optimization in kernel compile

[Valdis.Kletnieks at vt.edu]

On Fri, 22 Mar 2013 22:32:40 +0800, ishare said:

> > are a few places that for correctness, you *cannot* use -O0. For instance,
a
> > few places where we use builtin_return_address() inside an inline (-O0
> > won't inline so builtin_return_address() ends up returning a pointer to
> > a function when we want the function's parent).
>
>   So it will cause an error ?

Yes, there are places where failing to optimize causes errors.

Consider this code:

static inline foo (return builtin_return_address());

int bar ( x = foo());

If you don't optimize, x ends up with a pointer into bar.  If it
gets inlined because you're optimizing, x ends up pointing to bar's caller.
This breaks stuff like the function tracer.

> > Since gdb and friends are able to deal with -O2 compiled code just fine,
> > there's really no reason *not* to optimize the kernel.
>
>   the debug information will be stripped  by  -O2 ,for example ,you can not touch

No debug information is stripped by -O2.  Debug information isn't emitted if
you don't compile with -g.  At one time, long ago (quite possibly literally
"before you were born" for some of the younger readers on the list), gcc was
unable to generate -g output if the optimizer was invoked.  But that was
last century (gcc 2.95 era).

>   the value of  some varibles at stack , and debugging will not run line by line,
>   instead , the source jump in unexpectable order .

I'm probably going to piss a bunch of people off by saying this, but:

If your C skills aren't up to debugging code that's been compiled with
-O2, maybe you shouldn't be poking around inside the kernel.
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