#1138: Create a 'vivify' opcode
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Reporter: Austin_Hastings | Type: todo
Status: new | Priority: normal
Milestone: | Component: core
Version: 1.7.0 | Severity: medium
Keywords: | Lang:
Patch: | Platform:
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Comment(by pmichaud):
Having just written further details on TT #787, I thought I'd add a note
here: In general I'm in favor of coming up with opcode(s) that avoid or
simplify the "create if null" sequences that NQP/PCT currently end up
generating. But I'm opposed to the "vivify" opcode as proposed above for
two reasons:
1. It's misnamed -- it's really just defaulting a null PMC to a value, it
doesn't "vivify" a lexical symbol or aggregate element in the sense of
"create and bind" that HLLs normally have.
2. I think we can come up with a far more powerful set of opcodes than
this.
Just brainstorming a bit: I'm thinking that what might be very powerful
would be separate "fetch" and "vivify" opcodes that handle nearly all of
the rvalue and lvalue scenarios we currently have. The generic form of
each would be:
{{{
$P0 = fetch $P1[key], $P2 # rvalue fetch
$P0 = vivify $P1[key], $P2 # lvalue fetch
}}}
The first form does a lookup of 'key' in an aggregate $P1, if found it
returns it, otherwise it returns $P2. The second form is similar, but if
'key' isn't found then we clone $P2 and bind the aggregate to the clone
before returning it.
Since lexpads and namespaces effectively act as hashes, these two opcodes
should be sufficient for nearly all lexical, package, and keyed variable
accesses. For example:
{{{
# at beginning of sub
.local pmc lexpad, undef
$P0 = getinterp
lexpad = $P0['lexpad', 0]
undef = new ['Undef']
# say($foo)
$P99 = fetch lexpad['$foo'], undef
say $P99
# $foo = 4
$P99 = vivify lexpad['$foo'], undef
assign $P99, 4
}}}
Clearly these would also work well for namespace and aggregates:
{{{
# $ABC::Test::xyz = 'hello';
$P0 = get_namespace ['ABC';'Test']
$P99 = vivify $P0['$xyz'], undef
assign $P99, 'hello'
# %hash['key'] = $a
$P100 = vivify lexpad['%hash'], hash
$P101 = vivify $P0['key'], undef
$P102 = fetch lexpad['$a'], undef
assign $P101, $P102
}}}
Here I've proposed that the last argument to 'fetch' and 'vivify' be used
as values to be cloned instead of types to be created -- I think that
cloning a default value gives us more flexibility in what that default can
be (e.g., initial values or properties already in place).
Thoughts welcomed on the above. If fetch and vivify were made available
along the lines I've given here it would greatly simplify variable access
in PCT and be far more efficient than what we've been generating.
Pm
--
Ticket URL: <https://trac.parrot.org/parrot/ticket/1138#comment:2>
Parrot <https://trac.parrot.org/parrot/>
Parrot Development
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