Gather heap page size conditions combination

When similar combination of conditions are separated in two places, it
is harder to make sure the conditional blocks match each other,

gc.c

@@ -886,26 +886,46 @@ enum {
 # define INCREMENTAL_MARK_STEP_ALLOCATIONS 500
 #endif
 
-#ifdef HAVE_MMAP
+#undef INIT_HEAP_PAGE_ALLOC_USE_MMAP
+/* Must define either HEAP_PAGE_ALLOC_USE_MMAP or
+ * INIT_HEAP_PAGE_ALLOC_USE_MMAP. */
+
+#ifndef HAVE_MMAP
+/* We can't use mmap of course, if it is not available. */
+static const bool HEAP_PAGE_ALLOC_USE_MMAP = false;
+
+#elif defined(__wasm__)
 /* wasmtime does not have proper support for mmap.
  * See https://github.com/bytecodealliance/wasmtime/blob/main/docs/WASI-rationale.md#why-no-mmap-and-friends
  */
-# if defined(__wasm__)
 static const bool HEAP_PAGE_ALLOC_USE_MMAP = false;
-# elif HAVE_CONST_PAGE_SIZE
-/* If we have the HEAP_PAGE and it is a constant, then we can directly use it. */
+
+#elif HAVE_CONST_PAGE_SIZE
+/* If we have the PAGE_SIZE and it is a constant, then we can directly use it. */
 static const bool HEAP_PAGE_ALLOC_USE_MMAP = (PAGE_SIZE <= HEAP_PAGE_SIZE);
-# elif defined(PAGE_MAX_SIZE) && (PAGE_MAX_SIZE <= HEAP_PAGE_SIZE)
-/* PAGE_SIZE <= HEAP_PAGE_SIZE */
+
+#elif defined(PAGE_MAX_SIZE) && (PAGE_MAX_SIZE <= HEAP_PAGE_SIZE)
+/* If we can use the maximum page size. */
 static const bool HEAP_PAGE_ALLOC_USE_MMAP = true;
-# else
-/* Otherwise, fall back to determining if we can use mmap during runtime. */
-#  define HEAP_PAGE_ALLOC_USE_MMAP (heap_page_alloc_use_mmap != false)
+
+#elif defined(PAGE_SIZE)
+/* If the PAGE_SIZE macro can be used dynamically. */
+# define INIT_HEAP_PAGE_ALLOC_USE_MMAP (PAGE_SIZE <= HEAP_PAGE_SIZE)
+
+#elif defined(HAVE_SYSCONF) && defined(_SC_PAGE_SIZE)
+/* If we can use sysconf to determine the page size. */
+# define INIT_HEAP_PAGE_ALLOC_USE_MMAP (sysconf(_SC_PAGE_SIZE) <= HEAP_PAGE_SIZE)
+
+#else
+/* Otherwise we can't determine the system page size, so don't use mmap. */
+static const bool HEAP_PAGE_ALLOC_USE_MMAP = false;
+#endif
+
+#ifdef INIT_HEAP_PAGE_ALLOC_USE_MMAP
+/* We can determine the system page size at runtime. */
+# define HEAP_PAGE_ALLOC_USE_MMAP (heap_page_alloc_use_mmap != false)
 
 static bool heap_page_alloc_use_mmap;
-# endif
-#elif !defined(__MINGW32__) && !defined(_WIN32)
-static const bool HEAP_PAGE_ALLOC_USE_MMAP = false;
 #endif
 
 struct heap_page {
@@ -1970,19 +1990,17 @@ heap_page_body_free(struct heap_page_body *page_body)
 {
     GC_ASSERT((uintptr_t)page_body % HEAP_PAGE_ALIGN == 0);
 
-#ifdef HAVE_MMAP
     if (HEAP_PAGE_ALLOC_USE_MMAP) {
+#ifdef HAVE_MMAP
         GC_ASSERT(HEAP_PAGE_SIZE % sysconf(_SC_PAGE_SIZE) == 0);
         if (munmap(page_body, HEAP_PAGE_SIZE)) {
             rb_bug("heap_page_body_free: munmap failed");
         }
+#endif
     }
     else {
         rb_aligned_free(page_body, HEAP_PAGE_SIZE);
     }
-#else
-    rb_aligned_free(page_body, HEAP_PAGE_SIZE);
-#endif
 }
 
 static void
@@ -2037,8 +2055,8 @@ heap_page_body_allocate(void)
 {
     struct heap_page_body *page_body;
 
-#ifdef HAVE_MMAP
     if (HEAP_PAGE_ALLOC_USE_MMAP) {
+#ifdef HAVE_MMAP
         GC_ASSERT(HEAP_PAGE_ALIGN % sysconf(_SC_PAGE_SIZE) == 0);
 
         char *ptr = mmap(NULL, HEAP_PAGE_ALIGN + HEAP_PAGE_SIZE,
@@ -2069,13 +2087,11 @@ heap_page_body_allocate(void)
         }
 
         page_body = (struct heap_page_body *)aligned;
+#endif
     }
     else {
         page_body = rb_aligned_malloc(HEAP_PAGE_ALIGN, HEAP_PAGE_SIZE);
     }
-#else
-    page_body = rb_aligned_malloc(HEAP_PAGE_ALIGN, HEAP_PAGE_SIZE);
-#endif
 
     GC_ASSERT((uintptr_t)page_body % HEAP_PAGE_ALIGN == 0);
 
@@ -3696,18 +3712,9 @@ Init_heap(void)
 {
     rb_objspace_t *objspace = &rb_objspace;
 
-#if defined(HAVE_MMAP) && !defined(__wasm__) && !HAVE_CONST_PAGE_SIZE && !defined(PAGE_MAX_SIZE)
+#if defined(INIT_HEAP_PAGE_ALLOC_USE_MMAP)
     /* Need to determine if we can use mmap at runtime. */
-# ifdef PAGE_SIZE
-    /* If the PAGE_SIZE macro can be used. */
-    heap_page_alloc_use_mmap = PAGE_SIZE <= HEAP_PAGE_SIZE;
-# elif defined(HAVE_SYSCONF) && defined(_SC_PAGE_SIZE)
-    /* If we can use sysconf to determine the page size. */
-    heap_page_alloc_use_mmap = sysconf(_SC_PAGE_SIZE) <= HEAP_PAGE_SIZE;
-# else
-    /* Otherwise we can't determine the system page size, so don't use mmap. */
-    heap_page_alloc_use_mmap = FALSE;
-# endif
+    heap_page_alloc_use_mmap = INIT_HEAP_PAGE_ALLOC_USE_MMAP;
 #endif
 
     objspace->next_object_id = INT2FIX(OBJ_ID_INITIAL);