Re: [PATCH] x86, retpolines: raise limit for generating indirect calls from switch-case

[Daniel Borkmann]

On 02/21/2019 11:27 PM, Linus Torvalds wrote:
> On Thu, Feb 21, 2019 at 2:20 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> 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.

Thanks,
Daniel

  [0] https://www.gnu.org/software/gcc/gcc-4.7/changes.html
  [1] https://bugs.launchpad.net/ubuntu/+source/gcc-4.8/+bug/1749261
      https://github.com/hjl-tools/gcc/tree/hjl/indirect/gcc-4_8-branch/master