Date: Fri, 12 Aug 2016 12:22:18 -0700 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: "K. Macy" <kmacy@freebsd.org>, Andriy Gapon <avg@freebsd.org>, Samy Bahra <sbahra@backtrace.io>, Ryan Stone <rysto32@gmail.com> Subject: Re: How to get better debugging for the kernel. Message-ID: <1551519.RkbAThDAeZ@ralph.baldwin.cx> In-Reply-To: <CAHM0Q_OHE%2Bj1TfnYfJ7w1v7MpbRTcwE7mQnpnyY=GG%2BDHzwETg@mail.gmail.com> References: <CACpH0Mcw1oTatUX3d7gJ4ys=dAj_y9C9vF_g110MrHVEup4mjQ@mail.gmail.com> <5cc825d5-9ed7-efac-b711-60a8d4b18cc4@FreeBSD.org> <CAHM0Q_OHE%2Bj1TfnYfJ7w1v7MpbRTcwE7mQnpnyY=GG%2BDHzwETg@mail.gmail.com>
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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
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