Compiler optimization level

For Developers
1

Hi,

I see that, in menuconfig, when building for a tiny 4/32 WR-940 v6, kernel build option "Compiler optimization level" is missing. Frankly speaking, I didn't notice it until building for the new WR-841N v13 which is MediaTek-based and 8/64, where that option is available. So I need some guidelines.

I guess it's because, being a 4/32 device, optimizing for performance woludn't be smart? If so, should I go to advanced configuration options and select:
target options -->-Os -pipe -mno-branch-likely -mips32r2 -mtune=24kc) Target Optimizations

Or are these used by default anyway?

2

You can check with adding V=s to your make command.

jeff@deb-devel:~/devel/openwrt$ fgrep -r CFLAGS include

reveals at least (context not examined):

include/target.mk:  CPU_CFLAGS = -Os -pipe
include/target.mk:    CPU_CFLAGS += -mno-branch-likely
[...]
include/target.mk:    CPU_CFLAGS_24kc = -mips32r2 -mtune=24kc
[...]
include/target.mk:  DEFAULT_CFLAGS=$(strip $(CPU_CFLAGS) $(CPU_CFLAGS_$(CPU_TYPE)) $(CPU_CFLAGS_$(CPU_SUBTYPE)))
include/target.mk:	 echo 'Target-Optimization: $(if $(CFLAGS),$(CFLAGS),$(DEFAULT_CFLAGS))'; \
3

-Os is set by default
The kernel is also optimized for performance by default (probably a bad idea to change this however), there are also a few kernel options you can disable if you want to keep the size down.