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    SubjectRe: [tip:x86/build] x86, retpolines: Raise limit for generating indirect calls from switch-case
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    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@iogearbox.net>
    > AuthorDate: Thu, 21 Feb 2019 23:19:41 +0100
    > Committer: Thomas Gleixner <tglx@linutronix.de>
    > 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 Björn 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.
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