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
@@ -166,8 +166,11 @@
 #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;
+  static uptr kAltStackSize = 0;
+  if (__sync_bool_compare_and_swap(&kAltStackSize, 0, 1)) {
+    // SIGSTKSZ is not enough.
+    kAltStackSize = SIGSTKSZ * 4;
+  }
   return kAltStackSize;
 }