Re^5: Macro in perl code?

sub X() { 0 } print "$message1 \n" if( runtime cond);
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Erm, no. The sub and the print statement being on the same line is an artifact of the code being a oneliner :-)

The point is that

sub X() { 0 }
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is folded into a constant at compile time (more accurate after compiling - while optimizing), and the resulting constant is used by the optimizer to further reduce the optree throwing away code which, depending on that constant, is never reached.

So, it's not a (runtime cond). You get a mandatory warning if you redefine the constant defining sub X() { } at runtime, and you won't get back code that has been optimized away after compile.

I assumed that since the OP did preprocessing with m4 even before the perl compiler sees the code, the value for his $verbose doesn't change at runtime, but I may be mistaken. Conditional code that depends (or could depend) on runtime state can't be optimized that way, so there's a difference in writing

{
    my $debug = 0;
    if ($debug) {
        warn "debug set\n";
    }
}
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and

{
    sub debug () { 0 }
    if (debug) {
        warn "debug set\n";
    }
}
[download]

Only in the latter case the if() block is optimized away, even if, in the former, there's no place in the scope in which the my $debug is declared, where its value would change.

qwurx [shmem] ~ > perl -MO=Deparse,-x,-p
{
    my $debug = 0;
    if ($debug) {
        warn "debug set\n";
    }
}
__END__
{
    (my $debug = 0);
    if ($debug) {
        warn("debug set\n");
    }
}
__DATA__
(<Ctrl>D pressed)
- syntax OK
qwurx [shmem] ~ > perl -MO=Deparse,-x,-p
{
    sub debug () { 0 }
    if (debug) {
        warn "debug set\n";
    }
}
__END__
sub debug () { 0 }
{
    '???';
}
__DATA__
(<Ctrl>D pressed)
- syntax OK
[download]

--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}

Comment on Re^5: Macro in perl code? Select or Download Code

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Re^6: Macro in perl code? by Krambambuli (Curate) on Oct 17, 2007 at 09:21 UTC
Thank you. It took some time, but now [thanks god, finally...! :) ] I got it.	[reply]
Re^6: Macro in perl code? by pileofrogs (Priest) on Oct 17, 2007 at 16:01 UTC
Ah.. no wonder I couldn't make it work... I am using a runtime condition. The idea was rather than call a function which evaluates the cond and may simply exit, put the code inline and avoid the function call overhead.	[reply]
Re^7: Macro in perl code? by shmem (Chancellor) on Oct 17, 2007 at 19:44 UTC
Uh, sorry. Seems like I've been barking up the wrong tree all the time. But! since you wrote My script has the global $verbosity which is set by command line args. I ask - does this value change ever after? Because if not, you can make that a static via `BEGIN { my $verbosity = shift; # or Getopts::Std, whatever... eval "sub verbose () { $verbosity }"; }` [download] of course only after examining under severe scrutiny what $verbosity holds (untaint, perlsec). Since the BEGIN block is a separate compile & run, the sub is defined and a constant sub after that, and its value can be used for optimizations. If the values you compare $verbose with are plain numbers as the 1 in `print "Holy Crap = $holy_crap\n" if $verbose >= 1;` [download] then optimization should work, because then you have constant vs. constant comparisons. Let's see... qwurx [shmem] ~ > perl -MO=Deparse,-x,-p - 1 BEGIN { my $verbose; 1 while ($verbose = shift) !~ /^\d$/; # get rid of -MO=Dep... eval "sub verbose () { $verbose }"; } print "Holy Crap = $holy_crap\n" if verbose >= 1; __END__ sub BEGIN { my($verbose); '???' while (not (($verbose = shift(@ARGV)) =~ /^\d$/)); eval("sub verbose () { $verbose }"); } print("Holy Crap = $holy_crap\n"); __DATA__ - syntax OK qwurx [shmem] ~ > perl -MO=Deparse,-x,-p - 0 BEGIN { my $verbose; 1 while ($verbose = shift) !~ /^\d$/; # get rid of -MO=Dep... eval "sub verbose () { $verbose }"; } print "Holy Crap = $holy_crap\n" if verbose >= 1; __END__ sub BEGIN { my($verbose); '???' while (not (($verbose = shift(@ARGV)) =~ /^\d$/)); eval("sub verbose () { $verbose }"); } '???'; <----- hooray! gone :-) __DATA__ - syntax OK [download] bingo :-) --shmem _($_=" "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}	[reply] [d/l] [select]
Re^7: Macro in perl code? by ikegami (Patriarch) on Oct 17, 2007 at 19:53 UTC
It sounds like you have a limited number of conditions (the different levels of verbosity), so you could something like the following: use constant MAX_VERBOSITY => 5; sub vprint { # Or whatever. our $msg; local msg = \($_[0]); print("$msg\n"); } BEGIN { my $verbosity = $ARGV[0] \|\| 0; # Or whatever my $eval = ''; for (1..MAX_VERBOSITY) { $eval .= ($_ <= $verbosity ? "vprint$_ = \&vprint;\n" : "use constant vprint$_ => 1;\n" ); } eval $eval."1;\n" or die $@; } vprint1("msg1"); # Optimized away if arg is < 1 vprint2("msg2"); # Optimized away if arg is < 2 vprint3("msg3"); # Optimized away if arg is < 3 vprint4("msg4"); # Optimized away if arg is < 4 vprint5("msg5"); # Optimized away if arg is < 5 [download] Update: Ah woops! There'll be a prototype mismatch. The best you can do (without playing with the opcode tree) is: use constant MAX_VERBOSITY => 5; BEGIN { my $verbosity = $ARGV[0] \|\| 0; # Or whatever my $eval = ''; for (1..MAX_VERBOSITY) { my $allow = ($_ <= $verbosity) ? 1 : 0; $eval .= "use constant V$_ => $allow;\n"; } eval $eval."1;\n" or die $@; } sub vprint { # Or whatever. our $msg; local msg = \($_[0]); print("$msg\n"); } vprint("msg1") if V1; # Optimized away if arg is < 1 vprint("msg2") if V2; # Optimized away if arg is < 2 vprint("msg3") if V3; # Optimized away if arg is < 3 vprint("msg4") if V4; # Optimized away if arg is < 4 vprint("msg5") if V5; # Optimized away if arg is < 5 [download] Tested >perl -MO=Terse 645574.pl 3 LISTOP (0x1986d80) leave [1] OP (0x1986864) enter COP (0x1986da4) nextstate UNOP (0x1986e00) entersub [2] UNOP (0x1986e3c) null [141] OP (0x1986e20) pushmark SVOP (0x1986e60) const [7] PV (0x182ff6c) "msg1" UNOP (0x1986e80) null [17] PADOP (0x1986ea0) gv GV (0x191a7f4) vprint COP (0x1986c64) nextstate UNOP (0x1986cc0) entersub [4] UNOP (0x1986cfc) null [141] OP (0x1986ce0) pushmark SVOP (0x1986d20) const [8] PV (0x191a7d0) "msg2" UNOP (0x1986d40) null [17] PADOP (0x1986d60) gv GV (0x191a7f4) vprint COP (0x1986b24) nextstate UNOP (0x1986b80) entersub [6] UNOP (0x1986bbc) null [141] OP (0x1986ba0) pushmark SVOP (0x1986be0) const [9] PV (0x191a7b8) "msg3" UNOP (0x1986c00) null [17] PADOP (0x1986c20) gv GV (0x191a7f4) vprint COP (0x19869e4) nextstate OP (0x1986a20) null [5] COP (0x19868a4) nextstate OP (0x19868e0) null [5] 645574.pl syntax OK >perl -MO=Terse 645574.pl 3 msg1 msg2 msg3 [download] I'm betting this is the fastest you'll get.	[reply] [d/l] [select]