i#2037 re-walk-maps: refactor to share main walk with re-walk (#2887)

Refactors find_executable_vm_areas() to share its map entry skipping with
the re-takeover re-walk from b06a702, but not its module list or
executable area updates.  This entry skipping for vmheap turns out to make
a big performance difference when attaching.

Removes individual updates to memcache for entries inside vmheap which were
already bulk-added for the find_executable_vm_areas() walk.

Issue: #2037

core/os_shared.h

@@ -761,7 +761,9 @@ app_pc get_dynamorio_dll_start(void);
 app_pc get_dynamorio_dll_preferred_base(void);
 
 bool is_in_dynamo_dll(app_pc pc);
+/* Returns the number of separate regions added to the dynamo vm areas list. */
 int find_dynamo_library_vm_areas(void);
+/* Returns the number of executable regions found in the address space. */
 int find_executable_vm_areas(void);
 
 /* all_memory_areas is !HAVE_MEMINFO-only: nop elsewhere */

core/unix/memcache.c

@@ -587,11 +587,7 @@ memcache_update_all_from_os(void)
     memcache_lock();
     /* We clear the entire cache to avoid false positive queries. */
     vmvector_reset_vector(GLOBAL_DCONTEXT, all_memory_areas);
-    while (memquery_iterator_next(&iter)) {
-        /* We do a heavyweight overlap check with everything. */
-        memcache_update_locked(iter.vm_start, iter.vm_end, iter.prot,
-                               -1, false/*!exists*/);
-    }
+    os_walk_address_space(&iter, false);
     memcache_unlock();
     memquery_iterator_stop(&iter);
 }