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

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Date:   Sat, 21 May 2022 11:04:29 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     dan.carpenter@...cle.com, Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-kbuild@...r.kernel.org
Subject: [RFC PATCH] kbuild: Add an option to enable -O1 and speed-up compilation time

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