Merge pull request #1844 from MasterDuke17/array_slice_optimization

Rewrite @A[1,3] to @a.AT-POS(1),@a.AT-POS(3) for literal indices

src/Perl6/Optimizer.nqp

@@ -1397,6 +1397,27 @@ class Perl6::Optimizer {
             return self.optimize_array_variable_initialization($op);
         }
 
+        # Turn `@a[1, 3]` into `@a.AT-POS(1), @a.AT-POS(3)`
+        elsif
+            $!level > 0
+            && ($optype eq 'call' || $optype eq 'callstatic')
+
+            # workaround because op_eq_core needs to be "primed"
+            && ((try $!symbols.find_lexical($op.name)) || 1)
+
+            && self.op_eq_core($op, '&postcircumfix:<[ ]>')
+
+            # if it's 3 that means the slice is on the LHS of an assignment
+            && +@($op) == 2
+
+            && nqp::istype($op[0], QAST::Var)
+            && nqp::substr($op[0].name, 0, 1) eq '@'
+            && nqp::istype($op[1], QAST::WVal)
+            && nqp::istype($op[1].value, $!symbols.find_in_setting("List"))
+        {
+            return self.optimize_array_slice($op);
+        }
+
         # Calls are especially interesting as we may wish to do some
         # kind of inlining.
         elsif $optype eq 'call' {
@@ -1501,6 +1522,27 @@ class Perl6::Optimizer {
         }
     }
 
+    method optimize_array_slice($op) {
+        unless nqp::isconcrete($op[1].value) {
+            return $op
+        }
+
+        my $reified := nqp::getattr($op[1].value, $!symbols.find_in_setting("List"), '$!reified');
+
+        unless $reified.HOW.name($reified) eq 'BOOTArray' {
+            return $op
+        }
+
+        my $new_op    := QAST::Op.new(:op('callstatic'), :name('&infix:<,>'));
+        my $array_var := $op[0];
+
+        for $reified -> $var {
+            $new_op.push: QAST::Op.new(:op('callmethod'), :name('AT-POS'), $array_var, QAST::WVal.new(:value($var)));
+        }
+
+        $new_op;
+    }
+
     method optimize_array_variable_initialization($op) {
         my $Positional := $!symbols.find_in_setting('Positional');
         if $op[0].returns =:= $Positional {