Create BYTES_REMAINING_IN_WORD()

This macro encapsulates the task of finding how far until the next word
boundary the passed-in address is.

There are several places that could use this, but instead of converting
use this in those places, the next commit will create macros that depend
on this one and those places will instead convert to use those other new
macros.

Author: khwilliamson

inline.h

@@ -1459,14 +1459,21 @@ Perl_valid_utf8_to_uv(const U8 *s, STRLEN *retlen)
 #  define PERL_WORDSIZE            sizeof(PERL_UINTMAX_T)
 #  define PERL_WORD_BOUNDARY_MASK (PERL_WORDSIZE - 1)
 
-/* Evaluates to 0 if 'x' is at a word boundary; otherwise evaluates to 1, by
- * or'ing together the lowest bits of 'x'.  Hopefully the final term gets
- * optimized out completely on a 32-bit system, and its mask gets optimized out
- * on a 64-bit system */
-#  define PERL_IS_SUBWORD_ADDR(x) (1 & (       PTR2nat(x)                     \
-                                      |   (  PTR2nat(x) >> 1)                 \
-                                      | ( ( (PTR2nat(x)                       \
-                                           & PERL_WORD_BOUNDARY_MASK) >> 2))))
+/* Given an address of a byte 'x', how many bytes away is that address to the
+ * following closest full word boundary. */
+#  define BYTES_REMAINING_IN_WORD(x)                                        \
+              ( (PERL_WORDSIZE - (PTR2nat(x) & PERL_WORD_BOUNDARY_MASK))    \
+               & PERL_WORD_BOUNDARY_MASK)
+/* For example, consider two addresses in an 8 byte word size (the dots are
+ * don't cares):
+ *      0b...............010                0b...............000
+ *      ((8 - (0b1101010 & 0x7)) & 0x7)     ((8 - (0b1101000 & 0x7)) & 0x7)
+ *      ((8 - 0b10) & 0x7)                  ((8 - 0) & 0x7)
+ *      (6 & 0x7)                           (8 & 0x7)
+ *      6                                   0                              */
+
+/* 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)
 
 /*
 =for apidoc      is_utf8_invariant_string
@@ -2116,6 +2123,7 @@ S_variant_under_utf8_count(const U8* const s, const U8* const e)
 #  undef PERL_COUNT_MULTIPLIER
 #  undef PERL_WORD_BOUNDARY_MASK
 #  undef PERL_VARIANTS_WORD_MASK
+#  undef BYTES_REMAINING_IN_WORD
 #endif
 
 #define is_utf8_string(s, len)  is_utf8_string_loclen(s, len, NULL, NULL)