[PATCH v4 2/4] perf: Use a union to clear branch entry bitfields

[James Clark]

On 27/05/2026 1:11 pm, Puranjay Mohan wrote:
> perf_clear_branch_entry_bitfields() zeroes individual bitfields of struct
> perf_branch_entry but has repeatedly fallen out of sync when new fields
> were added (new_type and priv were missed).
> 
> Wrap the bitfields in an anonymous struct inside a union with a u64
> bitfields member, and clear them all with a single assignment. This
> avoids having to update the clearing function every time a new bitfield
> is added.
> 
> Fixes: bfe4daf850f4 ("perf/core: Add perf_clear_branch_entry_bitfields() helper")
> Signed-off-by: Puranjay Mohan <puranjay at kernel.org>
> ---
>   include/linux/perf_event.h            |  9 +--------
>   include/uapi/linux/perf_event.h       | 25 +++++++++++++++----------
>   tools/include/uapi/linux/perf_event.h | 25 +++++++++++++++----------
>   3 files changed, 31 insertions(+), 28 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 48d851fbd8ea..f7360c43f902 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1474,14 +1474,7 @@ static inline u32 perf_sample_data_size(struct perf_sample_data *data,
>    */
>   static inline void perf_clear_branch_entry_bitfields(struct perf_branch_entry *br)
>   {
> -	br->mispred	= 0;
> -	br->predicted	= 0;
> -	br->in_tx	= 0;
> -	br->abort	= 0;
> -	br->cycles	= 0;
> -	br->type	= 0;
> -	br->spec	= PERF_BR_SPEC_NA;
> -	br->reserved	= 0;
> +	br->bitfields = 0;

The comment above here says:

  * The to and from fields are not cleared because they are
  * systematically modified by caller.

But this is an inline function and the to/from fields are always 
assigned right after calling it. Surely the compiler can work that out 
and this was a premature optimization.

If we rename it to perf_clear_branch_entry() and do:

   *br = (struct perf_branch_entry) {0};

Then we can delete a big comment, remove a potential source of bugs if 
even more members are added, not have to update the uapi headers and 
simplify it instead of working around the original issue.

>   }
>   
>   extern void perf_output_sample(struct perf_output_handle *handle,
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index fd10aa8d697f..c2e7b1b1c4fa 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -1491,16 +1491,21 @@ union perf_mem_data_src {
>   struct perf_branch_entry {
>   	__u64	from;
>   	__u64	to;
> -	__u64	mispred   :  1, /* target mispredicted */
> -		predicted :  1, /* target predicted */
> -		in_tx     :  1, /* in transaction */
> -		abort     :  1, /* transaction abort */
> -		cycles    : 16, /* cycle count to last branch */
> -		type      :  4, /* branch type */
> -		spec      :  2, /* branch speculation info */
> -		new_type  :  4, /* additional branch type */
> -		priv      :  3, /* privilege level */
> -		reserved  : 31;
> +	union {
> +		struct {
> +			__u64	mispred   :  1, /* target mispredicted */
> +				predicted :  1, /* target predicted */
> +				in_tx     :  1, /* in transaction */
> +				abort     :  1, /* transaction abort */
> +				cycles    : 16, /* cycle count to last branch */
> +				type      :  4, /* branch type */
> +				spec      :  2, /* branch speculation info */
> +				new_type  :  4, /* additional branch type */
> +				priv      :  3, /* privilege level */
> +				reserved  : 31;
> +		};
> +		__u64	bitfields;
> +	};
>   };
>   
>   /* Size of used info bits in struct perf_branch_entry */
> diff --git a/tools/include/uapi/linux/perf_event.h b/tools/include/uapi/linux/perf_event.h
> index fd10aa8d697f..c2e7b1b1c4fa 100644
> --- a/tools/include/uapi/linux/perf_event.h
> +++ b/tools/include/uapi/linux/perf_event.h
> @@ -1491,16 +1491,21 @@ union perf_mem_data_src {
>   struct perf_branch_entry {
>   	__u64	from;
>   	__u64	to;
> -	__u64	mispred   :  1, /* target mispredicted */
> -		predicted :  1, /* target predicted */
> -		in_tx     :  1, /* in transaction */
> -		abort     :  1, /* transaction abort */
> -		cycles    : 16, /* cycle count to last branch */
> -		type      :  4, /* branch type */
> -		spec      :  2, /* branch speculation info */
> -		new_type  :  4, /* additional branch type */
> -		priv      :  3, /* privilege level */
> -		reserved  : 31;
> +	union {
> +		struct {
> +			__u64	mispred   :  1, /* target mispredicted */
> +				predicted :  1, /* target predicted */
> +				in_tx     :  1, /* in transaction */
> +				abort     :  1, /* transaction abort */
> +				cycles    : 16, /* cycle count to last branch */
> +				type      :  4, /* branch type */
> +				spec      :  2, /* branch speculation info */
> +				new_type  :  4, /* additional branch type */
> +				priv      :  3, /* privilege level */
> +				reserved  : 31;
> +		};
> +		__u64	bitfields;
> +	};
>   };
>   
>   /* Size of used info bits in struct perf_branch_entry */