Re: [PATCH 1/3] dwarf_loader: Initial support for DW_TAG_variant_part

Alan Maguire
2026-04-08T14:05:57Z

On 23/03/2026 21:15, Arnaldo Carvalho de Melo wrote:
> From: Arnaldo Carvalho de Melo <acme@redhat.com>
>
> Still doesn't handle its sub hierarchy, i.e. the DW_TAG_variant entries
> and its underlying DW_TAG_member entries.
>
> This was noticed when running the regression test that uses a debug
> build of perf to process a perf.data file and test pahole's pretty
> printing features, as now perf has a synthetic workload that is written
> in rust:
>
> <0><20c95a>: Abbrev Number: 1 (DW_TAG_compile_unit)
> <20c95b> DW_AT_producer : (indirect string, offset: 0x4ee5a): clang LLVM (rustc version 1.93.1 (01f6ddf75 2026-02-11) (Fedora 1.93.1-1.fc43))
> <20c95f> DW_AT_language : 28 (Rust)
> <20c961> DW_AT_name : (indirect string, offset: 0x4eeaa): tests/workloads/code_with_type.rs/@/code_with_type.d6e680867bfb8b27-cgu.0
> <20c965> DW_AT_stmt_list : 0x5e1ed
> <20c969> DW_AT_comp_dir : (indirect string, offset: 0x487f1): /home/acme/git/perf-tools/tools/perf
> <20c96d> DW_AT_low_pc : 0
> <20c975> DW_AT_ranges : 0x2d0
> ⬢ [acme@toolbx pahole]$
>
> So lets add some scaffolding for the Rust DWARF constructs involved for
> us to be able to continue using perf with DWARF to test the pretty
> printing features.
>
> Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> ---
> dwarf_loader.c | 26 +++++++++++++++++++++++++-
> dwarves.c | 6 ++++++
> dwarves.h | 7 +++++++
> 3 files changed, 38 insertions(+), 1 deletion(-)
>
> diff --git a/dwarf_loader.c b/dwarf_loader.c
> index 16fb7becffee56f6..f0833e8c44a944a8 100644
> --- a/dwarf_loader.c
> +++ b/dwarf_loader.c
> @@ -508,6 +508,8 @@ static void tag__init(struct tag *tag, struct cu *cu, Dwarf_Die *die)
>
> if (tag->tag == DW_TAG_imported_module || tag->tag == DW_TAG_imported_declaration)
> dwarf_tag__set_attr_type(dtag, type, die, DW_AT_import);
> + else if (tag->tag == DW_TAG_variant_part)
> + dwarf_tag__set_attr_type(dtag, type, die, DW_AT_discr);
> else
> dwarf_tag__set_attr_type(dtag, type, die, DW_AT_type);
>
> @@ -1170,6 +1172,18 @@ static struct template_parameter_pack *template_parameter_pack__new(Dwarf_Die *d
> return pack;
> }
>
> +static struct variant_part *variant_part__new(Dwarf_Die *die, struct cu *cu, struct conf_load *conf)
> +{
> + struct variant_part *vpart = tag__alloc(cu, sizeof(*vpart));
> +
> + if (vpart != NULL) {
> + tag__init(&vpart->tag, cu, die);
> + INIT_LIST_HEAD(&vpart->variants);
> + }
> +
> + return vpart;
> +}
> +
> /* Returns number of locations found or negative value for errors. */
> static ptrdiff_t __dwarf_getlocations(Dwarf_Attribute *attr,
> ptrdiff_t offset, Dwarf_Addr *basep,
> @@ -1990,9 +2004,19 @@ static int die__process_class(Dwarf_Die *die, struct type *class,
> case DW_TAG_GNU_template_template_param:
> #endif
> case DW_TAG_subrange_type: // XXX: ADA stuff, its a type tho, will have other entries referencing it...
> - case DW_TAG_variant_part: // XXX: Rust stuff
> tag__print_not_supported(die);
> continue;
> + case DW_TAG_variant_part: {
> + struct variant_part *vpart = variant_part__new(die, cu, conf);
> +
> + if (vpart == NULL)
> + return -ENOMEM;
> +
> + // For rust it seems we have just one, but DWARF, according to Gemini, support having
> + // more than one DW_TAG_variant_part for a given DW_TAG_structure_type, so future proof it
> + type__add_variant_part(class, vpart);
> + continue;
> + }
> case DW_TAG_template_type_parameter: {
> struct template_type_param *ttparm = template_type_param__new(die, cu, conf);
>
> diff --git a/dwarves.c b/dwarves.c
> index ef93239d26827711..efc7ae214376e1c1 100644
> --- a/dwarves.c
> +++ b/dwarves.c
> @@ -428,6 +428,7 @@ void __type__init(struct type *type)
> INIT_LIST_HEAD(&type->type_enum);
> INIT_LIST_HEAD(&type->template_type_params);
> INIT_LIST_HEAD(&type->template_value_params);
> + INIT_LIST_HEAD(&type->variant_parts);
> type->template_parameter_pack = NULL;
> type->sizeof_member = NULL;
> type->member_prefix = NULL;
> @@ -1366,6 +1367,11 @@ void type__add_template_value_param(struct type *type, struct template_value_par
> list_add_tail(&tvparam->tag.node, &type->template_value_params);
> }
>
> +void type__add_variant_part(struct type *type, struct variant_part *vpart)
> +{
> + list_add_tail(&vpart->tag.node, &type->variant_parts);
> +}
> +
> struct class_member *type__last_member(struct type *type)
> {
> struct class_member *pos;
> diff --git a/dwarves.h b/dwarves.h
> index d7c64742c29a9e9f..95d84b8ce3a6e95d 100644
> --- a/dwarves.h
> +++ b/dwarves.h
> @@ -1011,6 +1011,11 @@ static inline struct formal_parameter_pack *tag__formal_parameter_pack(const str
>
> void formal_parameter_pack__add(struct formal_parameter_pack *pack, struct parameter *param);
>
> +struct variant_part {
> + struct tag tag;
> + struct list_head variants;
> +};
> +
> /*
> * tag.tag can be DW_TAG_subprogram_type or DW_TAG_subroutine_type.
> */
> @@ -1281,6 +1286,7 @@ struct type {
> uint8_t is_signed_enum:1;
> struct list_head template_type_params;
> struct list_head template_value_params;
> + struct list_head variant_parts;
> struct template_parameter_pack *template_parameter_pack;
> };
>
> @@ -1401,6 +1407,7 @@ static inline struct class_member *class_member__next(struct class_member *membe
> void type__add_member(struct type *type, struct class_member *member);
> void type__add_template_type_param(struct type *type, struct template_type_param *ttparm);
> void type__add_template_value_param(struct type *type, struct template_value_param *tvparam);
> +void type__add_variant_part(struct type *type, struct variant_part *vpart);
>
> struct class_member *
> type__find_first_biggest_size_base_type_member(struct type *type,

do we also need some cleanup for the variant parts in type__delete() something like:

list_for_each_entry_safe(pos, n, type->variant_parts, node) {
list_del_int(&pos->node);
free(pos);
}

?