RE: [PATCH v5] net: optimize raw checksum computation

["Morten Brørup" <mb@smartsharesystems.com>]

+Aaron, please read up on this discussion, and step in if you can help.

(Aaron is the DPDK Project testing leader, and works at Red Hat.)

> From: Scott Mitchell [mailto:scott.k.mitch1@gmail.com]
> Sent: Friday, 9 January 2026 05.58
> 
> On Thu, Jan 8, 2026 at 7:01 PM Stephen Hemminger
> <stephen@networkplumber.org> wrote:
> >
> > On Thu, 8 Jan 2026 16:19:37 -0500
> > Scott Mitchell <scott.k.mitch1@gmail.com> wrote:
> >
> > > On Thu, Jan 8, 2026 at 11:12 AM Stephen Hemminger
> > > <stephen@networkplumber.org> wrote:
> > > >
> > > > On Thu,  8 Jan 2026 01:13:38 -0500
> > > > scott.k.mitch1@gmail.com wrote:
> > > >
> > > > > +#ifdef RTE_CC_GCC
> > > > > +     /* Suppress GCC -Wmaybe-uninitialized false positive. No
> assembly/runtime impacts. */
> > > > > +     asm volatile("" : "+m" (psd_hdr));
> > > > > +#endif
> > > > >
> > > >
> > > > Maybe rte_compiler_barrier() will do same thing?
> > >
> > > Agreed it feels like a compiler bug but looking for advice if I'm
> > > missing something :)
> > >
> > > My initial concern with rte_compiler_barrier is its a general
> barrier
> > > which may have broader impacts on
> > > optimizations and compiled code. Will that be an issue in this
> case? I
> > > wasn't sure and the approach
> > > in the patch is targeted at a specific variable and assembly from
> > > clang/gcc was the same. I will
> > > introduce a macro to make it cleaner and I can replace it with
> > > rte_compiler_barrier if preferred.
> >
> > Maybe try with -fanalyzer and it might tell you more.
> > I suspect some of the aliasing setting are causing issues.
> > Some drivers are turning on no-strict-aliasing
> 
> I have more evidence this is a GCC optimizer bug.
> The RTE_SUPPRESS_UNINITIALIZED_WARNING approach serves
> as a workaround to avoid the bug. I created a more minimal reproducer:
> https://gist.github.com/Scottmitch/bf23748b4588e68c9bdb8d124f92f1bd
> 
> Your suspicion was correct, -fno-strict-aliasing avoids the bug but I
> don't
> think it is desirable to enable this broadly for DPDK when we have a
> more targeted workaround.
> 
> I will reach out to RH to confirm but in the interim I suggest we keep
> RTE_SUPPRESS_UNINITIALIZED_WARNING (or similar alternative).

If this is a GCC compiler bug limited to the GCC version offered by RHEL 11, I prefer splitting the patch into a series with the following steps:
Patch 1/2: Add the optimization and new test cases in their minimal form, designed to work on normal compilers. Disregard bugs/warnings from the weird RHEL 11 compiler.
I.e. don't modify lib/eal/include/rte_common.h, lib/net/rte_ip6.h, lib/net/rte_ip4.h, drivers/net/hinic/hinic_pmd_tx.c, drivers/net/mlx5/mlx5_flow_dv.c.
Patch 2/2: Add the workarounds required by the RHEL 11 compiler.

Also, the change to drivers/net/hinic/hinic_pmd_tx.c should be moved to a patch independent of this series.
It's not directly related to this series, so let's not add more to the discussion than we need to. ;-)
And the implementation in the driver only considers RTE_MBUF_F_TX_TCP_SEG, whereas the DPDK function also considers RTE_MBUF_F_TX_UDP_SEG, so it warrants a separate discussion; it possibly fixes a bug.

Maybe even move the RHEL 11 related patches (my suggested patch 2/2) into a separate series, for the same conceptual reasons as moving the HINIC driver patch into a separate series.
You can use the Depends-On tag (https://doc.dpdk.org/guides/contributing/patches.html#patch-dependencies) for the follow-on changes to __rte_raw_cksum().