Re: [PATCH, re-send] Always trap on BUG()

[Ingo Molnar]

* 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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