On Fri Dec 23 11, John Baldwin wrote: > On Thursday, December 22, 2011 9:51:47 pm Garrett Cooper wrote: > > On Dec 22, 2011, at 4:59 PM, Alexander Best <arun...@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"
would making -O2 -fno-inline the default flags introduce any major slowdown? all that would be needed then to build a debugging kernel would be to add -g. cheers. alex > > Just adding -fno-inline makes a world of difference. > > -- > John Baldwin _______________________________________________ freebsd-current@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
