Re: [ARM] Add endless loop to avoid a compiler warning on noreturn functions.

[Christophe Lyon <christophe dot lyon at linaro dot org>]

On Tue, 2 Oct 2018 at 10:55, Richard Earnshaw (lists)
<Richard.Earnshaw@arm.com> wrote:
>
> On 02/10/18 07:52, Christophe Lyon wrote:
> > On Tue, 2 Oct 2018 at 00:25, Craig Howland <howland@lgsinnovations.com> wrote:
> >>
> >> On 10/01/2018 05:27 PM, Christophe Lyon wrote:
> >>> Hi,
> >>>
> >>> While building newlib for ARM, I noticed GCC warnings for _exit() that
> >>> the compiler thinks they return a value despite being noreturn.
> >>>
> >>> Like other targets, this small adds an endless loop to avoid the warning.
> >>>
> >>> OK?
> >>>
> >>> Christophe
> >> The proper fix (for both places) is to add noreturn to the _kill() prototype in
> >> the file.  (Which presumably is true, otherwise _exit() will return.  I did test
> >> that it fixes the warning.)  (It would not be surprising if it also needed to be
> >> added to the _kill() source, itself.)
> >
> > Well, when compiled with ARM_RDI_MONITOR, _kill does seem to return:
> > #if SEMIHOST_V2
> > if (_has_ext_exit_extended ())
> >   return do_AngelSWI (insn, block);
> > else
> > #endif
> >   return do_AngelSWI (insn, (void*)block[0]);
> >
>
> do_AngelSWI is a multi-purpose call that will normally return, so that
> can't be marked no-return.
Indeed.

>
> I think the right fix here is to remove the "return" from the statements
> and add __builtin_unreachable () at the end of the function.
>
By "function", do you mean _kill or _exit ?
IIUC the patch should:
- remove "return" from _kill
- add _builtin_unreachable to both _kill and _exit
- add "noreturn" to _kill prototype

Unless there are cases where this version of _kill can kill
another thread/process and thus actually return to its caller?


> R.
>
> > I guess the noreturn should not be added to
> > newlib/libc/include/sys/signal.h
> > because it depends on the actual target implementation of _kill?
> >
> >> Craig
>