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Postings from December 2021
I now have a partial attack on this, on a branch
https://github.com/leonerd/perl5/tree/no-snails
On Sun, 5 Dec 2021 20:39:04 +0000
Nicholas Clark <nick@ccl4.org> wrote:
> > It seems to me we need four pieces. The first two of these should be
> > quite simple:
> >
> > 1. Add a flag (maybe CvSIGNATURE(cv)) on CVs that is present when
> > the sub has a signature. The parser can easily turn this one
> > on at the time it creates the CV.
Done, easy.
> > 2. Get pp_entersub to check that flag, and skip setting up the
> > callee's @_ array if set. -- this is the core of the speedup.
TODO
> > 3. {handwavy something} to ensure that OP_ARGCHECK and OP_ARGELEM
> > can look somewhere else to find the arguments, now they can no
> > longer use GvAV(PL_defgv).
Done by just having them use PAD_SVl(0) instead of GvAV(PL_defgv). I'm
actually quite surprised how simple this was, and doesn't break any
tests :)
> > 4. {handwavy something else} to make accesses of @_ throw an
> > error at runtime.
This part I'm in the middle of doing now:
> instead having GvAV(PL_defgv) pointing to a tied array singleton that
> is built to croak.
Actually rather than a tied array (because tie is so annoying to create
from inside core), I just created a new kind of magic; "forbid" magic.
This totally disallows any kind of access - all of its magic vtable
calls immediately croak.
And already this code has caught something. I can't even build perl
right now, because the build system makes miniperl and then uses
miniperl to build the unicore tables, getting as far as:
./miniperl -Ilib lib/unicore/mktables -C lib/unicore -P pod -maketest -makelist -p
Access to the @_ variable is forbidden (lenmagic) at lib/unicore/mktables line 1983.
make: *** [makefile:408: uni.data] Error 25
Hah! It really works :)
Lets peek at the source code:
1883 sub set_access($name, $field, @accessors) {
...
1983 if (grep { /^a/i } @_
1984 or length($access) > length('readable_'))
Inside a real signatured sub, we already have code that tries to access
@_. Oops.
I suspect this code is supposed to read one of
if(grep { /^a/i } @accessors
or
if(grep { /^a/i } ($name, $field, @accessors)
but I can't immediately tell from context which. Perhaps someone
familiar with unicore can assist? For now I'll just rewrite it to my
best guess so I can get on with building perl. But it does appear that
overall this technique appears to have legs:
$ ./perl -Ilib -Mfeature=signatures -E 'sub f { say "slug0 is $_[0]" } f("hello")'
slug0 is hello
$ ./perl -Ilib -Mfeature=signatures -E 'sub f($x) { say "x is $x" } f("hello")'
The signatures feature is experimental at -e line 1.
x is hello
$ ./perl -Ilib -Mfeature=signatures -E 'sub f($x) { say "x is $x; slug0 is $_[0]" } f("hello")'
The signatures feature is experimental at -e line 1.
Access to the @_ variable is forbidden (getmagic) at -e line 1.
$ ./perl -Ilib -Mfeature=signatures -E 'sub f($x) { $_[0] = "boo" } f("hello")'
The signatures feature is experimental at -e line 1.
Access to the @_ variable is forbidden (getmagic) at -e line 1.
These are of course all runtime exceptions, and not compiletime ones,
but that's a further-out goal... :)
--
Paul "LeoNerd" Evans
leonerd@leonerd.org.uk | https://metacpan.org/author/PEVANS
http://www.leonerd.org.uk/ | https://www.tindie.com/stores/leonerd/
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