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

[Richard Biener <richard.guenther@gmail.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