Create WORTH_PER_WORD_LOOP(), WORTH_PER_WORD_LOOP_BINMODE()

There are several places in the perl core that, for performance,  use
word-at-a-time operations on byte data when the data to be processed is
long enough to overcome the extra setup overhead required.

The code that does this is not immediately obvious, and is currently
repeated at each such place.

This macro creates two macros that encapsulate this logic, making each
place that uses them easier to read.

One macro is for data that isn't dependent on the character set.  The
other is for character data.  EBCDIC data is not suitable for per-word
operation, so the this macro always returns false on an EBCDIC platform.
This allows for the removal of some EBCDIC #ifdefs in our code base.

Author: khwilliamson

inline.h

@@ -1475,6 +1475,48 @@ Perl_valid_utf8_to_uv(const U8 *s, STRLEN *retlen)
 /* Evaluates to 0 if 'x' is at a word boundary; otherwise evaluates to 1 */
 #  define PERL_IS_SUBWORD_ADDR(x) (BYTES_REMAINING_IN_WORD(x) != 0)
 
+/* Some tasks that are byte-oriented can be done as well a full word-at-a-time,
+ * running 8 times faster on an 8-byte word, for example.  But there is
+ * generally extra setup required to do this, and byte-at-a-time must be used
+ * anyway to get to the next word boundary.  This macro calculates whether the
+ * trade-off is worth doing.  If not, it returns NULL; if so, it returns a
+ * pointer to the first byte of the next word.  Code using this is typically
+ * structured like:
+ *      U8 * next_word_boundary = WORTH_PER_LOOP()
+ *      if (next_word_boundary) {
+ *          loop per-byte until next_word_boundary
+ *          loop per-word until less than a word left before upper boundary
+ *      }
+ *      loop per-byte until reach final boundary
+ *
+ * 's' is the current position in the string
+ * 'e' is the upper string bound
+ * 'full_words_needed' is the caller's determination of where to make the
+ *      trade-off between per-byte and per-word.  Only if the number of words
+ *      in the input string is at least this many, does the macro return
+ *      non-NULL.
+ *
+ * Because of EBCDIC, there are two forms of this macro.
+ * WORTH_PER_WORD_LOOP_BINMODE() is for use when the data being examined is
+ * not dependent on the character set.  The more usual form is plain
+ * WORTH_PER_WORD_LOOP() for character data.  Because EBCDIC needs an extra
+ * transformation, per-word operations are not appropriate on it, so the macro
+ * always returns NULL, meaning don't use a per-word loop on an EBCDIC
+ * platform. */
+#  define WORTH_PER_WORD_LOOP_BINMODE(s, e, full_words_needed)      \
+       /* Note multiple evaluations of 's' */                       \
+       ( ( ( (s) + BYTES_REMAINING_IN_WORD(s)                       \
+                 + (full_words_needed) * PERL_WORDSIZE) < (e) )     \
+        ? ((s) + BYTES_REMAINING_IN_WORD(s))                        \
+        : NULL)
+
+#  ifdef EBCDIC
+#    define WORTH_PER_WORD_LOOP(s, e, f)  NULL
+#  else
+#    define WORTH_PER_WORD_LOOP(s, e, f)                \
+        WORTH_PER_WORD_LOOP_BINMODE(s, e, f)
+#  endif
+
 /*
 =for apidoc      is_utf8_invariant_string
 =for apidoc_item is_utf8_invariant_string_loc