comment on

It may look like runtime, but it's not. Just change $[ to $a see what happens to the optree by running the source through B::Concise. You'll notice that the $a version has real assignments like

k     <2> sassign vKS/2 ->l
i        <$> const(IV 0) s ->j
-        <1> ex-rv2sv sKRM*/1 ->k
j           <$> gvsv(*a) s ->k
[download]

where in the $[ version all you get is a mere

i     <;> nextstate(main 2 x.pl:4) v ->j
[download]

The effect of changing the array base index is achieved through virtual machine state, which is actually independent from the runtime value of $[. The value accessible through $[ is kept synched to the internal state but not vice versa.

Full B::Concise output follows:

$ cat x.pl
my @elements = qw/ 0 1 2 3 4 5 6 7 8 9 /;

$[ = 0;
print $elements[1], "\n";

$[ = 1;
print $elements[1], "\n";

if ($[ == 1) {
  $[ = 0;
  print $elements[1], "\n";
}

$ perl -MO=Concise x.pl
18 <@> leave[@elements:1,4] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 x.pl:1) v ->3
g     <2> aassign[t2] vKS ->h
-        <1> ex-list lKP ->e
3           <0> pushmark s ->4
4           <$> const(PV "0") s ->5
5           <$> const(PV "1") s ->6
6           <$> const(PV "2") s ->7
7           <$> const(PV "3") s ->8
8           <$> const(PV "4") s ->9
9           <$> const(PV "5") s ->a
a           <$> const(PV "6") s ->b
b           <$> const(PV "7") s ->c
c           <$> const(PV "8") s ->d
d           <$> const(PV "9") s ->e
-        <1> ex-list lK ->g
e           <0> pushmark s ->f
f           <0> padav[@elements:1,4] lRM*/LVINTRO ->g
h     <;> nextstate(main 2 x.pl:3) v ->i
i     <;> nextstate(main 2 x.pl:4) v ->j
o     <@> print vK ->p
j        <0> pushmark s ->k
m        <2> aelem sK/2 ->n
k           <0> padav[@elements:1,4] sR ->l
l           <$> const(IV 1) s ->m
n        <$> const(PV "\n") s ->o
p     <;> nextstate(main 2 x.pl:6 $[=1) v ->q
q     <;> nextstate(main 2 x.pl:7 $[=1) v ->r
w     <@> print vK ->x
r        <0> pushmark s ->s
u        <2> aelem sK/2 ->v
s           <0> padav[@elements:1,4] sR ->t
t           <$> const(IV 1) s ->u
v        <$> const(PV "\n") s ->w
x     <;> nextstate(main 4 x.pl:9) v ->y
17    <@> leave vKP ->18
y        <0> enter v ->z
z        <;> nextstate(main 2 x.pl:10) v ->10
10       <;> nextstate(main 2 x.pl:11) v ->11
16       <@> print vK ->17
11          <0> pushmark s ->12
14          <2> aelem sK/2 ->15
12             <0> padav[@elements:1,4] sR ->13
13             <$> const(IV 1) s ->14
15          <$> const(PV "\n") s ->16
x.pl syntax OK
[download]

$ cat x.pl
my @elements = qw/ 0 1 2 3 4 5 6 7 8 9 /;

$a = 0;
print $elements[1], "\n";

$a = 1;
print $elements[1], "\n";

if ($a == 1) {
  $a = 0;
  print $elements[1], "\n";
}

$ perl -MO=Concise x.pl
1l <@> leave[@elements:1,4] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 x.pl:1) v ->3
g     <2> aassign[t2] vKS ->h
-        <1> ex-list lKP ->e
3           <0> pushmark s ->4
4           <$> const(PV "0") s ->5
5           <$> const(PV "1") s ->6
6           <$> const(PV "2") s ->7
7           <$> const(PV "3") s ->8
8           <$> const(PV "4") s ->9
9           <$> const(PV "5") s ->a
a           <$> const(PV "6") s ->b
b           <$> const(PV "7") s ->c
c           <$> const(PV "8") s ->d
d           <$> const(PV "9") s ->e
-        <1> ex-list lK ->g
e           <0> pushmark s ->f
f           <0> padav[@elements:1,4] lRM*/LVINTRO ->g
h     <;> nextstate(main 2 x.pl:3) v ->i
k     <2> sassign vKS/2 ->l
i        <$> const(IV 0) s ->j
-        <1> ex-rv2sv sKRM*/1 ->k
j           <$> gvsv(*a) s ->k
l     <;> nextstate(main 2 x.pl:4) v ->m
r     <@> print vK ->s
m        <0> pushmark s ->n
p        <2> aelem sK/2 ->q
n           <0> padav[@elements:1,4] sR ->o
o           <$> const(IV 1) s ->p
q        <$> const(PV "\n") s ->r
s     <;> nextstate(main 2 x.pl:6) v ->t
v     <2> sassign vKS/2 ->w
t        <$> const(IV 1) s ->u
-        <1> ex-rv2sv sKRM*/1 ->v
u           <$> gvsv(*a) s ->v
w     <;> nextstate(main 2 x.pl:7) v ->x
12    <@> print vK ->13
x        <0> pushmark s ->y
10       <2> aelem sK/2 ->11
y           <0> padav[@elements:1,4] sR ->z
z           <$> const(IV 1) s ->10
11       <$> const(PV "\n") s ->12
13    <;> nextstate(main 4 x.pl:9) v ->14
-     <1> null vKP/1 ->1l
17       <|> and(other->18) vK/1 ->1l
16          <2> eq sK/2 ->17
-              <1> ex-rv2sv sK/1 ->15
14                <$> gvsv(*a) s ->15
15             <$> const(IV 1) s ->16
1k          <@> leave vKP ->1l
18             <0> enter v ->19
19             <;> nextstate(main 2 x.pl:10) v ->1a
1c             <2> sassign vKS/2 ->1d
1a                <$> const(IV 0) s ->1b
-                 <1> ex-rv2sv sKRM*/1 ->1c
1b                   <$> gvsv(*a) s ->1c
1d             <;> nextstate(main 2 x.pl:11) v ->1e
1j             <@> print vK ->1k
1e                <0> pushmark s ->1f
1h                <2> aelem sK/2 ->1i
1f                   <0> padav[@elements:1,4] sR ->1g
1g                   <$> const(IV 1) s ->1h
1i                <$> const(PV "\n") s ->1j
[download]

Makeshifts last the longest.

In reply to Re^2: *a=*[; bug? by Aristotle
in thread *a=*[; bug? by ambrus

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