Re^5: Reference of constants and literals

For example, the following code continually allocates memory blocks at the same two locations. This is not related to my since we're forcing my to continually reallocate SVs by keeping the refcount greater than 1. There is no reuse of allocated memory at all.
perl -le"sub f{ \my $x } print f() for 1..10;"
[download]

What refcount is kept greater than 1? After the print, the result of f() is no longer reference to. Hence the ref to $x goes out of scope, and since nothing is refering to $x, that one goes out of scope as well. Here's a slight variation that shows this:

sub DESTROY {say "DESTROYED"}
sub f{ bless \my $x } say f() for 1..10;
__END__
main=SCALAR(0x9721530)
DESTROYED
main=SCALAR(0x9704c30)
DESTROYED
main=SCALAR(0x9721530)
DESTROYED
main=SCALAR(0x9704c30)
DESTROYED
main=SCALAR(0x9721530)
DESTROYED
main=SCALAR(0x9704c30)
DESTROYED
main=SCALAR(0x9721530)
DESTROYED
main=SCALAR(0x9704c30)
DESTROYED
main=SCALAR(0x9721530)
DESTROYED
main=SCALAR(0x9704c30)
DESTROYED
[download]

OTOH, if you do keep the refcount above 0, no address is reused:

my @a;
sub f{ push @a, \my $x; $a[-1] } say f() for 1..10;
__END__
SCALAR(0x9c34108)
SCALAR(0x9c33c1c)
SCALAR(0x9c41f8c)
SCALAR(0x9c33f50)
SCALAR(0x9c341a8)
SCALAR(0x9c34180)
SCALAR(0x9c33f00)
SCALAR(0x9c50534)
SCALAR(0x9c5050c)
SCALAR(0x9c504e4)
[download]

Comment on Re^5: Reference of constants and literals Select or Download Code

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Re^6: Reference of constants and literals by ikegami (Patriarch) on Nov 24, 2008 at 11:54 UTC
What refcount is kept greater than 1? This is what happens to scalar lexicals on scope exit: 848: ptr = (void)&PL_curpad[SSPOPLONG]; 849: sv = (SV*)ptr; 850: 858: / Can clear pad variable in place? / 859: if (SvREFCNT(sv) <= 1 && !SvOBJECT(sv)) { 860: / 861: * if a my variable that was made readonly is going out of 862: * scope, we want to remove the readonlyness so that it can 863: * go out of scope quietly 864: / 865: if (SvPADMY(sv) && !SvFAKE(sv)) 866: SvREADONLY_off(sv); 867: 868: if (SvTHINKFIRST(sv)) 869: sv_force_normal_flags(sv, SV_IMMEDIATE_UNREF); 870: if (SvMAGICAL(sv)) 871: mg_free(sv); 872: 885: SvOK_off(sv); 888: SvPADSTALE_on(sv); / mark as no longer live / 889: } 890: else { / Someone has a claim on this, so abandon it. / 891: const U32 padflags = SvFLAGS(sv) & (SVs_PADMY\|SVs_PADTMP); 895: (SV*)ptr = newSV(0); 897: SvREFCNT_dec(sv); / Cast current value to the winds. / 898: / preserve pad nature, but also mark as not live 899: * for any closure capturing / 900: SvFLAGS((SV)ptr) \|= padflags \| SVs_PADSTALE; 901: } [download] (Lifted from `Perl_leave_scope` in scope.c in perl.git.) Note line 895. It allocates a new value for `$x`. Lexicals are allocated on scope exit, not on scope entry. That means `$x`'s refcount at scope exit is what's meaningful. Its refcount is not checked on scope entry or when `my` is executed. And `$x`'s refcount at scope exit is 2. It is referenced by both the `f`'s pad and the reference. `>perl -MDevel::Peek -le"sub f{ my $rv = \my $x; Dump($x); $rv } print +f() for 1..5" SV = NULL(0x0) at 0x235fcc REFCNT = 2 FLAGS = (PADBUSY,PADMY) SCALAR(0x235fcc) SV = NULL(0x0) at 0x236038 REFCNT = 2 FLAGS = (PADBUSY,PADMY) SCALAR(0x236038) SV = NULL(0x0) at 0x235fcc REFCNT = 2 FLAGS = (PADBUSY,PADMY) SCALAR(0x235fcc) SV = NULL(0x0) at 0x236038 REFCNT = 2 FLAGS = (PADBUSY,PADMY) SCALAR(0x236038) SV = NULL(0x0) at 0x235fcc REFCNT = 2 FLAGS = (PADBUSY,PADMY) SCALAR(0x235fcc)` [download] Your bless doesn't prove anything since the presence of an object forces a reallocation. See line 859. Update**: Added code from `scope.c`	[reply] [d/l] [select]
Re^6: Reference of constants and literals by ikegami (Patriarch) on Nov 24, 2008 at 12:48 UTC
( Ignore this post. It's subjective and/or unclear, and it doesn't add anything to the conversation. ) and since nothing is refering to $x, that one goes out of scope as well. There is never a time when there's nothing referring to `$x`. `f`'s pad refers to `$x` even when `f` is not being executed.	[reply] [d/l] [select]