linux-kernel - Re: [GIT PULL] scheduler fixes

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Date:   Sun, 17 Nov 2019 08:29:37 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] scheduler fixes

On Sun, Nov 17, 2019 at 2:20 AM Valentin Schneider
<valentin.schneider@....com> wrote:
>
> AFAIUI the requirement for the enum type is that it has to be an int type that
> covers all its values, so I could see some funky optimization (e.g. check the
> returned value is < 512 but it's assumed the type for the enum is 8 bits so
> this becomes always true). Then again we don't have any explicit check on
> those returned values, plus they fit in 11 bits, so as you say it's
> mostly likely inconsequential (and I didn't see any compile diff).

Gcc can - and does - narrow enums to smaller integer types with the
'-fshort-enums' flag.

However, in practice nobody uses that, and it can cause interop
problems. So I think for us, enums are always at least 'int' (they can
be bigger).

That said, mixing enums and values that are bigger than the enumerated
ones is just a bad idea

It will, for example, cause us to miss compiler warnings (eg switch
statements with an enum will warn if you don't handle all cases, but
the 'all cases' is based on the actual enum range, not on the
_possible_ invalid values).

                     Linus