Re: [RFC] [VOTE] Constant scalar expressions

Hi, On Fri, Nov 22, 2013 at 9:30 AM, Yasuo Ohgaki <yohgaki@ohgaki.net> wrote: > Hi all, > > On Fri, Nov 22, 2013 at 10:19 AM, Yasuo Ohgaki <yohgaki@ohgaki.net> wrote: > > > On Fri, Nov 22, 2013 at 10:09 AM, Yasuo Ohgaki <yohgaki@ohgaki.net> > wrote: > > > >> Anyway, constant variables are useful for VM/op code optimization. > >> > >> include('foo.php'); // const a = 123; > >> const b = a; > >> > >> is allowed currently. We may restrict this behavior only to the same > file, > >> so that VM/op code optimization can be done. > >> > >> Currently, no expression is allowed with 'const'. Defining 'const' > >> using > >> another constant with the same value is useless. It's a bad coding > >> practice > >> in many case, I suppose. > >> > >> I understand use cases of scalar expressions with constants, but I'm > >> concerned > >> loosing door for further optimizations. (e.g. $x = $x + 123 is faster > >> than $x = $x + a, > >> where a is 123 in constant var hash) > >> > >> We may be better to restrict expressions with constants only to the same > >> file > >> even if we loose some benefits/flexibility in user code. > >> > >> Is this discussed? > >> > > > > I voted 'yes', but I change my mind. Sorry. > > If the RFC leaves door for constant optimization, I'll vote 'yes'. > > Constant look up is not a simple hash look up. > > > > One more additional note why I decided to vote 'no' for this. > We already have dynamic constant via define(). > > <?php > define('A', 123); > define('B', A*2); > > echo B; // 246 > ?> > > We can even use function return values with define(). > > <?php > function foo() { > return 123; > } > > define('A', foo()); > > echo A; // 123 > ?> > Yes, of course that's all possible, because define() is just a function and therefore the normal assignment rules apply. However, you didn't mention the biggest drawback to such an approach, which is that it creates *global* constants; there's no way to namespace them. > I prefer 'const' to be a static for better performance, since > we have dynamic constant by define() already. > Perhaps we'd have to consider how much performance is impacted on existing code, e.g. class A { const B = 123; } How much slower would that become if this RFC is accepted, cached or otherwise? > > Regards, > > -- > Yasuo Ohgaki > yohgaki@ohgaki.net > -- -- Tjerk