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 <[email protected]>
---
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 */
