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Re: [PATCH, MPX, 2/X] Pointers Checker [7/25] Suppress BUILT_IN_CHKP_ARG_BND optimizations.


On Wed, Nov 6, 2013 at 12:00 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
> 2013/11/6 Richard Biener <richard.guenther@gmail.com>:
>> On Tue, Nov 5, 2013 at 3:24 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>> 2013/11/5 Richard Biener <richard.guenther@gmail.com>:
>>>> On Tue, Nov 5, 2013 at 1:52 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>>>>
>>>>> For input parameter P I need to have
>>>>>   BOUNDS = __builtin_arg_bnd (P)
>>>>> to somehow refer to bounds of P in GIMPLE.  Optimizations may modify
>>>>> __builtin_arg_bnd (P) replacing P with its copy or some value. It
>>>>> makes call useless because removes information about parameter whose
>>>>> bounds we refer to. I want such optimization to ignore
>>>>> __builtin_arg_bnd calls and always leave default SSA_NAME of PARM_DECL
>>>>> there as arg.
>>>>
>>>> How does a compilable testcase look like that shows how the default def
>>>> is used?  And what transforms break that use?  I really cannot see
>>>> how this would happen (and you didn't add a testcase).
>>>
>>> Here is a test source:
>>>
>>> extern int bar1 (int *p);
>>> extern int bar2 (int);
>>>
>>> int foo (int *p)
>>> {
>>>   if (!p)
>>>     return bar1 (p);
>>>   return bar2 (10);
>>> }
>>>
>>> After instrumentation GIMPLE looks like:
>>>
>>> foo (int * p)
>>> {
>>>   <unnamed type> __bound_tmp.0;
>>>   int _1;
>>>   int _6;
>>>   int _8;
>>>
>>>   <bb 6>:
>>>   __bound_tmp.0_9 = __builtin_ia32_arg_bnd (p_3(D));
>>>
>>>   <bb 2>:
>>>   if (p_3(D) == 0B)
>>>     goto <bb 3>;
>>>   else
>>>     goto <bb 4>;
>>>
>>>   <bb 3>:
>>>   _6 = bar1 (p_3(D), __bound_tmp.0_9);
>>>   goto <bb 5>;
>>>
>>>   <bb 4>:
>>>   _8 = bar2 (10);
>>>
>>>   <bb 5>:
>>>   # _1 = PHI <_6(3), _8(4)>
>>>   return _1;
>>>
>>> }
>>>
>>> Here is optimized GIMPLE (if I do not apply my changes in tree-ssa-dom.c):
>>>
>>> foo (int * p)
>>> {
>>>   <unnamed type> __bound_tmp.0;
>>>   int _1;
>>>   int _6;
>>>   int _8;
>>>
>>>   <bb 2>:
>>>   if (p_3(D) == 0B)
>>>     goto <bb 3>;
>>>   else
>>>     goto <bb 4>;
>>>
>>>   <bb 3>:
>>>   __bound_tmp.0_9 = __builtin_ia32_arg_bnd (0B); [return slot optimization]
>>>   _6 = bar1 (0B, __bound_tmp.0_9); [tail call]
>>>   goto <bb 5>;
>>>
>>>   <bb 4>:
>>>   _8 = bar2 (10); [tail call]
>>>
>>>   <bb 5>:
>>>   # _1 = PHI <_6(3), _8(4)>
>>>   return _1;
>>>
>>> }
>>>
>>> Now during expand or inline of foo I cannot determine the value for
>>> __bound_tmp.0_9.
>>
>> Well, but clearly you can see that bounds for p == NULL are
>> useless and thus there is no problem with the transform.
>
> This example just demonstrates the issue. I may use some var's address
> in comparison with the similar result. And value does not always allow
> to determine bounds, because pointers with the same value may have
> different bounds.

Some vars address can be used for better bounds deriving though.
Well, it clearly shows it's a "get me good results" issue and not
a correctness issue.  And for "good results" you trade in missed
general optimizations.  That doesn't sound very good to me.

Richard.

>>
>> Also sth weird is going on as your arg_bnd call gets [return slot optimization].
>> It seems that your POINTER_BOUNDS_TYPE is seen as aggregate somehow.
>
> Target is responsible for having return slot optimization here. And
> target expands all such calls. So, do not see the problem here.
>
> Ilya
>
>>
>> Richard.
>>
>>> Ilya
>>>>
>>>> Richard.
>>>>


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