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Message-ID: <CAJUgMyK2wTiEZB__dtgCELmaNGFWhG1j0g9rv_C=cLD6Zq4_5w@mail.gmail.com>
Date: Thu, 28 Aug 2025 14:36:32 -0400
From: Zecheng Li <zecheng@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>,
"Liang, Kan" <kan.liang@...ux.intel.com>, Masami Hiramatsu <mhiramat@...nel.org>,
Xu Liu <xliuprof@...gle.com>, linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/10] perf dwarf-aux: More accurate variable type
match for breg
On Thu, Aug 28, 2025 at 3:19 AM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Mon, Aug 25, 2025 at 07:54:04PM +0000, Zecheng Li wrote:
> > Introduces the function is_breg_access_indirect to determine whether a
> > memory access involving a DW_OP_breg* operation refers to the variable's
> > value directly or requires dereferencing the variable's type as a
> > pointer based on the DWARF expression. Previously, all breg based
> > accesses were assumed to directly access the variable's value
> > (is_pointer = false).
> >
> > The is_breg_access_indirect function handles three cases:
> >
> > 1. Base register + offset only: (e.g., DW_OP_breg7 RSP+88) The
> > calculated address is the location of the variable. The access is
> > direct, so no type dereference is needed. Returns false.
>
> I'm afraid there may be cases that the base register doesn't point to
> the stack. In that case it may return true, right?
Hi Namhyung,
In this case, the DWARF specification for a DW_OP_breg* operation is
to always calculate a memory address. So, even if the base register
isn't the stack pointer, the expression still resolves to the
variable's location, meaning the access is direct (is_pointer =
false).
> I think struct find_var_data already has 'is_fbreg' field. Maybe you
> can add 'is_stack' or 'is_stack_reg' field if the target. Currently we
> hardcoded X86_REG_SP but it should be arch-dependent.
Therefore we don't need to check if the register is a stack or frame base.
> >
> > 2. Base register + offset, followed by other operations ending in
> > DW_OP_stack_value, including DW_OP_deref: (e.g., DW_OP_breg*,
> > DW_OP_deref, DW_OP_stack_value) The DWARF expression computes the
> > variable's value, but that value requires a dereference. The memory
> > access is fetching that value, so no type dereference is needed.
> > Returns false.
> >
> > 3. Base register + offset, followed only by DW_OP_stack_value: (e.g.,
> > DW_OP_breg13 R13+256, DW_OP_stack_value) This indicates the value at
> > the base + offset is the variable's value. Since this value is being
> > used as an address in the memory access, the variable's type is
> > treated as a pointer and requires a type dereference. Returns true.
> >
> > The is_pointer argument passed to match_var_offset is now set by
> > is_breg_access_indirect for breg accesses.
> >
> > There are more complex expressions that includes multiple operations and
> > may require additional handling, such as DW_OP_deref without a
> > DW_OP_stack_value, or including multiple base registers. They are less
> > common in the Linux kernel dwarf and are skipped in check_allowed_ops.
> >
> > Signed-off-by: Zecheng Li <zecheng@...gle.com>
> > ---
> > tools/perf/util/dwarf-aux.c | 38 ++++++++++++++++++++++++++++++++-----
> > 1 file changed, 33 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> > index 920054425578..449bc9ad7aff 100644
> > --- a/tools/perf/util/dwarf-aux.c
> > +++ b/tools/perf/util/dwarf-aux.c
> > @@ -1423,6 +1423,34 @@ static bool match_var_offset(Dwarf_Die *die_mem, struct find_var_data *data,
> > return true;
> > }
> >
> > +/**
> > + * is_breg_access_indirect - Check if breg based access implies type
> > + * dereference
> > + * @ops: DWARF operations array
> > + * @nops: Number of operations in @ops
> > + *
> > + * Returns true if the DWARF expression evaluates to the variable's
> > + * value, so the memory access on that register needs type dereference.
> > + * Returns false if the expression evaluates to the variable's address.
> > + * This is called after check_allowed_ops.
> > + */
> > +static bool is_breg_access_indirect(Dwarf_Op *ops, size_t nops)
> > +{
> > + /* only the base register */
> > + if (nops == 1)
> > + return false;
>
> Then it could be like below:
>
> if (nops == 1) {
> int reg = reg_from_dwarf_op(ops);
> return !(reg == DWARF_REG_FB || data->is_fbreg || reg == data->is_stack);
> }
>
> Thanks,
> Namhyung
>
> > +
> > + if (nops == 2 && ops[1].atom == DW_OP_stack_value)
> > + return true;
> > +
> > + if (nops == 3 && (ops[1].atom == DW_OP_deref ||
> > + ops[1].atom == DW_OP_deref_size) &&
> > + ops[2].atom == DW_OP_stack_value)
> > + return false;
> > + /* unreachable, OP not supported */
> > + return false;
> > +}
> > +
> > /* Only checks direct child DIEs in the given scope. */
> > static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
> > {
> > @@ -1451,7 +1479,7 @@ static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
> > if (data->is_fbreg && ops->atom == DW_OP_fbreg &&
> > check_allowed_ops(ops, nops) &&
> > match_var_offset(die_mem, data, data->offset, ops->number,
> > - /*is_pointer=*/false))
> > + is_breg_access_indirect(ops, nops)))
> > return DIE_FIND_CB_END;
> >
> > /* Only match with a simple case */
> > @@ -1463,11 +1491,11 @@ static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
> > /*is_pointer=*/true))
> > return DIE_FIND_CB_END;
> >
> > - /* Local variables accessed by a register + offset */
> > + /* variables accessed by a register + offset */
> > if (ops->atom == (DW_OP_breg0 + data->reg) &&
> > check_allowed_ops(ops, nops) &&
> > match_var_offset(die_mem, data, data->offset, ops->number,
> > - /*is_pointer=*/false))
> > + is_breg_access_indirect(ops, nops)))
> > return DIE_FIND_CB_END;
> > } else {
> > /* pointer variables saved in a register 32 or above */
> > @@ -1477,11 +1505,11 @@ static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
> > /*is_pointer=*/true))
> > return DIE_FIND_CB_END;
> >
> > - /* Local variables accessed by a register + offset */
> > + /* variables accessed by a register + offset */
> > if (ops->atom == DW_OP_bregx && data->reg == ops->number &&
> > check_allowed_ops(ops, nops) &&
> > match_var_offset(die_mem, data, data->offset, ops->number2,
> > - /*is_poitner=*/false))
> > + is_breg_access_indirect(ops, nops)))
> > return DIE_FIND_CB_END;
> > }
> > }
> > --
> > 2.51.0.261.g7ce5a0a67e-goog
> >
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