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    SubjectRe: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression
    On Tue, May 26, 2020 at 2:02 PM Will Deacon <will@kernel.org> wrote:
    > On Tue, May 26, 2020 at 12:42:16PM +0200, Arnd Bergmann wrote:
    > >
    > > I find this patch only solves half the problem: it's much faster than
    > > without the
    > > patch, but still much slower than the current mainline version. As far as I'm
    > > concerned, I think the build speed regression compared to mainline is not yet
    > > acceptable, and we should try harder.
    > >
    > > I have not looked too deeply at it yet, but this is what I found from looking
    > > at a file in a randconfig build:
    > >
    > > Configuration: see https://pastebin.com/raw/R9erCwNj
    >
    > So this .config actually has KCSAN enabled. Do you still see the slowdown
    > with that disabled?

    Yes, enabling or disabling KCSAN seems to make no difference to
    compile speed in this config and source file, I still get the 12 seconds
    preprocessing time and 9MB file size with KCSAN disabled, possibly
    a few percent smaller/faster. I actually thought that CONFIG_FTRACE
    had a bigger impact, but disabling that also just reduces the time
    by a few percent rather than getting it down to the expected milliseconds.

    > Although not ideal, having a longer compiler time when
    > the compiler is being asked to perform instrumentation doesn't seem like a
    > show-stopper to me.

    I agree in general, but building an allyesconfig kernel is still an important
    use case that should not take twice as long after a small kernel change
    regardless of whether a new feature is used or not. (I have not actually
    compared the overall build speed for allmodconfig, as this takes a really
    long time at the moment)

    Arnd

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