Re: [tip:x86/build] x86, retpolines: Raise limit for generating indirect calls from switch-case

[H.J. Lu]

On Thu, Feb 28, 2019 at 3:27 AM David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
>
> On Thu, 2019-02-28 at 03:12 -0800, tip-bot for Daniel Borkmann wrote:
> > Commit-ID:  ce02ef06fcf7a399a6276adb83f37373d10cbbe1
> > Gitweb:     https://git.kernel.org/tip/ce02ef06fcf7a399a6276adb83f37373d10cbbe1
> > Author:     Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > AuthorDate: Thu, 21 Feb 2019 23:19:41 +0100
> > Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > CommitDate: Thu, 28 Feb 2019 12:10:31 +0100
> >
> > x86, retpolines: Raise limit for generating indirect calls from switch-case
> >
> > From networking side, there are numerous attempts to get rid of indirect
> > calls in fast-path wherever feasible in order to avoid the cost of
> > retpolines, for example, just to name a few:
> >
> >   * 283c16a2dfd3 ("indirect call wrappers: helpers to speed-up indirect calls of builtin")
> >   * aaa5d90b395a ("net: use indirect call wrappers at GRO network layer")
> >   * 028e0a476684 ("net: use indirect call wrappers at GRO transport layer")
> >   * 356da6d0cde3 ("dma-mapping: bypass indirect calls for dma-direct")
> >   * 09772d92cd5a ("bpf: avoid retpoline for lookup/update/delete calls on maps")
> >   * 10870dd89e95 ("netfilter: nf_tables: add direct calls for all builtin expressions")
> >   [...]
> >
> > Recent work on XDP from BjÃrn and Magnus additionally found that manually
> > transforming the XDP return code switch statement with more than 5 cases
> > into if-else combination would result in a considerable speedup in XDP
> > layer due to avoidance of indirect calls in CONFIG_RETPOLINE enabled
> > builds.
>
> +HJL
>
> This is a GCC bug, surely? It should know how expensive each
> instruction is, and choose which to use accordingly. That should be
> true even when the indirect branch "instruction" is a retpoline, and
> thus enormously expensive.
>
> I believe this is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86952 so
> please at least reference that bug, and be prepared to turn this hack
> off when GCC is fixed.

We couldn't find a testcase to show jump table with indirect branch
is slower than direct branches.

-- 
H.J.