Re: How to swap scalar values without copies

[asarih]

Is this not what you need?

$a=\$x;
$b=\$y;
print "$a $b\n"
($a,$b)=($b,$a);
print "$a $b\n";

update: changed $a and $b to references.

[Anonymous Monk]

No, this violates the no-copy rule. If $a is a 10Mb string instead of 'a', then, during the assignement ($a,$b)=($b,$a) the process allocates 10 more Mb. I have verified the following resident memory counts:

print "stage 0\n"; 
sleep 5; # memory usage now: 1800 Kb
$a='a' x 10000000;
$b='b';
print "stage 1\n"; 
sleep 5; # memory usage now: 21336 Kb
($a,$b)=($b,$a);
print "stage 2\n";
sleep 5; # memory usage now: 31104 Kb

[Roger]

One method I think possible is at the XS level. A simple scalar is stored as SvPV internally by Perl, which looks like this:


SV            xpv
+--------+    +-----+
| ANY    |--->| PVX |---> char[]
| REFCNT |    | CUR |
| FLAGS  |    | LEN |
+--------+    +-----+

To do an immediate swap without copying strings, at XS level, do this -

SV                  xpv
+--------+          +-----+
| ANY    |-+      ->| PVX |---> char[]
| REFCNT |  \    /  | CUR |
| FLAGS  |   \  /   | LEN |
+--------+    \/    +-----+
              /\
SV           /  \   xpv
+--------+  /    \  +-----+
| ANY    |-+      ->| PVX |---> char[]
| REFCNT |          | CUR |
| FLAGS  |          | LEN |
+--------+          +-----+


I can provide an XS example if you are not sure how to do this at XS level.

[Anonymous Monk]

I think that Data::Swap does exactly this. The Swap.xs file, once stripped of all checks, looks like:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
MODULE = Data::Swap  PACKAGE = Data::Swap
void swap(foo, bar)
        SVref foo
        SVref bar
    PREINIT:
        void *any;
        U32 flags;
    CODE:
        any = foo->sv_any;
        flags = foo->sv_flags;
        foo->sv_any = bar->sv_any;
        foo->sv_flags = bar->sv_flags;
        bar->sv_any = any;
        bar->sv_flags = flags;

But for this you need to translate the .xs into a .c, compile the latter and place the resulting shared object in blib somewhere in the hierarchy. And you need also a Swap.pm module bootstrapping it.

I would like to avoid to ask people to install this module to run my code. Is it possible to hide these files in my user hierarchy?

I begin to believe that there is no native way to solve my original problem in Perl5.

[Anonymous Monk]

(I see now the update, and I don't know how to update my reply in turn ...).

Swapping the values of the references does not waste memory, but it violates rule n.2, namely that I want $a to be a string, not a reference to a string.

[hardburn]

I don't think you can have it both ways. You either need to work around having a reference or live with the major memory requirement. I'm not an XS-guru, but it appears Data::Swap just does the reference stuff for you.

[Anonymous Monk]

That is a very stupid requirement. You're creating a problem where one doesn't exist.

[Anonymous Monk]

Thanks for your insightful contribution