Date: Fri, 12 Aug 2016 20:39:57 +0000 From: Samy Bahra <sbahra@backtrace.io> To: John Baldwin <jhb@freebsd.org>, freebsd-hackers@freebsd.org Cc: "K. Macy" <kmacy@freebsd.org>, Andriy Gapon <avg@freebsd.org>, Ryan Stone <rysto32@gmail.com> Subject: Re: How to get better debugging for the kernel. Message-ID: <CA%2BGoQ8pYq7Bic8Jx=f_%2B-yrx59yE%2BnV27wF_rNWaefcOELwjBg@mail.gmail.com> In-Reply-To: <1551519.RkbAThDAeZ@ralph.baldwin.cx> References: <CACpH0Mcw1oTatUX3d7gJ4ys=dAj_y9C9vF_g110MrHVEup4mjQ@mail.gmail.com> <5cc825d5-9ed7-efac-b711-60a8d4b18cc4@FreeBSD.org> <CAHM0Q_OHE%2Bj1TfnYfJ7w1v7MpbRTcwE7mQnpnyY=GG%2BDHzwETg@mail.gmail.com> <1551519.RkbAThDAeZ@ralph.baldwin.cx>
next in thread | previous in thread | raw e-mail | index | archive | help
Slides up at: http://backtrace.io/blog/images/bbcon2016-sbahra.pdf On Fri, Aug 12, 2016 at 4:32 PM John Baldwin <jhb@freebsd.org> wrote: > On Thursday, August 04, 2016 01:07:39 AM K. Macy wrote: > > On Wed, Aug 3, 2016 at 12:53 PM, Andriy Gapon <avg@freebsd.org> wrote: > > > On 03/08/2016 20:14, Ryan Stone wrote: > > >> Are you using the kgdb from the base system or from ports(it's a part > of > > >> devel/gdb)? The kgdb in ports is significantly better. If you > haven't > > >> tried the version from ports already, definitely do that first. > > > > > > kgdb 7.x from ports is certainly more powerful than the old base kgdb, > > > but clang with O2 optimizations seems to be too much even for it. > > > > Samy did a good presentation about this issue. I'm hoping I can get > > him to put his slides on line. Evidently clang is much more simplistic > > about how it treats callee saved registers. In essence clang will > > always err on the side of saying "optimized out" even when it has > > sufficient state to know otherwise. Gcc, on the other hand will > > sometimes incorrectly infer that a value is valid when it is in fact > > not. > > > > I have been building some kernels with clang with dwarf4 enabled (and > > thus needed to use kgdb 7.x from ports). Contrary to what I have heard > > from some others I have found it to have virtually no added benefit. > > My understanding is that dwarf4 will not help with C programs like the > kernel, that the new idioms in dwarf4 are for declaring more complex > constructs in C++11, C++14, etc. I have heard that clang does not update > debug information during optimization passes causing it to loose track of > variables that are moved during optimization. I have not (yet) tried > using gcc as avg@ describes though I will likely start doing so soon. > > -- > John Baldwin >
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?CA%2BGoQ8pYq7Bic8Jx=f_%2B-yrx59yE%2BnV27wF_rNWaefcOELwjBg>