Index: sys/amd64/amd64/exec_machdep.c
===================================================================
--- sys/amd64/amd64/exec_machdep.c
+++ sys/amd64/amd64/exec_machdep.c
@@ -95,6 +95,13 @@
 #include <machine/specialreg.h>
 #include <machine/trap.h>
 
+_Static_assert(sizeof(mcontext_t) == 800, "mcontext_t bogusly changed size");
+_Static_assert(sizeof(ucontext_t) == 880, "mcontext_t bogusly changed size");
+#ifdef COMPAT_FREEBSD32
+_Static_assert(sizeof(mcontext32_t) == 640, "mcontext32_t bogusly changed size");
+_Static_assert(sizeof(ucontext32_t) == 704, "ucontext32_t bogusly changed size");
+#endif /* COMPAT_FREEBSD32 */
+
 /*
  * Send an interrupt to process.
  *