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References: <23e0ba7863d51ab629498762a97d477645aeafea.1653123744.git.christophe.jaillet@wanadoo.fr>
In-Reply-To: <23e0ba7863d51ab629498762a97d477645aeafea.1653123744.git.christophe.jaillet@wanadoo.fr>
From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Sun, 22 May 2022 00:38:17 +0900
Message-ID: <CAK7LNASHGyzKS-d+S1GYz_L0pBxxSwk05YfvV+LK4ghZ5xx=cA@mail.gmail.com>
Subject: Re: [RFC PATCH] kbuild: Add an option to enable -O1 and speed-up
 compilation time
To:     Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        kernel-janitors@vger.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Changbin Du <changbin.du@gmail.com>
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(+CC: Arnd, Changbin Du)


If -O1 support does not require additional effort,
I have no objection to this patch.

(but I do not have enough insight about
the compiler's inlining heuristic)


BTW, when we attempted to add the -Og support,
we fixed various parts, and Linus rejected it.

https://lore.kernel.org/linux-kbuild/CAK7LNARQggM3aKEPRKJqa4tunFAfmfErMZuS-rrnRv6UB1VpPQ@mail.gmail.com/


On Sat, May 21, 2022 at 6:04 PM Christophe JAILLET
<christophe.jaillet@wanadoo.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.
>
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.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
> +         memory.
> +
>  config CC_OPTIMIZE_FOR_PERFORMANCE
>         bool "Optimize for performance (-O2)"
>         help
> --
> 2.34.1
>


-- 
Best Regards
Masahiro Yamada