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authorTom Lane2011-10-27 17:50:57 +0000
committerTom Lane2011-10-27 17:50:57 +0000
commit756a4ed5ad3e57c26a247234de371a6ca21806cd (patch)
tree2134e627df4b72f4288eeb2419e9998ac9df6dad
parentbf82013631e32436c9abb23fee8be0a4ce46b3dd (diff)
Add simple script to check for right recursion in Bison grammars.
We should generally use left-recursion not right-recursion to parse lists. Bison hasn't got any built-in way to check for this type of inefficiency, and I didn't find anything on the net in a quick search, so I wrote a little Perl script to do it. Add to src/tools/ so we don't have to re-invent this wheel next time we wonder if we're doing anything stupid. Currently, the only place that seems to need fixing is plpgsql's stmt_else production, so the problem doesn't appear to be common enough to warrant trying to include such a test in our standard build process. If we did want to do that, we'd need a way to ignore some false positives, such as a_expr := '-' a_expr
-rwxr-xr-xsrc/tools/check_bison_recursion.pl74
1 files changed, 74 insertions, 0 deletions
diff --git a/src/tools/check_bison_recursion.pl b/src/tools/check_bison_recursion.pl
new file mode 100755
index 0000000000..34f6ed8466
--- /dev/null
+++ b/src/tools/check_bison_recursion.pl
@@ -0,0 +1,74 @@
+#! /usr/bin/perl
+
+#################################################################
+#
+# check_bison_recursion.pl -- check for right recursion in Bison grammars
+#
+# The standard way to parse list constructs in Bison grammars is via left
+# recursion, wherein a nonterminal symbol has itself as the first symbol
+# in one of its expansion rules. It is also possible to parse a list via
+# right recursion, wherein a nonterminal symbol has itself as the last
+# symbol of an expansion; but that's a bad way to write it because a long
+# enough list will result in parser stack overflow. Since Bison doesn't
+# have any built-in way to warn about use of right recursion, we use this
+# script when we want to check for the problem.
+#
+# To use: run bison with the -v switch, then feed the produced y.output
+# file to this script.
+#
+# Copyright (c) 2011, PostgreSQL Global Development Group
+#
+# src/tools/check_bison_recursion.pl
+#################################################################
+
+use strict;
+use warnings;
+
+my $debug = 0;
+
+# must retain this across input lines
+my $cur_nonterminal;
+
+# We parse the input and emit warnings on the fly.
+my $in_grammar = 0;
+
+while (<>) {
+ my $rule_number;
+ my $rhs;
+
+ # We only care about the "Grammar" part of the input.
+ if (m/^Grammar$/) {
+ $in_grammar = 1;
+ } elsif (m/^Terminal/) {
+ $in_grammar = 0;
+ } elsif ($in_grammar) {
+ if (m/^\s*(\d+)\s+(\S+):\s+(.*)$/) {
+ # first rule for nonterminal
+ $rule_number = $1;
+ $cur_nonterminal = $2;
+ $rhs = $3;
+ } elsif (m/^\s*(\d+)\s+\|\s+(.*)$/) {
+ # additional rule for nonterminal
+ $rule_number = $1;
+ $rhs = $2;
+ }
+ }
+
+ # Process rule if we found one
+ if (defined $rule_number) {
+ # deconstruct the RHS
+ $rhs =~ s|^/\* empty \*/$||;
+ my @rhs = split '\s', $rhs;
+ print "Rule $rule_number: $cur_nonterminal := @rhs\n" if $debug;
+ # We complain if the nonterminal appears as the last RHS element
+ # but not elsewhere, since "expr := expr + expr" is reasonable
+ my $lastrhs = pop @rhs;
+ if (defined $lastrhs &&
+ $cur_nonterminal eq $lastrhs &&
+ !grep { $cur_nonterminal eq $_ } @rhs) {
+ print "Right recursion in rule $rule_number: $cur_nonterminal := $rhs\n";
+ }
+ }
+}
+
+exit 0;