[PATCH 4/9] perf dlfilter: Add non-empty branch stack filter

[Amir Ayupov <aaupov@fb.com>]

--itrace=L adds decoded branch history to existing samples, but a sample
that was recorded while the decoder had no trace for that thread keeps an
empty branch stack. Consumers of the resulting perf script output, such
as profile generators for context-sensitive PGO, have no use for those
samples.

Add an opt-in dlfilter that drops samples whose parsed branch stack is
empty, so users can exclude them without changing default sample
semantics. Build and install it alongside perf's existing dlfilters.

Signed-off-by: Amir Ayupov <aaupov@fb.com>
---
 tools/perf/Makefile.perf                      |  1 +
 .../dlfilters/dlfilter-nonempty-brstack.c     | 26 +++++++++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 tools/perf/dlfilters/dlfilter-nonempty-brstack.c

diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index 0031112c036e8..aeb8085b0756d 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -427,6 +427,7 @@ PROGRAMS += $(OUTPUT)$(LIBJVMTI)
 endif
 
 DLFILTERS := dlfilter-test-api-v0.so dlfilter-test-api-v2.so dlfilter-show-cycles.so
+DLFILTERS += dlfilter-nonempty-brstack.so
 DLFILTERS := $(patsubst %,$(OUTPUT)dlfilters/%,$(DLFILTERS))
 
 # what 'all' will build and 'install' will install, in perfexecdir
diff --git a/tools/perf/dlfilters/dlfilter-nonempty-brstack.c b/tools/perf/dlfilters/dlfilter-nonempty-brstack.c
new file mode 100644
index 0000000000000..9e66205b841d5
--- /dev/null
+++ b/tools/perf/dlfilters/dlfilter-nonempty-brstack.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * dlfilter-nonempty-brstack.c: Filter out samples with no branch stack
+ * Copyright (c) 2026, Meta Platforms, Inc.
+ */
+#include <stddef.h>
+
+#include <perf/perf_dlfilter.h>
+
+int filter_event(void *data, const struct perf_dlfilter_sample *sample, void *ctx)
+{
+	/* Return 1 to filter out the sample, 0 to keep it */
+	return !sample->brstack_nr;
+}
+
+const char *filter_description(const char **long_description)
+{
+	static char *long_desc =
+		"Instruction trace decoders can add branch history to existing "
+		"samples, but samples that were recorded while no trace was "
+		"being collected get an empty branch stack. Filter those out so "
+		"that only samples carrying branch history remain.";
+
+	*long_description = long_desc;
+	return "Keep only samples with a non-empty branch stack";
+}
-- 
2.52.0