Cache file search results

preprocess.c

@@ -684,11 +684,18 @@ char *search_include_paths(char *filename) {
   if (filename[0] == '/')
     return filename;
 
+  static HashMap cache;
+  char *cached = hashmap_get(&cache, filename);
+  if (cached)
+    return cached;
+
   // Search a file from the include paths.
   for (int i = 0; i < include_paths.len; i++) {
     char *path = format("%s/%s", include_paths.data[i], filename);
-    if (file_exists(path))
-      return path;
+    if (!file_exists(path))
+      continue;
+    hashmap_put(&cache, filename, path);
+    return path;
   }
   return NULL;
 }