[EXT] Re: [PATCH] ring: compilation fix with GCC-12

[Amit Prakash Shukla]

Hi Thomas,

> -----Original Message-----
> From: Thomas Monjalon <thomas at monjalon.net>
> Sent: Friday, January 13, 2023 3:12 AM
> To: Konstantin Ananyev <konstantin.v.ananyev at yandex.ru>; Honnappa
> Nagarahalli <Honnappa.Nagarahalli at arm.com>; Amit Prakash Shukla
> <amitprakashs at marvell.com>
> Cc: dev at dpdk.org; Jerin Jacob Kollanukkaran <jerinj at marvell.com>;
> david.marchand at redhat.com; bruce.richardson at intel.com;
> ferruh.yigit at amd.com
> Subject: Re: [EXT] Re: [PATCH] ring: compilation fix with GCC-12
> 
> 23/08/2022 11:38, Amit Prakash Shukla:
> > From: Konstantin Ananyev <konstantin.v.ananyev at yandex.ru>
> > > 06/08/2022 19:35, Honnappa Nagarahalli пишет:
> > > >> Replacing memcpy with rte_memcpy fixes the GCC-12 compilation
> issue.
> > > >
> > > > Any reason why this replacement fixes the problem?
> > > > Do you have any performance numbers with this change?
> > > >
> > > >> Also it would be better to change to rte_memcpy as the function
> > > >> is called in fastpath.
> > > >
> > > > On Arm platforms, memcpy in the later versions has the best
> performance.
> > >
> > > I agree with Honnappa, it is better to keep memcpy() here.
> > > Actually what is strange - why it ends up in
> > > __rte_ring_dequeue_elems_128() at all?
> > > Inside rxa_intr_ring_dequeue() we clearly doing: rte_ring_dequeue(),
> > > which should boil down to ___rte_ring_dequeue_elems_64().
> > > it should go to __rte_ring_dequeue_elems_128() at all.
> >
> > I agree. After having close look and doing few experiments, ideally it
> > should not be going to __rte_ring_dequeue_elems_128().
> > Sizeof(in call of rte_ring_enqueue_elem) gets evaluated at compile
> > time which in this case it is evaluated to 8 bytes so
> > __rte_ring_dequeue_elems_128() shall not be in the path. Looks like more
> of a gcc-12 bug.?
> >
> > > Another q - is this warning happens only on arm platforms?
> >
> > Warning is observed on x86 with build type as debug.
> > "meson --werror --buildtype=debug build"
> 
> I confirm the compilation issue on x86 with GCC 12 in a debug build.
> 
> We need to find a workaround.
> Is it reported to GCC already?
> 
I found an old gcc bug reporting similar issue. This bug seems to be re-opened recently in Dec-2022. Not sure if it is reopened specifically for gcc-12.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89689

Kevin has push a work around for DPDK-21.11.3.
https://git.dpdk.org/dpdk-stable/commit/?h=21.11&id=e1d728588dc73af9ed60cc0074d51a7f24b2ba60