linux-kernel - Re: [RFC PATCH] kbuild: Add an option to enable -O1 and speed-up compilation time

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Date:   Mon, 23 May 2022 11:26:54 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     dan.carpenter@...cle.com, Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kbuild@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>
Subject: Re: [RFC PATCH] kbuild: Add an option to enable -O1 and speed-up
 compilation time

On Sat, May 21, 2022 at 2:04 AM Christophe JAILLET
<christophe.jaillet@...adoo.fr> wrote:
>
> Add a new compilation option which speeds-up compilation time.
> This can be useful when using static checker such as smatch or build-bots.
> In such cases, the speed and quality of the generated code is not
> important.
>
> Using -O0 would be even better, but unfortunately, building fails with
> this option.

Which is a tragedy.

As with the Rust series, I'm not a fan of this (or
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3):
https://lore.kernel.org/lkml/CAKwvOd=7QTUH69+ZbT7e8einvgcosTbDkyohmPaUBv6_y8RfrQ@mail.gmail.com/

These feel more like attempts to wrap every conceivable command line
flag in a kconfig option, which makes me think of that meme from
Jurassic Park: "your scientists were so preoccupied with whether or
not they could, they didn't stop to think if they should."
Not a fan.  I'd ask for measurements, but that would be a request for
a "rock fetching quest" for something I still wouldn't be a fan of.

>
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
>  Makefile     | 5 ++++-
>  init/Kconfig | 8 ++++++++
>  2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 1f8bef92868f..14467386f947 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -817,7 +817,10 @@ KBUILD_CFLAGS      += $(call cc-disable-warning, format-truncation)
>  KBUILD_CFLAGS  += $(call cc-disable-warning, format-overflow)
>  KBUILD_CFLAGS  += $(call cc-disable-warning, address-of-packed-member)
>
> -ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
> +ifdef CONFIG_CC_OPTIMIZE_FOR_COMPILATION_SPEED
> +KBUILD_CFLAGS += -O1
> +KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 1
> +else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
>  KBUILD_CFLAGS += -O2
>  KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 2
>  else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
> diff --git a/init/Kconfig b/init/Kconfig
> index a96776a9b080..3177a1830c9a 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1384,6 +1384,14 @@ choice
>         prompt "Compiler optimization level"
>         default CC_OPTIMIZE_FOR_PERFORMANCE
>
> +config CC_OPTIMIZE_FOR_COMPILATION_SPEED
> +       bool "Optimize for compilation speed (-O1)"
> +       help
> +         This option can be useful when running a static checker such as smatch
> +         or a build-bot.
> +         Compilation time is slighly faster than -O2 and it requires less

s/slighly/slightly/

> +         memory.
> +
>  config CC_OPTIMIZE_FOR_PERFORMANCE
>         bool "Optimize for performance (-O2)"
>         help
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers