Linux kernel gcc compile optimization level

wiki.gentoo.org 上的这篇文章Kernel/Optimization介绍了很多关于 Linux 内核编译优化的事情。其中有如下一段。

*FLAGS

Note

For more list of *FLAGS to play with, see the GCC manual and Clang manual or man 1 gcc and man 1 clang commands. By default, most of the kernel is build with C’s -O2 (some code, like Random Number Generation, does not work with optimizations and sometimes checked with the C macro OPTIMIZE). This can be changed via Kbuild. Before making any KCFLAGS and similar flags, please check the kernel’s Makefiles before it gets any changes. For example, -fallow-store-data-races is disabled on this Makefile.

-O3

The command to add this flag is: make KCFLAGS=”-O3”

There was a official attempt to add -O3 to the kernel but Linus Torvalds reject it due to -O3 historically outputting worse code than -O2. Phoronix ran a -O3 kernel benchmark and found nearly all tested programs to have no measurable benefit.

上面提到：By default, most of the kernel is build with C’s -O2,参考的是https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile#n892，注意写作本文时Linux内核主线版本是7.0-rc2，所以更新了一下这里Makefile对应的行号。具体Malefile内容如下。

ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
KBUILD_CFLAGS += -O2
KBUILD_RUSTFLAGS += -Copt-level=2
else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
KBUILD_CFLAGS += -Os
KBUILD_RUSTFLAGS += -Copt-level=s
endif

可以看出，如果定义了宏CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE则使用O2优化编译，如果定义了CONFIG_CC_OPTIMIZE_FOR_SIZE则使用Os优化编译，那么如何看出默认是哪钟编译呢？

在文件https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/init/Kconfig中，我们可以看到下面的内容：

choice
	prompt "Compiler optimization level"
	default CC_OPTIMIZE_FOR_PERFORMANCE

config CC_OPTIMIZE_FOR_PERFORMANCE
	bool "Optimize for performance (-O2)"
	help
	  This is the default optimization level for the kernel, building
	  with the "-O2" compiler flag for best performance and most
	  helpful compile-time warnings.

config CC_OPTIMIZE_FOR_SIZE
	bool "Optimize for size (-Os)"
	help
	  Choosing this option will pass "-Os" to your compiler resulting
	  in a smaller kernel.

endchoice

可以看出默认的选项就是CC_OPTIMIZE_FOR_PERFORMANCE。这个patchkconfig: Warn if choice default is not in choice还增加了对优化级别的检查。

这里有篇文章使用-O0编译Linux内核，专门讲解了如何配置Linux kernel使用-O0级别编译，因为内核本身没有提供原生支持，所以需要自己手动修改。对于-O1编译，也是一样的修改。

到此为止，我们也可以明白，为什么内核中有很多READ_ONCE,WRITE_ONCE这种设置内存屏障的读写接口了，为的就是在gcc优化开启的条件下代码依然可以正确运行。而对于我们很多应用层软件开发，即使代码没有这种内存屏障考虑，如果没有开启代码优化，也不会出什么问题。但是如果一旦开启，就需要对这种细节格外注意，否则出现莫名其妙的问题，还怪罪是gcc优化引入的，而不知道是自己的代码没有考虑gcc的优化编译。

其实，曾经有更激进的建议提出把默认级别修改为-O3，但是被linus拒绝了，理由是-O3在很长一段历史中生成的代码会更糟糕，而且没有充分的证据证明-O3有更好的效果，具体可以参考https://lore.kernel.org/lkml/20220621133526.29662-1-mikoxyzzz@gmail.com/，https://lore.kernel.org/lkml/20191211104619.114557-1-oleksandr@redhat.com/，https://lore.kernel.org/lkml/CA+55aFz2sNBbZyg-_i8_Ldr2e8o9dfvdSfHHuRzVtP2VMAUWPg@mail.gmail.com/。

这个patchkbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3中给出了如何使用-O3 编译内核的方法，同时说明了增加-O3 编译选项需要提供的依据。

Folks looking to experiment with `-O3` (or any compiler flag for that
matter) may pass them along to the command line invocation of make:

$ make KCFLAGS=-O3

Code that looks to re-add an explicit Kconfig option for `-O3` should
provide:
1. A rigorous and reproducible performance profile of a reasonable
   userspace workload that demonstrates a hot loop in the kernel that
   would benefit from `-O3` over `-O2`.
2. Disassembly of said loop body before and after.
3. Provides stats on terms of increase in file size.