Re: Additional debug info to aid cacheline analysis

[Namhyung Kim]

Hi Masami,

On Mon, Nov 2, 2020 at 5:27 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> Hi,
>
> On Fri, 30 Oct 2020 11:10:04 +0100
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > 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@xxxxxxxxx> 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.
>
> For this simple case, it is possible.
>
> This offset information is stored in the DWARF as a data-structure type
> information. (perf-probe uses it to find how to get the given local var's
> fields)
>
> So if we do this off-line, I think it is possible if it is recorded with
> instruction-pointers. For each place, we can do
>
>  - decode instruction and get the access address.
>  - get var assignment of %rax at that IP.
>  - get type information of var and find the field from offset.
>
> However, the problem is that if the DWARF has only assignment of "a",
> we need to decode the function body. (and usually this happens)
>
> func() {
>  struct xxx a;
>  ...
>  a.b->c->d++;
> }
>
> In this case, only "a" is the local variable. So DWARF records assignment of
> "a", not "b" nor "c" (since those are not a name of variables, just a name
> of fields). GCC may generate something like
>
>  mov    16(%rsp),%rdx   // rdx = a.b
>  mov    8(%rdx),%rax            // rax = b->c
>  inc    8(%rax)         // c->d++

Right, it'd be really nice if compiler can add information about the
(hidden) assignments in the rdx and rax here.

Thanks
Namhyung