[rtsan] Add exit statistics (#109885)

adds the flag `print_stats_on_exit` which mirrors nsan's same flag.

# Why?

Not only is this nice for the end users, this gives us a very trivial
way to test deduplication which is next up

Currently the style is something like:

```
RealtimeSanitizer exit stats:
    Total error count: 488
```

6 files changed, 94 insertions, 3 deletions

diff --git a/compiler-rt/lib/rtsan/CMakeLists.txt b/compiler-rt/lib/rtsan/CMakeLists.txt
index b7e2362d..af34fb6 100644
--- a/compiler-rt/lib/rtsan/CMakeLists.txt
+++ b/compiler-rt/lib/rtsan/CMakeLists.txt
@@ -5,7 +5,9 @@ set(RTSAN_CXX_SOURCES
   rtsan_context.cpp
   rtsan_diagnostics.cpp
   rtsan_flags.cpp
-  rtsan_interceptors.cpp)
+  rtsan_interceptors.cpp
+  rtsan_stats.cpp
+  )
 
 set(RTSAN_PREINIT_SOURCES
   rtsan_preinit.cpp)
@@ -16,7 +18,9 @@ set(RTSAN_HEADERS
   rtsan_context.h
   rtsan_diagnostics.h
   rtsan_flags.h
-  rtsan_flags.inc)
+  rtsan_flags.inc
+  rtsan_stats.h
+  )
 
 set(RTSAN_DEPS)
 
diff --git a/compiler-rt/lib/rtsan/rtsan.cpp b/compiler-rt/lib/rtsan/rtsan.cpp
index f02e894..87c3611 100644
--- a/compiler-rt/lib/rtsan/rtsan.cpp
+++ b/compiler-rt/lib/rtsan/rtsan.cpp
@@ -13,6 +13,7 @@
 #include "rtsan/rtsan_diagnostics.h"
 #include "rtsan/rtsan_flags.h"
 #include "rtsan/rtsan_interceptors.h"
+#include "rtsan/rtsan_stats.h"
 
 #include "sanitizer_common/sanitizer_atomic.h"
 #include "sanitizer_common/sanitizer_common.h"
@@ -46,7 +47,10 @@ static InitializationState GetInitializationState() {
 
 static auto OnViolationAction(DiagnosticsInfo info) {
   return [info]() {
-    __rtsan::PrintDiagnostics(info);
+    IncrementTotalErrorCount();
+
+    PrintDiagnostics(info);
+
     if (flags().halt_on_error)
       Die();
   };
@@ -62,6 +66,9 @@ SANITIZER_INTERFACE_ATTRIBUTE void __rtsan_init() {
   InitializeFlags();
   InitializeInterceptors();
 
+  if (flags().print_stats_on_exit)
+    Atexit(PrintStatisticsSummary);
+
   SetInitializationState(InitializationState::Initialized);
 }
 
diff --git a/compiler-rt/lib/rtsan/rtsan_flags.inc b/compiler-rt/lib/rtsan/rtsan_flags.inc
index 25d62cf..1df71127 100644
--- a/compiler-rt/lib/rtsan/rtsan_flags.inc
+++ b/compiler-rt/lib/rtsan/rtsan_flags.inc
@@ -17,3 +17,4 @@
 // See COMMON_FLAG in sanitizer_flags.inc for more details.
 
 RTSAN_FLAG(bool, halt_on_error, true, "Exit after first reported error.")
+RTSAN_FLAG(bool, print_stats_on_exit, false, "Print stats on exit.")
diff --git a/compiler-rt/lib/rtsan/rtsan_stats.cpp b/compiler-rt/lib/rtsan/rtsan_stats.cpp
new file mode 100644
index 0000000..7c1ccf2
--- /dev/null
+++ b/compiler-rt/lib/rtsan/rtsan_stats.cpp
@@ -0,0 +1,35 @@
+//===--- rtsan_stats.cpp - Realtime Sanitizer -------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Part of the RealtimeSanitizer runtime library
+//
+//===----------------------------------------------------------------------===//
+
+#include "rtsan/rtsan_stats.h"
+
+#include "sanitizer_common/sanitizer_atomic.h"
+#include "sanitizer_common/sanitizer_common.h"
+
+using namespace __sanitizer;
+using namespace __rtsan;
+
+static atomic_uint32_t rtsan_total_error_count{0};
+
+void __rtsan::IncrementTotalErrorCount() {
+  atomic_fetch_add(&rtsan_total_error_count, 1, memory_order_relaxed);
+}
+
+static u32 GetTotalErrorCount() {
+  return atomic_load(&rtsan_total_error_count, memory_order_relaxed);
+}
+
+void __rtsan::PrintStatisticsSummary() {
+  ScopedErrorReportLock l;
+  Printf("RealtimeSanitizer exit stats:\n");
+  Printf("    Total error count: %u\n", GetTotalErrorCount());
+}
diff --git a/compiler-rt/lib/rtsan/rtsan_stats.h b/compiler-rt/lib/rtsan/rtsan_stats.h
new file mode 100644
index 0000000..3aa30f6
--- /dev/null
+++ b/compiler-rt/lib/rtsan/rtsan_stats.h
@@ -0,0 +1,21 @@
+//===--- rtsan_stats.h - Realtime Sanitizer ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Part of the RealtimeSanitizer runtime library
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+
+namespace __rtsan {
+
+void IncrementTotalErrorCount();
+
+void PrintStatisticsSummary();
+
+} // namespace __rtsan
diff --git a/compiler-rt/test/rtsan/exit_stats.cpp b/compiler-rt/test/rtsan/exit_stats.cpp
new file mode 100644
index 0000000..b46a0fd
--- /dev/null
+++ b/compiler-rt/test/rtsan/exit_stats.cpp
@@ -0,0 +1,23 @@
+// RUN: %clangxx -fsanitize=realtime %s -o %t
+// RUN: env RTSAN_OPTIONS="halt_on_error=false,print_stats_on_exit=true" %run %t 2>&1 | FileCheck %s
+
+// UNSUPPORTED: ios
+
+// Intent: Ensure exits stats are printed on exit.
+
+#include <unistd.h>
+
+void violation() [[clang::nonblocking]] {
+  const int kNumViolations = 10;
+  for (int i = 0; i < kNumViolations; i++) {
+    usleep(1);
+  }
+}
+
+int main() {
+  violation();
+  return 0;
+}
+
+// CHECK: RealtimeSanitizer exit stats:
+// CHECK-NEXT: Total error count: 10