diff --git a/include/private/gc_priv.h b/include/private/gc_priv.h
index d9765caae..5063e22c1 100644
--- a/include/private/gc_priv.h
+++ b/include/private/gc_priv.h
@@ -1943,8 +1943,28 @@ GC_INNER void GC_push_all_register_sections(
 
 /* Important internal collector routines.       */
 
+/* Return the current stack pointer, approximately. */
 GC_INNER ptr_t GC_approx_sp(void);
 
+/* Same as GC_approx_sp() but a macro.  sp_ptr should be a pointer  */
+/* to a local variable of volatile ptr_t type.                      */
+#if (defined(E2K) && defined(__clang__)         \
+     || (defined(S390) && __clang_major__ < 8)) \
+    && !defined(CPPCHECK)
+/* Workaround some bugs in clang:                                   */
+/* "undefined reference to llvm.frameaddress" error (clang-9/e2k);  */
+/* a crash in SystemZTargetLowering of libLLVM-3.8 (s390).          */
+#  define APPROX_SP(sp_ptr) (void)(*(sp_ptr) = (ptr_t)(sp_ptr))
+#elif defined(CPPCHECK)                          \
+    || (__GNUC__ >= 4 /* GC_GNUC_PREREQ(4, 0) */ \
+        && !defined(STACK_NOT_SCANNED))
+/* TODO: Use GC_GNUC_PREREQ after fixing a bug in cppcheck. */
+#  define APPROX_SP(sp_ptr) \
+    (void)(*(sp_ptr) = (ptr_t)__builtin_frame_address(0))
+#else
+#  define APPROX_SP(sp_ptr) (void)(*(sp_ptr) = (ptr_t)(sp_ptr))
+#endif
+
 GC_INNER GC_bool GC_should_collect(void);
 
 /* Get the next block whose address is at least h.  Returned block is   */
diff --git a/mark_rts.c b/mark_rts.c
index 0456b2968..d08b34569 100644
--- a/mark_rts.c
+++ b/mark_rts.c
@@ -543,21 +543,7 @@ GC_approx_sp(void)
   /* This also forces stack to grow if necessary.  Otherwise the      */
   /* later accesses might cause the kernel to think we are doing      */
   /* something wrong.                                                 */
-#if ((defined(E2K) && defined(__clang__))         \
-     || (defined(S390) && (__clang_major__ < 8))) \
-    && !defined(CPPCHECK)
-  /* Workaround some bugs in clang:                                   */
-  /* "undefined reference to llvm.frameaddress" error (clang-9/e2k);  */
-  /* a crash in SystemZTargetLowering of libLLVM-3.8 (S390).          */
-  sp = (ptr_t)(&sp);
-#elif defined(CPPCHECK)                          \
-    || (__GNUC__ >= 4 /* GC_GNUC_PREREQ(4, 0) */ \
-        && !defined(STACK_NOT_SCANNED))
-  /* TODO: Use GC_GNUC_PREREQ after fixing a bug in cppcheck. */
-  sp = (ptr_t)__builtin_frame_address(0);
-#else
-  sp = (ptr_t)(&sp);
-#endif
+  APPROX_SP(&sp);
   return (/* no volatile */ ptr_t)sp;
 }
 
diff --git a/tools/setjmp_t.c b/tools/setjmp_t.c
index 73548feec..6f228d9a1 100644
--- a/tools/setjmp_t.c
+++ b/tools/setjmp_t.c
@@ -68,13 +68,10 @@ GC_ATTR_NOINLINE
 static word
 nested_sp(void)
 {
-#if defined(CPPCHECK) || GC_GNUC_PREREQ(4, 0)
-  return ADDR(__builtin_frame_address(0));
-#else
-  volatile word sp;
-  sp = ADDR(&sp);
-  return sp;
-#endif
+  volatile ptr_t sp;
+
+  APPROX_SP(&sp);
+  return ADDR(sp);
 }
 
 /* To prevent nested_sp inlining. */
@@ -91,7 +88,7 @@ const char *a_str = "a";
 int
 main(void)
 {
-  volatile word sp;
+  volatile ptr_t sp;
   unsigned ps = GETPAGESIZE();
 #ifndef WASI
   JMP_BUF b;
@@ -104,20 +101,21 @@ main(void)
   static volatile int y = 0;
 #endif
 
-  sp = ADDR(&sp);
+  APPROX_SP(&sp);
   printf("This appears to be a %s running %s\n", MACH_TYPE, OS_TYPE);
 #if defined(CPPCHECK)
   (void)nested_sp(); /* to workaround a bug in cppcheck */
 #endif
-  if (nested_sp_fn() < sp) {
+  if (nested_sp_fn() < ADDR(sp)) {
     printf("Stack appears to grow down, which is the default.\n");
     printf("A good guess for STACKBOTTOM on this machine is 0x%lx.\n",
-           ((unsigned long)sp + ps) & ~(unsigned long)(ps - 1));
+           ((unsigned long)ADDR(sp) + ps) & ~(unsigned long)(ps - 1));
   } else {
     printf("Stack appears to grow up.\n");
     printf("Define STACK_GROWS_UP in gc_priv.h\n");
+    /* Note: sp is rounded down. */
     printf("A good guess for STACKBOTTOM on this machine is 0x%lx.\n",
-           (unsigned long)sp & ~(unsigned long)(ps - 1)); /* round down */
+           (unsigned long)ADDR(sp) & ~(unsigned long)(ps - 1));
   }
   printf("Note that this may vary between machines of ostensibly\n");
   printf("the same architecture (e.g. Sun 3/50s and 3/80s).\n");