PERL_STRLEN_NEW_MIN - increase to multiple of pointer sizes

Major malloc implementations, including the popular dlmalloc
derivatives all return chunks of memory that are a multiple of
the platform's pointer size. Perl's traditional default string
allocation of 10 bytes will almost certainly result in a larger
allocation than requested. Consequently, the interpreter may try
to Renew() an allocation to increase the PV buffer size when it
does not actually need to do so.

This commit increases the default string size to the nearest
pointer multiple. (12 bytes for 32-bit pointers, 16 bytes for
64-bit pointers). This is almost certainly unnecessarily small
for 64-bit platforms, since most common malloc implementations
seem to return 3*pointer size (i.e. 24 bytes) as the smallest
allocation. However, 16 bytes was chosen to prevent an increase
in memory usage in memory-constrained platforms which might have
a smaller minimum memory allocation.

Author: richardleach

perl.h

@@ -1603,11 +1603,20 @@ Use L</UV> to declare variables of the maximum usable size on this platform.
 
 /* Functions like Perl_sv_grow mandate a minimum string size.
  * This was 10 bytes for a long time, the rationale for which seems lost
- * to the mists of time. However, since this does not correlate to what
- * modern malloc implementations will actually return, it can be revised
- * to be more appropriate. */
+ * to the mists of time. However, this does not correlate to what modern
+ * malloc implementations will actually return, in particular the fact
+ * that chunks are almost certainly some multiple of pointer size. The
+ * default has therefore been revised to a more useful approximation.
+ * Notes: The following is specifically conservative for 64 bit, since
+ * most dlmalloc derivatives seem to serve a 3xPTRSIZE minimum chunk,
+ * so the below perhaps should be:
+ *     ((PTRSIZE == 4) ? 12 : 24)
+ * Configure probes for malloc_good_size, malloc_actual_size etc.
+ * could be revised to record the actual minimum chunk size, to which
+ * PERL_STRLEN_NEW_MIN could then be set.
+ */
 #ifndef PERL_STRLEN_NEW_MIN
-#define PERL_STRLEN_NEW_MIN 10
+#define PERL_STRLEN_NEW_MIN ((PTRSIZE == 4) ? 12 : 16)
 #endif
 
 /* Round all values passed to malloc up, by default to a multiple of