On Wed, Aug 12, 2026 at 06:48:24PM +0300, Adrian Hunter wrote:

On 11/08/2026 18:58, Adrian Hunter wrote:

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On 03/08/2026 12:06, Amir Ayupov wrote:

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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@fb.com

Fixes tag?

Otherwise:

Reviewed-by: Adrian Hunter adrian.hunter@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.

This is one of those options that few people use as its so specialized, do you think we could auto-enable it if we notice it is a good idea for some specific machine and request from the user? I.e. user requests callchains, unlimited or with a limit that is more than what we can do without stitching: we auto stich?

- Arnaldo

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

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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;