regexes: speed up scanning for literal matches

if an unanchored regex (not ignoring case) starts with a literal,
we now use the "index" opcode (at least on parrot) to search for
the start position.
Speeds up the fairly artifical benchmark nqp::x(abc, 500) ~ def ~~ /def/
by a factor of 4

Author: moritz

src/QRegex/P6Regex/Actions.nqp

@@ -667,9 +667,21 @@ class QRegex::P6Regex::Actions is HLL::Actions {
         self.store_regex_nfa($code_obj, $block, QRegex::NFA.new.addnode($qast));
         self.alt_nfas($code_obj, $block, $qast);
 
+        my $scan := QAST::Regex.new( :rxtype<scan> );
+        {
+            my $q := $qast;
+            if $q.rxtype eq 'concat' && $q[0] {
+                $q := $q[0]
+            }
+            if $q.rxtype eq 'literal' {
+                nqp::push($scan, $q[0]);
+                $scan.subtype($q.subtype);
+            }
+        }
+
         $block<orig_qast> := $qast;
         $qast := QAST::Regex.new( :rxtype<concat>,
-                     QAST::Regex.new( :rxtype<scan> ),
+                     $scan,
                      $qast,
                      ($anon
                           ?? QAST::Regex.new( :rxtype<pass> )

src/vm/parrot/QAST/Compiler.nqp

@@ -1564,6 +1564,10 @@ class QAST::Compiler is HLL::Compiler {
         $ops.push_pirop('goto', $scanlabel);
         $ops.push($looplabel);
         $ops.push_pirop('inc', %*REG<pos>);
+        if nqp::elems($node.list) && $node.subtype ne 'ignorecase' {
+            $ops.push_pirop('index', %*REG<pos>, %*REG<tgt>, self.rxescape($node[0]), %*REG<pos>);
+            $ops.push_pirop('eq', %*REG<pos>, -1, %*REG<fail>);
+        }
         $ops.push_pirop('gt', %*REG<pos>, %*REG<eos>, %*REG<fail>);
         $ops.push_pirop('repr_bind_attr_int', %*REG<cur>, %*REG<curclass>, '"$!from"', %*REG<pos>);
         $ops.push($scanlabel);