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

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

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.

Thanks
Gui Haochen

> 
> Richard.
> 
>>
>> Thanks
>> Gui Haochen