Newsgroups: php.internals
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Xref: news.php.net php.internals:107208
Date: Wed, 18 Sep 2019 17:10:30 +0200
To: Dmitry Stogov <dmitry@zend.com>
Cc: PHP Internals <internals@lists.php.net>,
	Nikita Popov <nikita.ppv@gmail.com>
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Message-ID: <20190918151030.D79495FB1A@mx.zeyos.com>
Subject: Constant propagation inside same compilation unit
From: ben.coutu@zeyos.com (Benjamin Coutu)

Hello,=0A=0ADuring performance testing of the (awesome) PHP 7.4 preloading =
feature I stumbled upon a lost opportunity I'd like to point out (though th=
is has nothing to do with preloading per se).=0A=0APlease consider the foll=
owing snippet:=0A=0Aconst TEST =3D 14;=0A=0Afunction d() {=0A  echo TEST;=
=0A}=0A=0AThe function "d" yields the following opcodes:=0A=0A#0 FETCH_CONS=
TANT "TEST" ~0=0A#1 ECHO ~0=0A#2 RETURN<-1> null=0A=0Ameaning, that the con=
stant "TEST" does not propagate into the function, though both, the constan=
t and the function, are defined in the same file.=0A=0ASame goes for this k=
ind of snippet:=0A=0Aclass D {=0A=09const C =3D TEST;=0A=0A=09public functi=
on d() {=0A=09=09echo self::C;=0A=09}=0A}=0A=0AHere the method "d" yields t=
he following opcodes:=0A=0A#0 FETCH_CLASS_CONSTANT "C" ~0=0A#1 ECHO ~0=0A#2=
 RETURN<-1> null=0A=0AInterestingly enough, class constants propagate as on=
e would expect:=0A=0Aclass D {=0A=09const C =3D 14;=0A=0A=09public function=
 d() {=0A=09=09echo self::C;=0A=09}=0A}=0A=0A#0 ECHO 14=0A#1 RETURN<-1> nul=
l=0A=0AI don't see why constants defined in the same file couldn't propagat=
e throughout the same compilation unit during compile time, as they can nev=
er have a different value (locally in that particular defining file) at run=
time.=0AIs this just an oversight or is there some deeper reasoning behind =
this that I'm simply missing?=0A=0AIn terms of preloading this would be par=
ticularly beneficial, cause one cloud then include/require files that defin=
e global constants before compiling files that contain those constants, the=
reby propagating their values throughout the code base during preloading. T=
hat would eliminate a lot of the runtime cost of (not so truly constant) co=
nstants.=0A=0APlease let me know your thoughts.=0A=0ACheers,=0A=0ABen=0A=0A=
-- =0A=0ABenjamin Coutu=0Aben.coutu@zeyos.com