[PATCH 2/9] perf thread-stack: Report branch stack hw_idx as not available

[Adrian Hunter]

On 11/08/2026 18:58, Adrian Hunter wrote:
> On 03/08/2026 12:06, Amir Ayupov wrote:
>> thread_stack__br_sample() and thread_stack__br_sample_late() fill a
>> caller-supplied branch_stack that is typically allocated with zalloc(),
>> leaving hw_idx as 0. Zero is a valid hardware index, so consumers that
>> honour PERF_SAMPLE_BRANCH_HW_INDEX see a reconstructed branch stack
>> claiming to start at LBR TOS entry 0.
>>
>> These branch stacks are reconstructed from instruction trace and have no
>> hardware index at all. Set hw_idx to -1ULL, which is the established way
>> to say "not available" and matches what intel-pt and cs-etm already put
>> in the branch stacks they synthesise directly.
>>
>> Signed-off-by: Amir Ayupov <aaupov at fb.com>
> 
> Fixes tag?
> 
> Otherwise:
> 
> Reviewed-by: Adrian Hunter <adrian.hunter at intel.com>

On second thoughts, it seems that hw_idx is only used for stitching
LBRs which is anyway disabled by default and only enabled by --stitch-lbr.

Setting -1ULL will prevent has_stitched_lbr() making a match, but we can
rely on the user to decide that for themselves via --stitch-lbr.

So, in fact, it doesn't look like this change should be needed?

> 
>> ---
>>  tools/perf/util/thread-stack.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/tools/perf/util/thread-stack.c b/tools/perf/util/thread-stack.c
>> index c5ce741b07446..1a3dffa83bde2 100644
>> --- a/tools/perf/util/thread-stack.c
>> +++ b/tools/perf/util/thread-stack.c
>> @@ -624,6 +624,7 @@ void thread_stack__br_sample(struct thread *thread, int cpu,
>>  	unsigned int nr;
>>  
>>  	dst->nr = 0;
>> +	dst->hw_idx = -1ULL;
>>  
>>  	if (!ts)
>>  		return;
>> @@ -686,6 +687,7 @@ void thread_stack__br_sample_late(struct thread *thread, int cpu,
>>  	bool start = false;
>>  
>>  	dst->nr = 0;
>> +	dst->hw_idx = -1ULL;
>>  
>>  	if (!ts)
>>  		return;
>