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

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

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

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