diff --git a/ext/msgpack/buffer.c b/ext/msgpack/buffer.c
index 9b7fd4e8..d1583702 100644
--- a/ext/msgpack/buffer.c
+++ b/ext/msgpack/buffer.c
@@ -300,7 +300,7 @@ static inline void _msgpack_buffer_add_new_chunk(msgpack_buffer_t* b)
 static inline void _msgpack_buffer_append_reference(msgpack_buffer_t* b, VALUE string)
 {
     VALUE mapped_string;
-    if(ENCODING_GET(string) == msgpack_rb_encindex_ascii8bit && RTEST(rb_obj_frozen_p(string))) {
+    if(ENCODING_GET_INLINED(string) == msgpack_rb_encindex_ascii8bit && RB_OBJ_FROZEN_RAW(string)) {
         mapped_string = string;
     } else {
         mapped_string = rb_str_dup(string);
@@ -309,8 +309,9 @@ static inline void _msgpack_buffer_append_reference(msgpack_buffer_t* b, VALUE s
 
     _msgpack_buffer_add_new_chunk(b);
 
-    char* data = RSTRING_PTR(mapped_string);
-    size_t length = RSTRING_LEN(mapped_string);
+    char* data;
+    size_t length;
+    RSTRING_GETMEM(mapped_string, data, length);
 
     b->tail.first = (char*) data;
     b->tail.last = (char*) data + length;
@@ -330,7 +331,7 @@ void _msgpack_buffer_append_long_string(msgpack_buffer_t* b, VALUE string)
 {
     if(b->io != Qnil) {
         msgpack_buffer_flush(b);
-        if (ENCODING_GET(string) == msgpack_rb_encindex_ascii8bit) {
+        if (ENCODING_GET_INLINED(string) == msgpack_rb_encindex_ascii8bit) {
             rb_funcall(b->io, b->io_write_all_method, 1, string);
         } else {
             msgpack_buffer_append(b, RSTRING_PTR(string), RSTRING_LEN(string));
diff --git a/ext/msgpack/buffer.h b/ext/msgpack/buffer.h
index c2ed46c4..1c8efe54 100644
--- a/ext/msgpack/buffer.h
+++ b/ext/msgpack/buffer.h
@@ -237,13 +237,14 @@ void _msgpack_buffer_append_long_string(msgpack_buffer_t* b, VALUE string);
 
 static inline size_t msgpack_buffer_append_string(msgpack_buffer_t* b, VALUE string)
 {
-    size_t length = RSTRING_LEN(string);
+    size_t length;
+    char *ptr;
+    RSTRING_GETMEM(string, ptr, length);
 
     if(length > b->write_reference_threshold) {
         _msgpack_buffer_append_long_string(b, string);
-
     } else {
-        msgpack_buffer_append(b, RSTRING_PTR(string), length);
+        msgpack_buffer_append(b, ptr, length);
     }
 
     return length;
diff --git a/ext/msgpack/extconf.rb b/ext/msgpack/extconf.rb
index f6ff2d47..b1b921c8 100644
--- a/ext/msgpack/extconf.rb
+++ b/ext/msgpack/extconf.rb
@@ -9,12 +9,13 @@
   "-fvisibility=hidden",
   "-I..",
   "-Wall",
-  "-O3",
   "-std=gnu99"
 ])
-append_cflags(RbConfig::CONFIG["debugflags"]) if RbConfig::CONFIG["debugflags"]
 
-append_cflags("-DRUBY_DEBUG=1") if ENV["MSGPACK_DEBUG"]
+if ENV["MSGPACK_DEBUG"]
+  append_cflags(RbConfig::CONFIG["debugflags"]) if RbConfig::CONFIG["debugflags"]
+  append_cflags("-DRUBY_DEBUG=1")
+end
 
 if RUBY_VERSION.start_with?('3.0.') && RUBY_VERSION <= '3.0.5'
   # https://bugs.ruby-lang.org/issues/18772
diff --git a/ext/msgpack/packer.h b/ext/msgpack/packer.h
index 5061b40c..f3441f5f 100644
--- a/ext/msgpack/packer.h
+++ b/ext/msgpack/packer.h
@@ -25,6 +25,11 @@
 #define MSGPACK_PACKER_IO_FLUSH_THRESHOLD_TO_WRITE_STRING_BODY (1024)
 #endif
 
+#ifndef UNREACHABLE_RETURN
+// Ruby 2.5
+#define UNREACHABLE_RETURN() return
+#endif
+
 struct msgpack_packer_t;
 typedef struct msgpack_packer_t msgpack_packer_t;
 
@@ -404,27 +409,33 @@ static inline bool msgpack_packer_is_utf8_compat_string(VALUE v, int encindex)
 {
     return encindex == msgpack_rb_encindex_utf8
         || encindex == msgpack_rb_encindex_usascii
-        || (rb_enc_asciicompat(rb_enc_from_index(encindex)) && ENC_CODERANGE_ASCIIONLY(v));
+        || ENC_CODERANGE_ASCIIONLY(v);
 }
 
 static inline void msgpack_packer_write_string_value(msgpack_packer_t* pk, VALUE v)
 {
-    /* actual return type of RSTRING_LEN is long */
-    unsigned long len = RSTRING_LEN(v);
-    if(len > 0xffffffffUL) {
-        // TODO rb_eArgError?
-        rb_raise(rb_eArgError, "size of string is too long to pack: %lu bytes should be <= %lu", len, 0xffffffffUL);
+    long len = RSTRING_LEN(v);
+
+    if(RB_UNLIKELY(len > 0xffffffffL)) {
+        rb_raise(rb_eArgError, "size of string is too long to pack: %lu bytes should be <= %ld", len, 0xffffffffL);
+        UNREACHABLE_RETURN();
+    }
+
+    if (RB_UNLIKELY(pk->compatibility_mode)) {
+        msgpack_packer_write_raw_header(pk, (unsigned int)len);
+        msgpack_buffer_append_string(PACKER_BUFFER_(pk), v);
+        return;
     }
 
-    int encindex = ENCODING_GET(v);
-    if(msgpack_packer_is_binary(v, encindex) && !pk->compatibility_mode) {
+    int encindex = ENCODING_GET_INLINED(v);
+    if(msgpack_packer_is_binary(v, encindex)) {
         /* write ASCII-8BIT string using Binary type */
         msgpack_packer_write_bin_header(pk, (unsigned int)len);
         msgpack_buffer_append_string(PACKER_BUFFER_(pk), v);
     } else {
         /* write UTF-8, US-ASCII, or 7bit-safe ascii-compatible string using String type directly */
         /* in compatibility mode, packer packs String values as is */
-        if(!pk->compatibility_mode && !msgpack_packer_is_utf8_compat_string(v, encindex)) {
+        if(RB_UNLIKELY(!msgpack_packer_is_utf8_compat_string(v, encindex))) {
             /* transcode other strings to UTF-8 and write using String type */
             VALUE enc = rb_enc_from_encoding(rb_utf8_encoding()); /* rb_enc_from_encoding_index is not extern */
             v = rb_str_encode(v, enc, 0, Qnil);