Cheat to speedup using nqp::iterator+nqp::shift on a list

[MasterDuke17]

We can assume the backing array is in a good state so reach in directly during iteration.

time MVM_SPESH_BLOCKING=1 nqp -e 'my $l := nqp::list(); my int $p := 0; while $p++ < 10_000 { nqp::push($l, $p) }; my int $j := 10000; while $j-- { my $iter := nqp::iterator($l); nqp::while($iter, my $a := nqp::shift($iter)) }' drops from ~1.46s to ~1.25s and time MVM_SPESH_BLOCKING=1 nqp -e 'my $l := nqp::list_s(); my int $p := 0; while $p++ < 10_000 { nqp::push_s($l, $p) }; my int $j := 10000; while $j-- { my $iter := nqp::iterator($l); nqp::while($iter, my $a := nqp::shift_s($iter)) }' drops from ~2.01s to ~1.83s.

[MasterDuke17]

I noticed this when looking at Raku/nqp#685.

[niner]

Why can we assume that the array is in a good state?

[MasterDuke17]

Why can we assume that the array is in a good state?

Well, I believe we’re allowed to act as if it’s in a good state. Modifying an array during iteration is not allowed AIUI. Also, the iterator keeps track of the index compared to the size of the array.