Re: [patch] Cleaning up amd64 kernel optimization options

On Thursday, December 22, 2011 9:51:47 pm Garrett Cooper wrote:
> On Dec 22, 2011, at 4:59 PM, Alexander Best <arundel_at_freebsd.org> wrote:
> 
> > On Thu Dec 22 11, Benjamin Kaduk wrote:
> >> On Thu, 22 Dec 2011, Alexander Best wrote:
> >> 
> >>> On Thu Dec 22 11, Dimitry Andric wrote:
> >>>> Hi,
> >>>> 
> >>>> I would like to ask some feedback on the attached patch, which cleans up
> >>>> the kernel optimization options for amd64.  This was touched upon
> >>>> earlier by Alexander Best in freebsd-toolchain, here:
> >>> 
> >>> i've been using such settings for a few months now and haven't noticed any
> >>> problems.
> >>> 
> >>> however bruce evans raised a good point (in a private mail). when you 
> >>> compile a
> >>> kernel without debugging enabled, -O2 is the default. if you experience 
> >>> issues,
> >>> and enable debugging, -O0 now becomes the default. in case the problems 
> >>> with
> >>> the kernel were caused by the -O2 option and aren't present with the -O0
> >>> option, the newly built kernel with debugging enabled will not help you 
> >>> fix the
> >>> problems. in that case you would need to set -O2 explicitly in CFLAGS. his
> >>> exact words were:
> >>> 
> >>> "
> >>> I don't like -O2 for anything.  However, if it is only a default, it is
> >>> not a problem provided it can be canceled easily.  And for debugging, you
> >>> want the default to be the same as without debugging, so that (by default)
> >>> you debug the same code that caused the problem.
> >>> "
> >>> 
> >>> however i don't think this is fixable. using -O0 for debuggable and
> >>> non-debuggable kernels will cause too much of a slowdown.
> >>> 
> >>> having -O2 as the default flag for non-debuggable kernels and -O2 -g for
> >>> debuggable kernels might cause situations, where debugging isn't possible,
> >>> where with -O0 -g it would have been.
> >>> 
> >>> so i guess although bruces concerns are valid, they are impossible to 
> >>> solve.
> >> 
> >> Where does -O0 come in?  I only see talk of -O (i.e. -O1) versus -O2.
> > 
> > sorry. of course i meant -O:
> > 
> > .if defined(DEBUG)
> > _MINUS_O=       -O
> > CTFFLAGS+=      -g
> > .else
> > [..]
> 
> Back in the 7.x days, I ran into some code that wasn't easily to debug because the compiler optimized things out with -O2 by inlining and 
otherwise shifting around code, so setting breakpoints in gdb became difficult. So from that point on I've gotten into the habit of doing -O 
explicitly in make.conf if DEBUG_FLAGS was specified. Just a thought..

I still leave -O2 in, but what I do is this:

  make DEBUG_FLAGS="-g -fno-inline"

Just adding -fno-inline makes a world of difference.

-- 
John Baldwin