How to get better debugging for the kernel.

[John Baldwin]

On Thursday, August 04, 2016 01:07:39 AM K. Macy wrote:
> On Wed, Aug 3, 2016 at 12:53 PM, Andriy Gapon <avg at 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