[ruby/erb] html_escape: Avoid buffer allocation for strings with no

escapable character
(ruby/erb#87)

This change improves reduces allocations and makes `html_escape` ~35% faster in
a benchmark with escaped strings taken from the `test_html_escape` test in
`test/test_erb.rb`.

- Perform buffer allocation on first instance of escapable character.
- Instead of copying characters one at a time, copy unescaped segments using
  `memcpy`.

ruby/erb@aa482890fe

Author: noteflakes

ext/erb/escape/escape.c

@@ -39,29 +39,44 @@ static VALUE
 optimized_escape_html(VALUE str)
 {
     VALUE vbuf;
-    char *buf = ALLOCV_N(char, vbuf, escaped_length(str));
+    char *buf = NULL;
     const char *cstr = RSTRING_PTR(str);
     const char *end = cstr + RSTRING_LEN(str);
 
-    char *dest = buf;
+    const char *segment_start = cstr;
+    char *dest = NULL;
     while (cstr < end) {
         const unsigned char c = *cstr++;
         uint8_t len = html_escape_table[c].len;
         if (len) {
+            uint16_t segment_len = cstr - segment_start - 1;
+            if (!buf) {
+                buf = ALLOCV_N(char, vbuf, escaped_length(str));
+                dest = buf;
+            }
+            if (segment_len) {
+                memcpy(dest, segment_start, segment_len);
+                dest += segment_len;
+            }
+            segment_start = cstr;
             memcpy(dest, html_escape_table[c].str, len);
             dest += len;
         }
-        else {
-            *dest++ = c;
+    }
+    if (buf) {
+        uint16_t segment_len = cstr - segment_start;
+        if (segment_len) {
+            memcpy(dest, segment_start, segment_len);
+            dest += segment_len;
         }
     }
 
     VALUE escaped = str;
-    if (RSTRING_LEN(str) < (dest - buf)) {
+    if (buf) {
         escaped = rb_str_new(buf, dest - buf);
         preserve_original_state(str, escaped);
+        ALLOCV_END(vbuf);
     }
-    ALLOCV_END(vbuf);
     return escaped;
 }