perf/x86: Make branch classifier fusion-aware

With branch fusion and other optimizations, branch sampling hardware in
some processors can report a branch from address that points to an
instruction preceding the actual branch by several bytes.

In such cases, the classifier cannot determine the branch type which leads
to failures such as with the recently added test from commit b55878c90ab9
("perf test: Add test for branch stack sampling"). Branch information is
also easier to consume and annotate if branch from addresses always point
to branch instructions.

Add a new variant of the branch classifier that can account for instruction
fusion. If fusion is expected and the current branch from address does not
point to a branch instruction, it attempts to find the first branch within
the next (MAX_INSN_SIZE - 1) bytes and if found, additionally provides the
offset between the reported branch from address and the address of the
expected branch instruction.

Signed-off-by: Sandipan Das <sandipan.das@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/b6bb0abaa8a54c0b6d716344700ee11a1793d709.1660211399.git.sandipan.das@amd.com

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 2de9cd66347e..93263b98cd6e 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -1271,6 +1271,8 @@ enum {
 
 int common_branch_type(int type);
 int branch_type(unsigned long from, unsigned long to, int abort);
+int branch_type_fused(unsigned long from, unsigned long to, int abort,
+		      int *offset);
 
 ssize_t x86_event_sysfs_show(char *page, u64 config, u64 event);
 ssize_t intel_event_sysfs_show(char *page, u64 config);