Paul writes:
>
> --- Brian Ingerson <[EMAIL PROTECTED]> wrote:
> > Garrett Goebel wrote:
> > >
> > > From: Paul [mailto:[EMAIL PROTECTED]]
> > > >
> > > > Anybody know offhand *why* my() lexicals are supposedly faster?
>
> ....
>
> > Yes this is OT, but I'll contribute to the problem as well...
> >
> > My coworker Gisle Aas (maybe you've heard of him ;) says that globals
> > and lexicals have identical speed because Perl optimizes out the
> > symbol-table lookup.
> >
> > Trust Gisle.
>
> lol -- now *there's* an answer.
> So for my details I should go to the parse tree docs, and the code, I'm
> thinking.
>
> BTW -- with many thanks to everyone -- my question was "why are they
> faster", but the reason was never the speed -- it was to understand the
> way Perl stores and *accesses* lexicals.
>
> Any input? =o)
If you have a reasonably recent Perl, you can do the following:
% perl -MO=Terse,exec -e '$f = 123'
OP (0x8180688) enter
COP (0x8180628) nextstate
SVOP (0x8175298) const IV (0x80f8770) 123
SVOP (0x817b488) gvsv GV (0x81017b0) *f
BINOP (0x8180600) sassign
LISTOP (0x8180660) leave
% perl -MO=Terse,exec -e 'my $f = 123'
OP (0x81805d0) enter
COP (0x8180598) nextstate
SVOP (0x8104b88) const IV (0x8104c9c) 123
OP (0x817b490) padsv [1]
BINOP (0x81761f0) sassign
LISTOP (0x81752a0) leave
As you can see from the output, for a non-lexical $f, Perl uses an
opcode "gvsv GV *f". The gvsv instruction gets a pointer to the
entire glob (*f) from which it dereferences the SV (scalar) part and
pushes it on the stack. See pp_hot.c:
PP(pp_gvsv)
{
djSP;
EXTEND(SP,1);
if (PL_op->op_private & OPpLVAL_INTRO)
PUSHs(save_scalar(cGVOP_gv));
else
PUSHs(GvSV(cGVOP_gv));
RETURN;
}
For the lexical, Perl has already determined at compile time that
$f is in pad slot number 1 (think stack or register allocation).
padsv is:
PP(pp_padsv)
{
djSP; dTARGET;
XPUSHs(TARG);
...
If you navigate all the macros, you'll find that takes curpad
(a pointer to an array of SV pointers: the current "stack frame"
where "stack" is in the sense of a traditional compiler, not the
(main) Perl stack) and pushes curpad[1] (remember $f was allocated
slot 1 at compile time) onto the (main Perl) stack.
--Malcolm
--
Malcolm Beattie <[EMAIL PROTECTED]>
Unix Systems Programmer
Oxford University Computing Services
