Index: compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
===================================================================
--- compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
+++ compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
@@ -15,28 +15,29 @@
 
 #if SANITIZER_POSIX
 
-#include "sanitizer_common.h"
-#include "sanitizer_flags.h"
-#include "sanitizer_platform_limits_netbsd.h"
-#include "sanitizer_platform_limits_posix.h"
-#include "sanitizer_platform_limits_solaris.h"
-#include "sanitizer_posix.h"
-#include "sanitizer_procmaps.h"
-
-#include <errno.h>
-#include <fcntl.h>
-#include <pthread.h>
-#include <signal.h>
-#include <stdlib.h>
-#include <sys/mman.h>
-#include <sys/resource.h>
-#include <sys/stat.h>
-#include <sys/time.h>
-#include <sys/types.h>
-#include <sys/wait.h>
-#include <unistd.h>
-
-#if SANITIZER_FREEBSD
+#  include <errno.h>
+#  include <fcntl.h>
+#  include <pthread.h>
+#  include <signal.h>
+#  include <stdlib.h>
+#  include <sys/mman.h>
+#  include <sys/resource.h>
+#  include <sys/stat.h>
+#  include <sys/time.h>
+#  include <sys/types.h>
+#  include <sys/wait.h>
+#  include <unistd.h>
+
+#  include "sanitizer_atomic.h"
+#  include "sanitizer_common.h"
+#  include "sanitizer_flags.h"
+#  include "sanitizer_platform_limits_netbsd.h"
+#  include "sanitizer_platform_limits_posix.h"
+#  include "sanitizer_platform_limits_solaris.h"
+#  include "sanitizer_posix.h"
+#  include "sanitizer_procmaps.h"
+
+#  if SANITIZER_FREEBSD
 // The MAP_NORESERVE define has been removed in FreeBSD 11.x, and even before
 // that, it was never implemented.  So just define it to zero.
 #undef MAP_NORESERVE
@@ -166,9 +167,13 @@
 #if !SANITIZER_GO
 // TODO(glider): different tools may require different altstack size.
 static uptr GetAltStackSize() {
-  // SIGSTKSZ is not enough.
-  static const uptr kAltStackSize = SIGSTKSZ * 4;
-  return kAltStackSize;
+  static atomic_uintptr_t kAltStackSize{0};
+  uptr ret = atomic_load(&kAltStackSize, memory_order_relaxed);
+  if (ret == 0) {
+    // SIGSTKSZ is not enough.
+    atomic_store(&kAltStackSize, SIGSTKSZ * 4, memory_order_relaxed);
+  }
+  return ret;
 }
 
 void SetAlternateSignalStack() {