* Arnd Bergmann <arnd@xxxxxxxx> wrote:
> I've run some size analyis using the ARM 'multi_v7_defconfig'
> and gcc-4.8, using various definitions for BUG() and BUG_ON(), to
> see how big the size improvement actually gets
>
> 1. Baseline: normal bug plus CONFIG_BUG_VERBOSE
> text data bss dec hex filename
> 3743196 224396 206812 4174404 3fb244 vmlinux-bugverbose
>
> 2. disabling CONFIG_BUG_VERBOSE, saving 0.6%
> 3716920 224380 206812 4148112 3f4b90 vmlinux-nobugverbose
>
> 3. #define BUG() panic(__func__), #define BUG_ON(c) if(c) BUG(), saving 1.0%
> 3701076 224384 206812 4132272 3f0db0 vmlinux-bug-panicfunc
>
> 3. #define BUG() panic(__func__), #define BUG_ON(c) if(c) BUG(), saving 1.5%
> 3678884 224400 206812 4110096 3eb710 vmlinux-bug-panic
>
> 4. #define BUG() unreachable(), saving 2.1%
> 3652636 224384 206812 4083832 3e5078 vmlinux-bug-unreachable
>
> 5. as 4, plus #define BUG_ON(c) if(c) unreachable(), saving 2.2%
> 3651108 224380 206812 4082300 3e4a7c vmlinux-bugon-unreachable
>
> 6. #define BUG() do{}while(0), saving 2.1%
> 3654264 224380 206812 4085456 3e56d0 vmlinux-nobug
>
> 7. as 6, plus #define BUG_ON if(0 && (c)) unreachable, saving 2.2%
> 3648392 223996 206748 4079136 3e3e20 vmlinux-no-bugon
>
> 8. my patch below, saving 1.8%
> 3666532 224380 206812 4097724 3e86bc obj-tmp/vmlinux
>
> The gain of doing unreachable and letting the code run off whereever
> is minimal compared to the current state of doing nothing, but it
> avoids the warnings.
>
> Same test using x86_defconfig:
>
> 1. CONFIG_BUG=y, CONFIG_BUGVERBOSE=n
> 10797859 1395648 1175552 13369059 cbfee3 vmlinux-x86-bug
>
> 2. CONFIG_BUG=n, saves 1.0%
> 10658553 1395584 1175552 13229689 c9de79 vmlinux-x86-nobug
>
> 3. with my patch, saves 0.8%
> 10684186 1395584 1175552 13255322 ca429a vmlinux-x86-archbug
>
> Getting 1-2% savings in kernel size seems absolutely worth keeping the
> option, but 0.2-0.4% left for getting reproducible behavior also seems
> worthwhile. The result in the patch below.
>
> This basically loses any of the BUG() reporting, but leaves the
> logic to trap and kill the task in place when CONFIG_BUG is disabled.
This also kills a boatload of compiler warnings when CONFIG_BUG is
disabled.
Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
Thanks,
Ingo
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