Re: [PATCH, MPX, 2/X] Pointers Checker [9/25] Bound constants

[Ilya Enkovich <enkovich dot gnu at gmail dot com>]

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2013/11/7 Ilya Enkovich <enkovich.gnu@gmail.com>:
> 2013/11/7 Jeff Law <law@redhat.com>:
>> On 10/31/13 03:15, Ilya Enkovich wrote:
>>>
>>> Hi,
>>>
>>> Here is a patch which adds support for bound constant to be used as
>>> DECL_INITIAL for constant static bounds generated by compiler.
>>>
>>> Thanks,
>>> Ilya
>>> --
>>>
>>> gcc/
>>>
>>> 2013-10-23  Ilya Enkovich  <ilya.enkovich@intel.com>
>>>
>>>         * emit-rtl.c (immed_double_const): Support MODE_POINTER_BOUNDS.
>>>         * explow.c (trunc_int_for_mode): Likewise.
>>>         * varpool.c (ctor_for_folding): Do not fold constant
>>>         bounds vars.
>>
>> I'm having a bit of trouble reconciling "add support for bound constant to
>> be used as DECL_INITIAL" rationale text and the actual patch.
>>
>> From reading the patch it appears that you want to allow generation of
>> immediate constants for objects with MODE_POINTER_BOUNDS.  OK, I can see how
>> that is useful.
>>
>> I can kindof see how you want to error out if someone asks for a constant to
>> be truncated to MODE_POINTER_BOUNDS.  Did this trip in practice or is it
>> preemptive?
>
> As far as I remember change in trunc_int_mode was required to expand
> bound constants on 32bit target. Size of the constant is equal to size
> of the HOST_WIDE_INT and thus constant generation goes through
> gen_int_mode and trunc_int_for_mode.
>
>>
>>
>>
>>> diff --git a/gcc/varpool.c b/gcc/varpool.c
>>> index 2eb1fc1..d9c08c1 100644
>>> --- a/gcc/varpool.c
>>> +++ b/gcc/varpool.c
>>> @@ -254,6 +254,12 @@ ctor_for_folding (tree decl)
>>>         && TREE_CODE (decl) != CONST_DECL)
>>>       return error_mark_node;
>>>
>>> +  /* Static constant bounds are created to be
>>> +     used instead of constants and therefore
>>> +     do not let folding it.  */
>>> +  if (POINTER_BOUNDS_P (decl))
>>> +    return error_mark_node;
>>
>> Here's the part I'm struggling a bit with.    Why did you need this?
>>
>> Isn't this going to prevent that DECL from being used in folding?   The
>> bounds shouldn't really affect that AFAICT.
>
> Bounds constants were introduced only for initialization of constant
> bound vars. Such vars are used to hold commonly used zero bounds (for
> cases when bounds are unknown) values and null bounds (for null
> pointers). Usage of such vars is optional and is controlled via
> compiler flag. It is used to try to decrease overhead on bounds
> creation. E.g. for MPX we need two instructions to create zero bounds
> and also it require one GPR. One of these instructions does not become
> nop when MPX is off which additionally increases overhead.  Having
> constant var we can just load bounds using one MPX instruction.  And
> if I do not prevent folding for these vars then all constant bounds
> vars usages are replaced with immediate bounds constant usages and I
> do not get desired effect.  Since there are no instructions working
> with bounds immediates, I do not see reasons for folding.
>
> Thanks,
> Ilya
>
>>
>> jeff