[lsan] Add a stress test.

[kcc]

LGTM

lib/lsan/lit_tests/TestCases/high_allocator_contention.cc

+// A benchmark that executes malloc/free pairs in parallel.
+// Usage: ./a.out number_of_threads total_number_of_allocations
+// RUN: %clangxx_lsan %s -o %t
+// RUN: %t 5 1000000 2>&1
+#include <assert.h>
+#include <pthread.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+int num_threads;
+int total_num_alloc;
+
+pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
+pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
+bool go = false;
+
+void *thread_fun(void *) {
+  pthread_mutex_lock(&mutex);
+  while (!go) pthread_cond_wait(&cond, &mutex);
+  pthread_mutex_unlock(&mutex);
+  for (int i = 0; i < total_num_alloc / num_threads; i++) {
+    void *p = malloc(10);
+    __asm__ __volatile__("" : : "r"(p) : "memory");
+    free((void *)p);
+  }
+  return 0;
+}
+
+int main(int argc, char** argv) {
+  assert(argc == 3);
+  num_threads = atoi(argv[1]);
+  assert(num_threads > 0);
+  total_num_alloc = atoi(argv[2]);
+  assert(total_num_alloc > 0);
+  pthread_t tid[num_threads];
+  printf("%d threads, %d allocations in each\n", num_threads,
+         total_num_alloc / num_threads);
+  for (int i = 0; i < num_threads; i++)
+    pthread_create(&tid[i], 0, thread_fun, 0);
+  pthread_mutex_lock(&mutex);
+  go = true;
+  pthread_cond_broadcast(&cond);
+  pthread_mutex_unlock(&mutex);
+  for (int i = 0; i < num_threads; i++) pthread_join(tid[i], 0);
+  return 0;
+}