comment on

$#a adds magic to the associated array. This costs time and memory. I've written a patch to prevent that when $#a is used as an rvalue.

speed_test.pl:

use strict;
use warnings;

use Devel::Peek qw( Dump );
use Time::HiRes qw( time );

my @a = qw( a b c d e f g h i j );

my $stime = time;
my $x; $x = $#a for 1..1_000_000;
my $etime = time;

print("time delta = ", $etime-$stime, "\n");

Dump(\@a, 1) if $ARGV[0];
[download]

Before change:

$ ./perl -Ilib speed_test.pl 0
time delta = 0.623134136199951

$ ./perl -Ilib speed_test.pl 0
time delta = 0.621975898742676

$ ./perl -Ilib speed_test.pl 0
time delta = 0.626943826675415

$ ./perl -Ilib speed_test.pl 1
time delta = 0.636697053909302
SV = IV(0x8400afc) at 0x8400b00
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x83ca0f0
  SV = PVAV(0x83bb1b4) at 0x83ca0f0
    REFCNT = 2
    FLAGS = (PADMY,RMG)
    MAGIC = 0x83c5210
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x83ca280
      SV = PVMG(0x840f42c) at 0x83ca280
        REFCNT = 1
        FLAGS = (GMG,SMG,pIOK)
        IV = 9
        NV = 0
        PV = 0
        MAGIC = 0x83c5230
          MG_VIRTUAL = &PL_vtbl_arylen
          MG_TYPE = PERL_MAGIC_arylen(#)
          MG_OBJ = 0x83ca0f0
    ARRAY = 0x83c5180
    FILL = 9
    MAX = 9
    ARYLEN = 0x83ca280
    FLAGS = (REAL)
[download]

After change:

$ ./perl -Ilib speed_test.pl 0
time delta = 0.463121175765991

$ ./perl -Ilib speed_test.pl 0
time delta = 0.498654127120972

$ ./perl -Ilib speed_test.pl 0
time delta = 0.462605953216553

$ ./perl -Ilib speed_test.pl 1
time delta = 0.464058876037598
SV = IV(0x83cb27c) at 0x83cb280
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x83cb0f0
  SV = PVAV(0x83bc1b4) at 0x83cb0f0
    REFCNT = 2
    FLAGS = (PADMY)
    ARRAY = 0x83c6180
    FILL = 9
    MAX = 9
    ARYLEN = 0x0
    FLAGS = (REAL)
[download]

Unfortunately, I'm not very confident about my patch

From c045b931b2afb3b2af6641a9943f65f295ee7da1 Mon Sep 17 00:00:00 2001
From: Eric Brine <ikegami@adaelis.com>
Date: Fri, 23 Oct 2009 19:05:40 -0400
Subject: [PATCH] Avoid adding magic with rvalue $#a

---
 pp.c |   17 ++++++++++++-----
 1 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/pp.c b/pp.c
index 6f56368..b9b8ba6 100644
--- a/pp.c
+++ b/pp.c
@@ -321,12 +321,19 @@ PP(pp_av2arylen)
 {
     dVAR; dSP;
     AV * const av = MUTABLE_AV(TOPs);
-    SV ** const sv = Perl_av_arylen_p(aTHX_ MUTABLE_AV(av));
-    if (!*sv) {
-       *sv = newSV_type(SVt_PVMG);
-       sv_magic(*sv, MUTABLE_SV(av), PERL_MAGIC_arylen, NULL, 0);
+    const I32 lvalue = PL_op->op_flags & OPf_MOD || LVRET;
+    if (lvalue) {
+       SV ** const sv = Perl_av_arylen_p(aTHX_ MUTABLE_AV(av));
+       if (!*sv) {
+           *sv = newSV_type(SVt_PVMG);
+           sv_magic(*sv, MUTABLE_SV(av), PERL_MAGIC_arylen, NULL, 0);
+       }
+       SETs(*sv);
+    } else {
+       SETs(sv_2mortal(newSViv(
+           AvFILL(MUTABLE_AV(av)) + CopARYBASE_get(PL_curcop)
+       )));
     }
-    SETs(*sv);
     RETURN;
 }

--
1.6.5
[download]

Specifically,

Is it ok to define sv inside the curlies?
Is the arg guaranteed to be an AV?
Is the sv_2mortal appropriate?

Also, I initially tried to avoid creating a new SV by using

        dTARGET;
        SETi( AvFILL(MUTABLE_AV(av)) + CopARYBASE_get(PL_curcop) );
[download]

instead of

        SETs(sv_2mortal(newSViv(
            AvFILL(MUTABLE_AV(av)) + CopARYBASE_get(PL_curcop)
        )));
[download]

but I got the following while building the rest of Perl:

panic: pad_sv po at lib/re.pm line 99.
[download]

Is using TARG the wrong way to go, or did I just do it wrong? I based my code on other ops.

In reply to Review of patch to avoid adding magic with rvalue $#a (XS gurus wanted) by ikegami

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