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Re^3: push to array without copying

by BrowserUk (Patriarch)
on Nov 15, 2016 at 21:51 UTC ( #1175959=note: print w/replies, xml ) Need Help??

in reply to Re^2: push to array without copying
in thread push to array without copying

Are you sure that doesn't make a copy?

Yes. Quite sure. I have two independent demonstrations for proof of that.

  1. Using Devel::Peek::Dump():

    This is not a Perl-level 'aliasing' effect; it is a C-level pointer swap, so you're looking at the wrong thing. It isn't the Perl reference values you should be considering, but rather the PV component of the two scalars as shown below.

    Note that whilst the the two scalars have different heads and bodies (the two hex values on the first line of each dump); the address of the actual data on the fourth line of each dump is identical in both:

    use Devel::Peek;; $h{ XXX } = 'test';; Dump $h{ XXX };; SV = PV(0x2ab2a0) at 0x3dc99d8 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x3e28f98 "test"\0 CUR = 4 LEN = 8 $a[0] = delete $h{ XXX };; Dump $a[ 0 ];; SV = PV(0x2ab2b0) at 0x3dc9978 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x3e28f98 "test"\0 CUR = 4 LEN = 8

    Contrast that with a normal assignment where all three hex values in the scalar Dump()s are different:

    use Devel::Peek;; $h{ XXX } = 'test';; Dump $h{ XXX };; SV = PV(0x11b2a0) at 0x3e199d8 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x3e78f98 "test"\0 CUR = 4 LEN = 8 $a[0] = $h{ XXX };; Dump $a[ 0 ];; SV = PV(0x11b2b0) at 0x3e19978 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x3e78f68 "test"\0 CUR = 4 LEN = 8
  2. Using empirical observation.

    In the trace below mem is a function that returns the process memory utilisation in K bytes.

    • When the REPL session has just started, the memory used is just under 10MB.
    • After I create the hash key with a 100e6 byte value, the memory has grown to 107MB.
    • After I transfer that value to the array element, the memory has change by a few kb, but is still basically the same 107MB.
    C:\test>p1 Perl> print mem;; 9,340 K Perl> $h{ XXX } = 'X' x 100e6;; Perl> print mem;; 107,248 K Perl> $a[ 0 ] = delete $h{ XXX };; Perl> print mem;; 107,308 K Perl>

    If the data had been copied, the footprint would have been over 200MB.

    (I cannot Dump() the scalars in this latter case because Dump() would spend a week trying to format a 100e6 byte string nicely, before dumping it to the console, which would take another week (or two:)!)

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Re^4: push to array without copying
by chris212 (Scribe) on Nov 16, 2016 at 18:39 UTC
    Very thorough explanation! Thanks, I appreciate it.

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