +static hash_map< cgraph_node *, vec<cgraph_node *>* > *return_callees_map;
>>>>>> +
>>>>>
>>>>> Hehe, a special case of return jump function.  We ought to support those more generally.
>>>>> How do you keep it up to date over callgraph changes?
>>>>>> @@ -921,6 +1055,23 @@ end:
>>>>>>    if (TREE_NOTHROW (decl))
>>>>>>      l->can_throw = false;
>>>>>>
>>>>>> +  if (ipa)
>>>>>> +    {
>>>>>> +      vec<cgraph_node *> v = vNULL;
>>>>>> +      l->malloc_state = PURE_CONST_MALLOC_BOTTOM;
>>>>>> +      if (DECL_IS_MALLOC (decl))
>>>>>> +     l->malloc_state = PURE_CONST_MALLOC;
>>>>>> +      else if (malloc_candidate_p (DECL_STRUCT_FUNCTION (decl), v))
>>>>>> +     {
>>>>>> +       l->malloc_state = PURE_CONST_MALLOC_TOP;
>>>>>> +       vec<cgraph_node *> *callees_p = new vec<cgraph_node *> (vNULL);
>>>>>> +       for (unsigned i = 0; i < v.length (); ++i)
>>>>>> +         callees_p->safe_push (v[i]);
>>>>>> +       return_callees_map->put (fn, callees_p);
>>>>>> +     }
>>>>>> +      v.release ();
>>>>>> +    }
>>>>>> +
>>>>>
>>>>> I would do non-ipa variant, too.  I think most attributes can be detected that way
>>>>> as well.
>>>>>
>>>>> The patch generally makes sense to me.  It would be nice to make it easier to write such
>>>>> a basic propagators across callgraph (perhaps adding a template doing the basic
>>>>> propagation logic). Also I think you need to solve the problem with keeping your
>>>>> summaries up to date across callgraph node removal and duplications.
>>>> Thanks for the suggestions, I will try to address them in a follow-up patch.
>>>> IIUC, I would need to modify ipa-pure-const cgraph hooks -
>>>> add_new_function, remove_node_data, duplicate_node_data
>>>> to keep return_callees_map up-to-date across callgraph node insertions
>>>> and removal ?
>>>>
>>>> Also, if instead of having a separate data-structure like return_callees_map,
>>>> should we rather have a flag within cgraph_edge, which marks that the
>>>> caller may return the value of the callee ?
>>> Hi,
>>> Sorry for the very late response. I have attached an updated version
>>> of the prototype patch,
>>> which adds a non-ipa variant, and keeps return_callees_map up-to-date
>>> across callgraph
>>> node insertions and removal. For the non-ipa variant,
>>> malloc_candidate_p() additionally checks
>>> that all the "return callees" have DECL_IS_MALLOC set to true.
>>> Bootstrapped+tested and LTO bootstrapped+tested on x86_64-unknown-linux-gnu.
>>> Does it look OK so far ?
>>>
>>> Um sorry for this silly question, but I don't really understand how
>>> does indirect call propagation
>>> work in ipa-pure-const ? For example consider propagation of nothrow
>>> attribute in following
>>> test-case:
>>>
>>> __attribute__((noinline, noclone, nothrow))
>>> int f1(int k) { return k; }
>>>
>>> __attribute__((noinline, noclone))
>>> static int foo(int (*p)(int))
>>> {
>>>   return p(10);
>>> }
>>>
>>> __attribute__((noinline, noclone))
>>> int bar(void)
>>> {
>>>   return foo(f1);
>>> }
>>>
>>> Shouldn't foo and bar be also marked as nothrow ?
>>> Since foo indirectly calls f1 which is nothrow and bar only calls foo ?
>>> The local-pure-const2 dump shows function is locally throwing  for
>>> "foo" and "bar".
>>>
>>> Um, I was wondering how to get "points-to" analysis for function-pointers,
>>> to get list of callees that may be indirectly called from that
>>> function pointer ?
>>> In the patch I just set node to bottom if it contains indirect calls
>>> which is far from ideal :(
>>> I would be much grateful for suggestions on how to handle indirect calls.
>>> Thanks!
>> ping https://gcc.gnu.org/ml/gcc-patches/2017-07/msg02063.html
> ping * 2 https://gcc.gnu.org/ml/gcc-patches/2017-07/msg02063.html
ping * 3 https://gcc.gnu.org/ml/gcc-patches/2017-07/msg02063.html

Thanks,
Prathamesh
>
> Thanks,
> Prathamesh
>>
>> Thanks,
>> Prathamesh
>>>
>>> Regards,
>>> Prathamesh
>>>>
>>>> Thanks,
>>>> Prathamesh
>>>>>
>>>>> Honza