[Backport] metatrace: remove memset and trivial-ctor assumption

Turns out that on MSVC std::atomic<int> is not trivially constructible
(although I think is still a plain old int, it just fails the check).
Fall back on resetting each element individually.
Thankfully the compiler can see through and eventually figures out
it can do a memset: https://godbolt.org/z/wMre8O

Bug: chromium:1010616
Fixes: QTBUG-82272
Change-Id: I971ff888306d6bdbaf6e6b886f9ca506ddc1b30a
Reviewed-by: Michael Brüning <michael.bruning@qt.io>

2 files changed, 19 insertions, 10 deletions

diff --git a/chromium/third_party/perfetto/include/perfetto/ext/base/metatrace.h b/chromium/third_party/perfetto/include/perfetto/ext/base/metatrace.h
index d560c3e5a24..9e9c95f6584 100644
--- a/chromium/third_party/perfetto/include/perfetto/ext/base/metatrace.h
+++ b/chromium/third_party/perfetto/include/perfetto/ext/base/metatrace.h
@@ -106,22 +106,33 @@ struct Record {
     timestamp_ns_high = static_cast<uint16_t>(diff >> 32);
   }
 
+  // We can't just memset() this class because on MSVC std::atomic<> is not
+  // trivially constructible anymore. Also std::atomic<> has a deleted copy
+  // constructor so we cant just do "*this = Record()" either.
+  // See http://bit.ly/339Jlzd .
+  void clear() {
+    this->~Record();
+    new (this) Record();
+  }
+
   // This field holds the type (counter vs event) in the MSB and event ID (as
   // defined in metatrace_events.h) in the lowest 15 bits. It is also used also
   // as a linearization point: this is always written after all the other
   // fields with a release-store. This is so the reader can determine whether it
   // can safely process the other event fields after a load-acquire.
-  std::atomic<uint16_t> type_and_id;
+  std::atomic<uint16_t> type_and_id{};
 
   // Timestamp is stored as a 48-bits value diffed against g_enabled_timestamp.
   // This gives us 78 hours from Enabled().
-  uint16_t timestamp_ns_high;
-  uint32_t timestamp_ns_low;
+  uint16_t timestamp_ns_high = 0;
+  uint32_t timestamp_ns_low = 0;
 
-  uint32_t thread_id;
+  uint32_t thread_id = 0;
 
   union {
-    uint32_t duration_ns;   // If type == event.
+    // Only one of the two elements can be zero initialized, clang complains
+    // about "initializing multiple members of union" otherwise.
+    uint32_t duration_ns = 0;  // If type == event.
     int32_t counter_value;  // If type == counter.
   };
 };
diff --git a/chromium/third_party/perfetto/src/base/metatrace.cc b/chromium/third_party/perfetto/src/base/metatrace.cc
index ca59b125c25..8c62c0017f1 100644
--- a/chromium/third_party/perfetto/src/base/metatrace.cc
+++ b/chromium/third_party/perfetto/src/base/metatrace.cc
@@ -84,11 +84,9 @@ void Disable() {
 
 // static
 void RingBuffer::Reset() {
-  static_assert(PERFETTO_IS_TRIVIALLY_CONSTRUCTIBLE(Record) &&
-                    std::is_trivially_destructible<Record>::value,
-                "Record must be trivial");
-  memset(&records_[0], 0, sizeof(records_));
-  memset(&bankruptcy_record_, 0, sizeof(bankruptcy_record_));
+  bankruptcy_record_.clear();
+  for (Record& record : records_)
+    record.clear();
   wr_index_ = 0;
   rd_index_ = 0;
   has_overruns_ = false;