Re^2: named sub, subref and magic goto speed

Oops! Good catch. I wonder how perl's not bailing out complaining e.g.

Can't find label CODE(0x81678a0) at sub.pl line 13.
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and seems to be doing the right thing instead. The corrected code yields

            Rate refcall   named    goto
refcall 708190/s      --     -1%    -10%
named   717414/s      1%      --     -9%
goto    790676/s     12%     10%      --
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but your previous reply (Re: named sub, subref and magic goto speed) explains that well: the goto &$ref essentially saves the building of one stack frame; that makes sense, thank you.

Uhm, no, goto &sub does create a new stack frame. From what I read in pp_ctl.c without knowing all those macros, I guess it's working like this:

A new frame is created, the old frame is unwound (sort of return), the old frame inspected (whether inside a sub, an eval, a loop and so on), then @_ pushed onto the stack again and the new frame is entered (sort of call).

It seems that the magic goto &function works along the semantics of

    my $ref = *function{'CODE'}; # get a reference 
    goto $ref;                   # magic goto to that reference
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which explains why the goto &$coderef is more expensive - it does an extra deref and ref step:

    my $ref = \&{$coderef};      # get a reference (\&{&$ref} without 
+the call)
    goto $ref;                   # magic goto to that reference
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whereas whithout the & we have just

    goto $ref;                   # we have a code reference, just use 
+that
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Roughly. I mean, explaining what's going on in C speaking perl is sort of... well ;-)

The relevant part of a diff of the output of two runs of the same code (foo.pl), once with goto $ref and once with goto &$ref (with -Dtls) shows what's going on:

--- g_ref.Dtls
+++ g_amp.Dtls
@@ -69,7 +69,13 @@
     =>  
 (foo.pl:5)     nextstate
     =>  
+(foo.pl:5)     pushmark
+    =>  *  
 (foo.pl:5)     padsv($ref)
+    =>  *  \CV(__ANON__)  
+(foo.pl:5)     rv2cv
+    =>  *  CV(__ANON__)  
+(foo.pl:5)     refgen
     =>  \CV(__ANON__)  
 (foo.pl:5)     goto
 (foo.pl:5)     (Found sub #1)
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--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

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Re^3: named sub, subref and magic goto speed by ikegami (Patriarch) on Dec 14, 2006 at 09:01 UTC
Oops! Good catch. I wonder how perl's not bailing out complaining Because it DWYM instead. The docs say a label is expected, but if a code ref is provided, `goto $ref` is treated as `goto &$ref`. Personally, I consider it a bug in the docs (not in Perl or your code) because there's no reason to treat `goto $ref` as it if were `goto "$ref"`. `$\ = "\n"; sub foo { print @_ } my $ref = \&foo; sub { @_=('$ref->(@_)'); $ref->(@_); }->(); sub { @_=('foo(@_)'); foo(@_); }->(); sub { @_=('goto $ref'); goto $ref; }->(); sub { @_=('goto &$ref'); goto &$ref; }->();` [download]	[reply] [d/l] [select]
Re^4: named sub, subref and magic goto speed by ysth (Canon) on Dec 14, 2006 at 09:51 UTC
Sure there is; goto $label, where $label is an object expected to return a label string, and the object happens to be based on a coderef.	[reply]