This intrinsic is slightly different from others in that it needs some data
flow analysis to tell the constant it folded to. In the long run, I think
the BE try to fold it whenever possible. Apparently,
CG is too late. If you check the definition of the intrinsic definition, I
flagged it as non-CG intrinsic.
I did't hard-code to prevent future change. If one want to pass it down to
BE, simply remove the
the Fold_object_size().
Also note that this intrinsic is marked as side-effect-free intrinsic op, it
can be optimized in the backend like an arithmetic expression.
Shuxin
On Wed, Dec 8, 2010 at 12:18 PM, Jian-Xin Lai <laij...@gmail.com> wrote:
> I wonder it's better to pass this intrinsic to BE and lower it just before
> CGEXP phase. So that we keep the opportunity to optimize it in wopt in the
> future.
>
> Another suggestion, for the ID of the intrin, "INTRN_OBJECT_SIZE" is better
> than "INTRN_OBJ_SZ".
>
> 2010/12/8 shuxin yang <shuxin.ope...@gmail.com>
>
>> Hi, there:
>>
>> Bug 586 is about back-end folding gcc extension
>> __builtin_object_size() into a compile time constant.
>>
>> If I understand right, "__builtin_object_size(void*p, int type)"
>> depends on compiler's capability
>> of data flow analysis; if the compiler is able to reveal the object O that
>> <p> points to, it will returns
>> sizeof(typeof(O)); otherwise, it returns "type < 2? -1 : 0".
>>
>> gcc tries to fold this built-in function in many places; it calls
>> fold_builtin_object_size()@builtin.c
>> prior to generic -> gspin conversion. However, fold_builtin_object_size()
>> is only able to handle simple cases;
>> general cases are taken cared by a pass named "objsz" (see
>> tree-object-size.c) which rely on
>> SSA and it is called after the generic->gspin conversion.
>>
>> My fix, honestly kludge, is simply to replace the builtin function with
>> "type < 2 ? -1 : 0".
>> In the long run, the back-end should have full-blown support.
>>
>> Thanks
>> Shuxin
>>
>>
>>
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>
>
> --
> Regards,
> Lai Jian-Xin
>
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