From: Stephen McCamant <smcc@mit.edu>
Date: Wed, 9 Jan 2002 23:04:01 +0000 (-0800)
Subject: Magic numbers in B::Concise
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=213a1a263469325697166cf306c9ca76600e54dd;p=p5sagit%2Fp5-mst-13.2.git

Magic numbers in B::Concise
Message-ID: <15421.15585.813580.211907@soda.csua.berkeley.edu>

p4raw-id: //depot/perl@14162
---

diff --git a/ext/B/B/Concise.pm b/ext/B/B/Concise.pm
index ccf326c..46a8cfe 100644
--- a/ext/B/B/Concise.pm
+++ b/ext/B/B/Concise.pm
@@ -499,10 +499,40 @@ sub tree {
            map(" " x (length($name)+$size) . $_, @lines));
 }
 
-# This is a bit of a hack; the 2 and 15 were determined empirically.
-# These need to stay the last things in the module.
-$cop_seq_base = svref_2object(eval 'sub{0;}')->START->cop_seq + 2;
-$seq_base = svref_2object(eval 'sub{}')->START->seq + 15;
+# *** Warning: fragile kludge ahead ***
+# Because the B::* modules run in the same interpreter as the code
+# they're compiling, their presence tends to distort the view we have
+# of the code we're looking at. In particular, perl gives sequence
+# numbers to both OPs in general and COPs in particular. If the
+# program we're looking at were run on its own, these numbers would
+# start at 1. Because all of B::Concise and all the modules it uses
+# are compiled first, though, by the time we get to the user's program
+# the sequence numbers are alreay at pretty high numbers, which would
+# be distracting if you're trying to tell OPs apart. Therefore we'd
+# like to subtract an offset from all the sequence numbers we display,
+# to restore the simpler view of the world. The trick is to know what
+# that offset will be, when we're still compiling B::Concise!  If we
+# hardcoded a value, it would have to change every time B::Concise or
+# other modules we use do. To help a little, what we do here is
+# compile a little code at the end of the module, and compute the base
+# sequence number for the user's program as being a small offset
+# later, so all we have to worry about are changes in the offset.
+
+# When you say "perl -MO=Concise -e '$a'", the output should look like:
+
+# 4  <@> leave[t1] vKP/REFC ->(end)
+# 1     <0> enter ->2
+ #^ smallest OP sequence number should be 1
+# 2     <;> nextstate(main 1 -e:1) v ->3
+ #                         ^ smallest COP sequence number should be 1
+# -     <1> ex-rv2sv vK/1 ->4
+# 3        <$> gvsv(*a) s ->4
+
+# If either of the marked numbers there aren't 1, it means you need to
+# update the corresponding magic number in the next two lines.
+# Reember, these need to stay the last things in the module.
+$cop_seq_base = svref_2object(eval 'sub{0;}')->START->cop_seq + 11;
+$seq_base = svref_2object(eval 'sub{}')->START->seq + 84;
 
 1;
 
diff --git a/ext/B/O.pm b/ext/B/O.pm
index af9885b..0cdf436 100644
--- a/ext/B/O.pm
+++ b/ext/B/O.pm
@@ -3,7 +3,7 @@ package O;
 our $VERSION = '1.00';
 
 use B qw(minus_c save_BEGINs);
-use Carp;    
+use Carp;
 
 sub import {
     my ($class, @options) = @_;
@@ -31,6 +31,11 @@ sub import {
 		open (STDOUT, ">&SAVEOUT");
 		close SAVEOUT;
 	    }
+
+	    # Note: if you change the code after this 'use', please
+	    # change the fudge factors in B::Concise (grep for
+	    # "fragile kludge") so that its output still looks
+	    # nice. Thanks. --smcc
 	    use B::].$backend.q[ ();
 	    if ($@) {
 		croak "use of backend $backend failed: $@";