Very clever ... thanks bob :) Cheers Joe
On Tue, Feb 17, 2015 at 6:46 AM, Dmitry Stogov <dmi...@zend.com> wrote: > On Tue, Feb 17, 2015 at 2:04 AM, Bob Weinand <bobw...@hotmail.com> wrote: > > > I'd like to show you my recent work on a jumptable optimization for > > switches. > > > > https://github.com/php/php-src/pull/1048 < > > https://github.com/php/php-src/pull/1048> > > > > It is a fully transparent optimization of switches, by putting a new > > ZEND_SWITCH opcode at the top of the switch in case we can build a > > jumptable, which is the case if the cases are only scalars (no doubles) > or > > evaluate to them at compile-time. > > > > Switches tend to be big sometimes with a lot of static (literals or > > constants usually) cases, which was a comparison for each case, and now > > just a simple hashtable lookup is needed, which greatly improves > > performance. In case where the result of the switch can be determined at > > compile-time, it even replaces the comparisons with a simple ZEND_JMP > > opcode. > > > > In synthetic benchmarks the results are mind blowing, but in practice, > > benefits mostly are stateful userland parsers (often called with > sometimes > > big switches), where it makes it a few percent faster. (For more concrete > > numbers see > https://github.com/php/php-src/pull/1048#issuecomment-73032647 > > ) > > > > As the optimization is only done if there are 3 or more cases, the lookup > > is always faster than the single comparisons. So there's no negative > > performance impact. > > > > It already works, except with opcache where the CFG optimization pass > > still lacks support. Dmitry or Laruence volunteered to add support for > > opcache. > > > > I'd like to know if there are any technical objections to the PR. If not, > > they will add opcache support and merge it in. > > > > Thanks, > > Bob > > > The patch looks interesting. > We will implement support for opcache and then analyze its impact on > performance. > I see it as a self-containing feature that won't affect anyone (except > opcache and may be debuggers). > I'm not going to work on it in nearest days or weeks, because of more > critical tasks. > > Thanks. Dmitry. >
