Re: [PATCH nf-next v4 1/5] net: sched: Micro-optimize egress handling

[Lukas Wunner]

On Sat, Jan 23, 2021 at 07:26:24PM -0800, Jakub Kicinski wrote:
> On Fri, 22 Jan 2021 09:47:01 +0100 Lukas Wunner wrote:
> > sch_handle_egress() returns either the skb or NULL to signal to its
> > caller __dev_queue_xmit() whether a packet should continue to be
> > processed.
> > 
> > The skb is always non-NULL, otherwise __dev_queue_xmit() would hit a
> > NULL pointer deref right at its top.
> > 
> > But the compiler doesn't know that.  So if sch_handle_egress() signals
> > success by returning the skb, the "if (!skb) goto out;" statement
> > results in a gratuitous NULL pointer check in the Assembler output.
> 
> Which exact compiler are we talking about it? Did you report this?
> As Eric pointed the compiler should be able to figure this out quite
> easily.

I tested with gcc 8, 9, 10.

No need to report as it's the expected behavior with
-fno-delete-null-pointer-checks, whose motivation appears
questionable though (per my preceding e-mail).

Thanks,

Lukas