Re: Additional debug info to aid cacheline analysis

Peter Zijlstra
2020-10-30T10:10:25Z

On Fri, Oct 30, 2020 at 10:16:49AM +0100, Mark Wielaard wrote:
> Hi Namhyung,
>
> On Fri, Oct 30, 2020 at 02:26:19PM +0900, Namhyung Kim wrote:
> > On Thu, Oct 8, 2020 at 6:38 PM Mark Wielaard <mark@klomp.org> wrote:
> > > GCC using -fvar-tracking and -fvar-tracking-assignments is pretty good
> > > at keeping track of where variables are held (in memory or registers)
> > > when in the program, even through various optimizations.
> > >
> > > -fvar-tracking-assignments is the default with -g -O2.
> > > Except for the upstream linux kernel code. Most distros enable it
> > > again, but you do want to enable it by hand when building from the
> > > upstream linux git repo.
> >
> > Please correct me if I'm wrong. This seems to track local variables.
> > But I'm not sure it's enough for this purpose as we want to know
> > types of any memory references (not directly from a variable).
> >
> > Let's say we have a variable like below:
> >
> > struct xxx a;
> >
> > a.b->c->d++;
> >
> > And we have a sample where 'd' is updated, then how can we know
> > it's from the variable 'a'? Maybe we don't need to know it, but we
> > should know it accesses the 'd' field in the struct 'c'.
> >
> > Probably we can analyze the asm code and figure out it's from 'a'
> > and accessing 'd' at the moment. I'm curious if there's a way in
> > the DWARF to help this kind of work.
>
> DWARF does have that information, but it stores it in a way that is
> kind of opposite to how you want to access it. Given a variable and an
> address, you can easily get the location where that variable is
> stored. But if you want to map back from a given (memory) location and
> address to the variable, that is more work.

The principal idea in this thread doesn't care about the address of the
variables. The idea was to get the data type and member information from
the instruction.

So in the above example: a.b->c->d++; what we'll end up with is
something like:

inc 8(%rax)

Where %rax contains c, and the offset of d in c is 8.

So what we want to (easily) find for that instruction is c::d.

So given any instruction with a memop (either load or store) we want to
find: type::member.