linux-kernel - Re: [PATCH v1 3/6] perf dwarf-aux: Better type matching for stack variables

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Message-ID: <CAP-5=fWbKj6fjxTccpszgMoVCc17e+2ZmixU5B3W0WU5wo_zDg@mail.gmail.com>
Date: Fri, 25 Jul 2025 18:17:33 -0700
From: Ian Rogers <irogers@...gle.com>
To: Zecheng Li <zecheng@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, "Liang, Kan" <kan.liang@...ux.intel.com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Zecheng Li <zli94@...u.edu>, Xu Liu <xliuprof@...gle.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/6] perf dwarf-aux: Better type matching for stack variables

On Fri, Jul 25, 2025 at 1:28 PM Zecheng Li <zecheng@...gle.com> wrote:
>
> Utilizes the previous is_breg_access_indirect function to determine if
> the stack location stores the variable itself or the struct it points
> to.
>
> If the DWARF expression shows DW_OP_stack_value without DW_OP_deref, it
> indicates the variable value is the reg + offset itself, and the stack
> location it points to is the dereferenced type.
>
> Signed-off-by: Zecheng Li <zecheng@...gle.com>
> ---
>  tools/perf/util/dwarf-aux.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index 814c96ea509f..4039dbd2b8c0 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -1635,6 +1635,14 @@ static int __die_collect_vars_cb(Dwarf_Die *die_mem, void *arg)
>         if (die_get_real_type(die_mem, &type_die) == NULL)
>                 return DIE_FIND_CB_SIBLING;
>
> +       if ((ops->atom == DW_OP_fbreg || ops->atom == DW_OP_breg7) &&

A comment saying the significance of DW_OP_fbreg and DW_OP_breg7 would
be useful, for example, why not DW_OP_breg6? Isn't breg7 going to be
x86 specific?

Thanks,
Ian


> +           dwarf_tag(&type_die) == DW_TAG_pointer_type &&
> +           is_breg_access_indirect(ops, nops)) {
> +               /* Get the target type of the pointer */
> +               if (die_get_real_type(&type_die, &type_die) == NULL)
> +                       return DIE_FIND_CB_SIBLING;
> +       }
> +
>         vt = malloc(sizeof(*vt));
>         if (vt == NULL)
>                 return DIE_FIND_CB_END;
> --
> 2.50.1.470.g6ba607880d-goog
>