Use stable_sort instead of sort in PerfUnwind::flushEventBuffer

This is required to keep the order of events with the same time.
Most notably, this happens when we analyze data from a
runtime-attached perf record, i.e. `perf record -p ...`. There,
the data file will contain lots of mmap events with time 0. The
order is important though, so keep that via stable_sort.

Change-Id: Ibaa3a816b4921e404105f3d01f500f9ffb8ddd59
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 5 insertions, 2 deletions

diff --git a/app/perfunwind.cpp b/app/perfunwind.cpp
index ddb6774..90d3f8c 100644
--- a/app/perfunwind.cpp
+++ b/app/perfunwind.cpp
@@ -833,8 +833,11 @@ void PerfUnwind::flushEventBuffer(uint desiredBufferSize)
     auto sortByTime = [](const PerfRecord &lhs, const PerfRecord &rhs) {
         return lhs.time() < rhs.time();
     };
-    std::sort(m_mmapBuffer.begin(), m_mmapBuffer.end(), sortByTime);
-    std::sort(m_sampleBuffer.begin(), m_sampleBuffer.end(), sortByTime);
+    // stable sort here to keep order of events with the same time
+    // esp. when we runtime-attach, we will get lots of mmap events with time 0
+    // which we must not shuffle
+    std::stable_sort(m_mmapBuffer.begin(), m_mmapBuffer.end(), sortByTime);
+    std::stable_sort(m_sampleBuffer.begin(), m_sampleBuffer.end(), sortByTime);
 
     if (m_stats.enabled) {
         for (const auto &sample : m_sampleBuffer) {