Re: [PATCH v1 5/6] perf dwarf-aux: Find pointer type to a type

[Namhyung Kim <namhyung@kernel.org>]

On Fri, Jul 25, 2025 at 08:28:08PM +0000, Zecheng Li wrote:
> Implement die_find_pointer_to_type that searches for the pointer type to
> a given type in the CU. There's no guarantee that a pointer DIE exists
> for every possible base type. Compilers only generate DIEs for types
> actually used or defined.

Please describe why it's necessary first.  What happens if it fails to
fine one?

Thanks,
Namhyung

> 
> The performance could be improved by adding a cache for the pointer
> types. Currently its impact on the annotation time for vmlinux is low.
> 
> Signed-off-by: Zecheng Li <zecheng@google.com>
> ---
>  tools/perf/util/dwarf-aux.c | 69 +++++++++++++++++++++++++++++++++++++
>  tools/perf/util/dwarf-aux.h |  4 +++
>  2 files changed, 73 insertions(+)
> 
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index 4039dbd2b8c0..49d509839a85 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -2121,3 +2121,72 @@ Dwarf_Die *die_deref_ptr_type(Dwarf_Die *ptr_die, int offset,
>  
>  	return die_get_member_type(&type_die, offset, die_mem);
>  }
> +
> +struct find_pointer_type_data {
> +	/* DIE offset of the type we want to point to */
> +	Dwarf_Off target_type_offset;
> +	Dwarf_Die *found_die;
> +};
> +
> +static int __die_find_pointer_to_type_cb(Dwarf_Die *die_mem, void *arg)
> +{
> +	struct find_pointer_type_data *data = arg;
> +	Dwarf_Attribute type_attr;
> +	Dwarf_Die type_die;
> +	Dwarf_Off ref_type_offset;
> +
> +	if (dwarf_tag(die_mem) != DW_TAG_pointer_type)
> +		return DIE_FIND_CB_CONTINUE;
> +
> +	if (!dwarf_attr(die_mem, DW_AT_type, &type_attr))
> +		return DIE_FIND_CB_SIBLING;
> +
> +	/* Get the DIE this pointer points to */
> +	if (!dwarf_formref_die(&type_attr, &type_die))
> +		return DIE_FIND_CB_SIBLING;
> +
> +	ref_type_offset = dwarf_dieoffset(&type_die);
> +
> +	if (ref_type_offset != 0 && ref_type_offset == data->target_type_offset) {
> +		/* This die_mem is a pointer to the target type */
> +		if (data->found_die)
> +			*data->found_die = *die_mem;
> +		return DIE_FIND_CB_END;
> +	}
> +
> +	return DIE_FIND_CB_SIBLING;
> +}
> +
> +/**
> + * die_find_pointer_to_type - Find a DIE for a pointer to a given type
> + * @cu_die: The compilation unit to search within.
> + * @target_type: The DIE of the type you want to find a pointer to.
> + * @result_die: Buffer to store the found DW_TAG_pointer_type DIE.
> + *
> + * Scans the children of the @cu_die for a DW_TAG_pointer_type DIE
> + * whose DW_AT_type attribute references the @target_type.
> + *
> + * Return: @result_die if found, NULL otherwise.
> + */
> +Dwarf_Die *die_find_pointer_to_type(Dwarf_Die *cu_die, Dwarf_Die *target_type,
> +				   Dwarf_Die *result_die)
> +{
> +	struct find_pointer_type_data data;
> +	Dwarf_Die search_mem;
> +
> +	if (!cu_die || !target_type || !result_die)
> +		return NULL;
> +
> +	data.target_type_offset = dwarf_dieoffset(target_type);
> +	if (data.target_type_offset == 0) {
> +		pr_debug("Target type DIE has no offset\n");
> +		return NULL;
> +	}
> +	data.found_die = result_die;
> +
> +	if (die_find_child(cu_die, __die_find_pointer_to_type_cb, &data, &search_mem))
> +		return result_die;
> +
> +	return NULL;
> +}
> +
> diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
> index 892c8c5c23fc..7e1336ff276e 100644
> --- a/tools/perf/util/dwarf-aux.h
> +++ b/tools/perf/util/dwarf-aux.h
> @@ -156,6 +156,10 @@ Dwarf_Die *die_get_member_type(Dwarf_Die *type_die, int offset, Dwarf_Die *die_m
>  /* Return type info where the pointer and offset point to */
>  Dwarf_Die *die_deref_ptr_type(Dwarf_Die *ptr_die, int offset, Dwarf_Die *die_mem);
>  
> +/* Find a DIE for a pointer to a given type */
> +Dwarf_Die *die_find_pointer_to_type(Dwarf_Die *cu_die, Dwarf_Die *target_type,
> +				   Dwarf_Die *result_die);
> +
>  /* Get byte offset range of given variable DIE */
>  int die_get_var_range(Dwarf_Die *sp_die, Dwarf_Die *vr_die, struct strbuf *buf);
>  
> -- 
> 2.50.1.470.g6ba607880d-goog
>