Add Perl_expected_size stub macro

Passed a number of bytes, this macro will return:
* PERL_STRLEN_NEW_MIN if that is larger than the argument value
* the argument value rounded up to PTRSIZE

The intention is to later convert `Perl_expected_size` into a
function that has some knowledge of the underlying malloc library's
behaviour and can return values more reflective of the actual
useable allocation size for the specified number of bytes.

This can aid in reducing unnecessary calls to realloc().

Author: richardleach

embed.fnc

@@ -1157,6 +1157,7 @@ AOdp	|SV *	|eval_pv	|NN const char *p			\
 				|I32 croak_on_error
 AOdp	|SSize_t|eval_sv	|NN SV *sv				\
 				|I32 flags
+EMTpx	|Size_t |expected_size	|UV size
 ATdmp	|bool	|extended_utf8_to_uv					\
 				|NN const U8 * const s			\
 				|NN const U8 * const e			\

perl.h

@@ -1686,6 +1686,23 @@ Use L</UV> to declare variables of the maximum usable size on this platform.
 #  define PERL_STRLEN_EXPAND_SHIFT 2
 #endif
 
+/* "expected_size" is a functional stub for building on in the future.
+ * The intention is to pass it a number of bytes, prior to using that
+ * number in a call to something like malloc() or realloc(), and a best-
+ * guess at the size to be allocated will be returned. ("Best-guess"
+ * may vary by platform and malloc implementation.)
+ * This will be useful to (1) grow strings (or anything else) in a way
+ * that results in SvLEN more accurately reflecting the usable space
+ * that has been allocated, and (2) we don't try to shrink an
+ * allocation that won't actually shrink in practice.
+ * Right now, this macro just rounds up a given number to the nearest
+ * multiple of PTRSIZE, for a minimum of PERL_STRLEN_NEW_MIN. This is
+ * not entirely useless, just not terribly accurate.
+ */
+#define expected_size(n) ( ((n) > PERL_STRLEN_NEW_MIN)               \
+                            ? (((n) + PTRSIZE - 1) & ~(PTRSIZE - 1)) \
+                            : PERL_STRLEN_NEW_MIN )
+
 /* This use of offsetof() requires /Zc:offsetof- for VS2017 (and presumably
  * onwards) when building Socket.xs, but we can just use a different definition
  * for STRUCT_OFFSET instead. */