How to compile Linux kernel with -O0 flag

[Mike Krinkin]

On Sat, Aug 13, 2016 at 09:56:05PM +0300, Aleksander Alekseev wrote:
> > The kernel will not run with -O0, sorry, just live with the build
> > optimization levels that is currently used and you should be fine.
> 
> Oh, I see. Fortunately I'm not afraid of assembler :) Thanks.
> 
> Just out of curiosity - is there a technical reason why -O0 couldn't
> be used in Linux kernel? I don't know, spinlocks would not work in this
> case because it's how GCC was written or something. Or just nobody
> compiles and tests kernel like this so it most likely would not work?

Just fixed Makefile and tried to build it with -O0, it doesn't even
compile, i got errors like this:

./include/linux/compiler-gcc.h:243:38: error: impossible constraint in ‘asm’
 #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)
                                      ^
./arch/x86/include/asm/cpufeature.h:146:3: note: in expansion of macro ‘asm_volatile_goto’
   asm_volatile_goto("1: jmp 6f\n"

Probably gcc cannot figure out that an macro argument can be evaluated at
compile time with optimizations disabled.

> 
> > But why do you want to use a debugger on the kernel?  That's not a
> > normal task unless you are bringing up a new hardware platform.
> 
> It's just something I always do when I learn new things. Trying to
> figure out how to debug something in this new environment. No real task
> so far.
> 
> -- 
> Best regards,
> Aleksander Alekseev
> 
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