Re: [patch] Cleaning up amd64 kernel optimization options

On Fri, Dec 23, 2011 at 12:34 PM, Garrett Cooper <yanegomi_at_gmail.com> wrote:
> On Fri, Dec 23, 2011 at 6:37 AM, John Baldwin <jhb_at_freebsd.org> wrote:
>> 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.
>
> Sweet -- thanks for the tip ;).

    Just as a sidenote, this option doesn't work when compiling
mii(4), re(4), etc [as modules at least].
Thanks,
-Garrett