The reason a change was made:

Hm. Pre-5.10, every array that didn't use $#array, wasted 1 pointer.

Post 5.10, every array uses that pointer to point to a 12 or 24-byte block of memory whether it is ever used or not.

And the very act of attempting to check to see if there is any magic attached, causes there to be magic attached.

That makes no sense at all to me.

Now the only way D::S can avoid causing the magic to be attached is to either not bother looking to see if there is any; or to dereference the magic pointer directly...

A microscopic optimisation attempt that causes a macroscopic [sic] pessimisation. Autovivifying magic just because someone wants to know if there is any is...

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

RIP PCW It is as I've been saying!(Audio until 20090817)

Commits 02d85cc37a4acecafdc2f0b45640b03cd1f4ac71 and 28c5b5bcd7f52e6b2219508a1066cd0ccc8dd19a fix this when $#array is used as a rvalue. You'll see these changes in 5.11.2 (next month's dev release) and 5.12 (early to mid 2010)

Any operation that queries the size does the same thing:

>perl -MDevel::Peek -le"my @a = qw(a b c); print @a-1; Dump \@a,1"
2
SV = RV(0x238c60) at 0x238c54
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x182a22c
  SV = PVAV(0x239a4c) at 0x182a22c
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x182521c
    FILL = 2
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
[download]

If it's a bug, it's not attributable to Devel::Size.

If it's a bug that reading $#a adds magic to @a, then that's not attributable to D::S.

The bug in D::S is that it should be using the method that doesn't add magic.

I see you removed the "rvalue" reference...

Anyway, the code of D::S does this:

   if (AvMAX(thing) != -1) {
      /* an array with 10 slots has AvMax() set to 9 - te 2007-04-22 *
+/
      total_size += sizeof(SV *) * (AvMAX(thing) + 1);
[download]

And AvMAX is defined as:

#define AvMAX(av)    ((XPVAV*)  SvANY(av))->xav_max
[download]

Ie. It simply gets the value from a struct member.

It also references AvALLOC & AvARYLEN, but neither of those does anything with magic either:

#define AvARRAY(av)    ((av)->sv_u.svu_array)
#define AvALLOC(av)    (*((SV***)&((XPVAV*)  SvANY(av))->xav_alloc))
#define AvMAX(av)    ((XPVAV*)  SvANY(av))->xav_max
#define AvFILLp(av)    ((XPVAV*)  SvANY(av))->xav_fill
#define AvARYLEN(av)    (*Perl_av_arylen_p(aTHX_ MUTABLE_AV(av)))
[download]

The only "method" that touches magic is AvFILL:

#define AvFILL(av)    ((SvRMAGICAL((const SV *) (av))) \
             ? mg_size(MUTABLE_SV(av)) : AvFILLp(av))
[download]

And AvFILL doesn't appear anywhere in the D::S source.

So, I say again, the bug does not lie with D::S.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

RIP PCW It is as I've been saying!(Audio until 20090817)

D::S also uses AvARRAY, AvALLOC and AvARYLEN.

use strict;
use warnings;

use Devel::Peek qw( Dump );

use Inline C => <<'__EOI__';
void call_AvMAX(AV* av)    { AvMAX(av);    }
void call_AvARRAY(AV* av)  { AvARRAY(av);  }
void call_AvALLOC(AV* av)  { AvALLOC(av);  }
void call_AvARYLEN(AV* av) { AvARYLEN(av); }
__EOI__

my @a = qw( a b c );  print("After init:\n");        Dump(\@a, 1);
call_AvMAX(\@a);      print("\nAfter AvMAX:\n");     Dump(\@a, 1);
call_AvARRAY(\@a);    print("\nAfter AvARRAY:\n");   Dump(\@a, 1);
call_AvALLOC(\@a);    print("\nAfter AvALLOC:\n");   Dump(\@a, 1);
call_AvARYLEN(\@a);   print("\nAfter AvARYLEN:\n");  Dump(\@a, 1);
[download]

After init:
SV = RV(0x238c18) at 0x238c0c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x182a68c
  SV = PVAV(0x2399f4) at 0x182a68c
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x1824504
    FILL = 2
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)

After AvMAX:
SV = RV(0x238c18) at 0x238c0c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x182a68c
  SV = PVAV(0x2399f4) at 0x182a68c
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x1824504
    FILL = 2
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)

After AvARRAY:
SV = RV(0x238c18) at 0x238c0c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x182a68c
  SV = PVAV(0x2399f4) at 0x182a68c
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x1824504
    FILL = 2
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)

After AvALLOC:
SV = RV(0x238c18) at 0x238c0c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x182a68c
  SV = PVAV(0x2399f4) at 0x182a68c
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x1824504
    FILL = 2
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)

After AvARYLEN:
SV = RV(0x238c18) at 0x238c0c
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x182a68c
  SV = PVAV(0x2399f4) at 0x182a68c
    REFCNT = 2
    FLAGS = (PADMY,RMG)
    MAGIC = 0x23fdbc
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
    ARRAY = 0x1824504
    FILL = 2
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
[download]

So AvARYLEN is the culprit. In 5.10.1, AvARYLEN is defined as

#define AvARYLEN(av)    (*Perl_av_arylen_p(aTHX_ MUTABLE_AV(av)))
[download]

Sounds familiar.