diff --git a/libc/private/bionic_constants.h b/libc/private/bionic_constants.h
index 09294b6..d7f4474 100644
--- a/libc/private/bionic_constants.h
+++ b/libc/private/bionic_constants.h
@@ -14,18 +14,18 @@
  * limitations under the License.
  */
 
-#ifndef _BIONIC_CONSTANTS_H_
-#define _BIONIC_CONSTANTS_H_
+#pragma once
 
 #define NS_PER_S 1000000000
 
-// Size of the shadow call stack. This must be a power of 2.
+// Size of the shadow call stack. This can be small because these stacks only
+// contain return addresses. This must be a power of 2 so the mask trick works.
+// See the SCS commentary in pthread_internal.h for more detail.
 #define SCS_SIZE (8 * 1024)
+#define SCS_MASK (SCS_SIZE - 1)
 
 // The shadow call stack is allocated at an aligned address within a guard region of this size. The
 // guard region must be large enough that we can allocate an SCS_SIZE-aligned SCS while ensuring
 // that there is at least one guard page after the SCS so that a stack overflow results in a SIGSEGV
 // instead of corrupting the allocation that comes after it.
 #define SCS_GUARD_REGION_SIZE (16 * 1024 * 1024)
-
-#endif // _BIONIC_CONSTANTS_H_