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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Wed, Apr 08, 2026 at 03:05:35PM +0100, Alan Maguire wrote:
> 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);
> 	}

Right, I'll fix it in v2, there are some other issues that Claude
detected that I'll address in the enumeration case, one of them can be
seen here:

⬢ [acme@toolbx pahole]$ pahole -C ProgramKind /tmp/build/perf-tools-next/tests/workloads/code_with_type.a
enum ProgramKind {
	PathLookup = 0,
	Relative   = 1,
	Absolute   = 2,
	�3"��     = 140698500347136,
} __attribute__((__packed__));

⬢ [acme@toolbx pahole]$

Namely this Rust enum has a DW_TAG_subprogram, that
enumeration__fprintf() doesn't know about, fixing it now.

- Arnaldo