Add a realpath cache to reduce number of syscalls.

Number of lstat and stat syscalls for each 'require'd file is doubled,
because rb_realpath_internal is called from two places with the same
arguments in require_internal; once for checking the realpaths cache,
and once in load_iseq_eval when iseq is not found.

Introduce rb_realpath_internal_cached function to reuse the realpath_map
cache which memoizes rb_realpath_internal function, leading to less
syscalls and increased startup performance depending on the cost of the syscalls
in a particular environment.

load.c

@@ -689,6 +689,19 @@ rb_provide(const char *feature)
 
 NORETURN(static void load_failed(VALUE));
 
+static inline VALUE
+realpath_internal_cached(VALUE hash, VALUE path)
+{
+    VALUE ret = rb_hash_aref(hash, path);
+    if(RTEST(ret)) {
+        return ret;
+    }
+
+    VALUE realpath = rb_realpath_internal(Qnil, path, 1);
+    rb_hash_aset(hash, rb_fstring(path), rb_fstring(realpath));
+    return realpath;
+}
+
 static inline void
 load_iseq_eval(rb_execution_context_t *ec, VALUE fname)
 {
@@ -701,8 +714,12 @@ load_iseq_eval(rb_execution_context_t *ec, VALUE fname)
         VALUE parser = rb_parser_new();
         rb_parser_set_context(parser, NULL, FALSE);
         ast = (rb_ast_t *)rb_parser_load_file(parser, fname);
+
+        rb_thread_t *th = rb_ec_thread_ptr(ec);
+        VALUE realpath_map = get_loaded_features_realpath_map(th->vm);
+
         iseq = rb_iseq_new_top(&ast->body, rb_fstring_lit("<top (required)>"),
-                               fname, rb_realpath_internal(Qnil, fname, 1), NULL);
+                               fname, realpath_internal_cached(realpath_map, fname), NULL);
         rb_ast_dispose(ast);
         rb_vm_pop_frame(ec);
         RB_GC_GUARD(v);
@@ -1208,7 +1225,7 @@ require_internal(rb_execution_context_t *ec, VALUE fname, int exception, bool wa
                 result = TAG_RETURN;
             }
             else if (RTEST(rb_hash_aref(realpaths,
-                                        realpath = rb_realpath_internal(Qnil, path, 1)))) {
+                                        realpath = realpath_internal_cached(realpath_map, path)))) {
                 result = 0;
             }
             else {
@@ -1268,7 +1285,6 @@ require_internal(rb_execution_context_t *ec, VALUE fname, int exception, bool wa
         if (real) {
             real = rb_fstring(real);
             rb_hash_aset(realpaths, real, Qtrue);
-            rb_hash_aset(realpath_map, path, real);
         }
     }
     ec->errinfo = saved.errinfo;