On 9/18/19 7:18 PM, Nikita Popov wrote: > On Wed, Sep 18, 2019 at 5:10 PM Benjamin Coutu <ben.co...@zeyos.com > <mailto:ben.co...@zeyos.com>> wrote: > > Hello, > > During performance testing of the (awesome) PHP 7.4 preloading > feature I stumbled upon a lost opportunity I'd like to point out > (though this has nothing to do with preloading per se). > > Please consider the following snippet: > > const TEST = 14; > > function d() { > echo TEST; > } > > The function "d" yields the following opcodes: > > #0 FETCH_CONSTANT "TEST" ~0 > #1 ECHO ~0 > #2 RETURN<-1> null > > meaning, that the constant "TEST" does not propagate into the > function, though both, the constant and the function, are defined in > the same file. > > Same goes for this kind of snippet: > > class D { > const C = TEST; > > public function d() { > echo self::C; > } > } > > Here the method "d" yields the following opcodes: > > #0 FETCH_CLASS_CONSTANT "C" ~0 > #1 ECHO ~0 > #2 RETURN<-1> null > > Interestingly enough, class constants propagate as one would expect: > > class D { > const C = 14; > > public function d() { > echo self::C; > } > } > > #0 ECHO 14 > #1 RETURN<-1> null > > I don't see why constants defined in the same file couldn't > propagate throughout the same compilation unit during compile time, > as they can never have a different value (locally in that particular > defining file) at runtime. > Is this just an oversight or is there some deeper reasoning behind > this that I'm simply missing? > > In terms of preloading this would be particularly beneficial, cause > one cloud then include/require files that define global constants > before compiling files that contain those constants, thereby > propagating their values throughout the code base during preloading. > That would eliminate a lot of the runtime cost of (not so truly > constant) constants. > > Please let me know your thoughts. > > > This optimization is unsafe, because the constant may already be > previously defined, in which case the "const" will be ignored with a > notice. There is an optimization flag that enables it, try > opcache.optimization_level=-1. > > Unfortunately I just noticed that I missed this notice during "engine > warning reclassification" RFC, because it occurs in zend_constants. > Otherwise we might have changed it to an exception. (Though given recent > discussions, probably not.) > > Nikita
I proposed this 3 years ago (for PHP-7.1), but then delayed for PHP-8. https://wiki.php.net/rfc/constant_redefinition Thanks. Dmitry.
