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Date:   Thu, 21 Feb 2019 23:56:55 +0100
From:   Daniel Borkmann <>
To:     Linus Torvalds <>
Cc:     Thomas Gleixner <>,
        Linux List Kernel Mailing <>,
        Netdev <>,
        "David S . Miller" <>,
        Björn Töpel <>,
        Magnus Karlsson <>,
        Jesper Dangaard Brouer <>,
        Alexei Starovoitov <>,
        Peter Zijlstra <>,
        David Woodhouse <>,
        Andy Lutomirski <>,
        Borislav Petkov <>
Subject: Re: [PATCH] x86, retpolines: raise limit for generating indirect
 calls from switch-case

On 02/21/2019 11:27 PM, Linus Torvalds wrote:
> On Thu, Feb 21, 2019 at 2:20 PM Daniel Borkmann <> wrote:
>> In case of gcc, this setting is controlled by case-values-threshold
>> which has an architecture global default that selects 4 or 5 (
> Ack. For retpoline, that's much too low.
> Patch looks sane, although it would be good to verify just which
> versions of gcc this works for. All versions with retpoline?

The feature was first added in gcc 4.7 [0], under "General Optimizer Improvements":

  Support for a new parameter --param case-values-threshold=n was added to allow
  users to control the cutoff between doing switch statements as a series of if
  statements and using a jump table.

>From what I can tell, original author (H.J. Lu) provided backports up to gcc 4.8
and distros seem to have pulled it from his github branch [1] as upstream gcc does
not handle backports for stable versions that old.



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