Re: Fwd: [PATCH, expand] Call misaligned memory reference in expand_builtin_return [PR112417]

[HAO CHEN GUI <guihaoc@linux.ibm.com>]

Sorry, forgot to cc gcc-patches.

在 2023/11/13 16:05, HAO CHEN GUI 写道:
> Andrew,
>   Could you kindly inform us what's the functionality of __objc_forward?
> Does it change the memory content pointed by args? Thanks a lot.
> 
> Thanks
> Gui Haochen
> 
> 
> libobjc/sendmsg.c.
> 
>    void *args, *res;
> 
>    args = __builtin_apply_args ();
>    res = __objc_forward (rcv, op, args);
>    if (res)
>      __builtin_return (res);
>    else
>      ...
> 
> -------- 转发的消息 --------
> 主题: Re: [PATCH, expand] Call misaligned memory reference in expand_builtin_return [PR112417]
> 日期: Fri, 10 Nov 2023 14:39:02 +0100
> From: Richard Biener <richard.guenther@gmail.com>
> 收件人: HAO CHEN GUI <guihaoc@linux.ibm.com>
> 抄送: gcc-patches <gcc-patches@gcc.gnu.org>, Kewen.Lin <linkw@linux.ibm.com>
> 
> On Fri, Nov 10, 2023 at 11:10 AM HAO CHEN GUI <guihaoc@linux.ibm.com> wrote:
>>
>> Hi Richard,
>>
>> 在 2023/11/10 17:06, Richard Biener 写道:
>>> On Fri, Nov 10, 2023 at 8:52 AM HAO CHEN GUI <guihaoc@linux.ibm.com> wrote:
>>>>
>>>> Hi Richard,
>>>>   Thanks so much for your comments.
>>>>
>>>> 在 2023/11/9 19:41, Richard Biener 写道:
>>>>> I'm not sure if the testcase is valid though?
>>>>>
>>>>> @defbuiltin{{void} __builtin_return (void *@var{result})}
>>>>> This built-in function returns the value described by @var{result} from
>>>>> the containing function.  You should specify, for @var{result}, a value
>>>>> returned by @code{__builtin_apply}.
>>>>> @enddefbuiltin
>>>>>
>>>>> I don't see __builtin_apply being used here?
>>>>
>>>> The prototype of the test case is from "__objc_block_forward" in
>>>> libobjc/sendmsg.c.
>>>>
>>>>   void *args, *res;
>>>>
>>>>   args = __builtin_apply_args ();
>>>>   res = __objc_forward (rcv, op, args);
>>>>   if (res)
>>>>     __builtin_return (res);
>>>>   else
>>>>     ...
>>>>
>>>> The __builtin_apply_args puts the return values on stack by the alignment.
>>>> But the forward function can do anything and return a void* pointer.
>>>> IMHO the alignment might be broken. So I just simplified it to use a
>>>> void* pointer as the input argument of  "__builtin_return" and skip
>>>> "__builtin_apply_args".
>>>
>>> But doesn't __objc_forward then break the contract between
>>> __builtin_apply_args and __builtin_return?
>>>
>>> That said, __builtin_return is a very special function, it's not supposed
>>> to deal with what you are fixing.  At least I think so.
>>>
>>> IMHO the bug is in __objc_block_forward.
>>
>> If so, can we document that the memory objects pointed by input argument of
>> __builtin_return have to be aligned? Then we can force the alignment in
>> __builtin_return. The customer function can do anything if gcc doesn't state
>> that.
> 
> I don't think they have to be aligned - they have to adhere to the ABI
> which __builtin_apply_args ensures.  But others might know more details
> here.
> 
>> Thanks
>> Gui Haochen
>>
>>>
>>> Richard.
>>>
>>>>
>>>> Thanks
>>>> Gui Haochen