Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:108017
To: Rowan Tommins <rowan.collins@gmail.com>, "internals@lists.php.net"
	<internals@lists.php.net>
Thread-Topic: [PHP-DEV] Initializing constants once, with code?
Thread-Index: AQHVwZMGVGGpEwzQ00OYU/q+965QOqfcl9gCgADlCgCAAKjxAIAAHwMAgAARm1Q=
Date: Tue, 7 Jan 2020 00:11:09 +0000
Message-ID:
 <DM5PR07MB3067EF32FCE09B76DA2662A3F93C0@DM5PR07MB3067.namprd07.prod.outlook.com>
References: <5e0d723f.1c69fb81.e2ae8.24e2SMTPIN_ADDED_MISSING@mx.google.com>
 <74F2DBFC-E63C-428C-A37F-2D0CEE15AD0F@newclarity.net>
 <DM5PR07MB306700E1BEEB2F7D25B445EBF93D0@DM5PR07MB3067.namprd07.prod.outlook.com>
 <CAMuFt-6PYPyZ8ri-19dP1tE1TWmcW0j0utfmDvZtmn3cGe24kA@mail.gmail.com>
 <8D8F3E7E-137E-459B-B13A-9C7940731243@newclarity.net>,<f55c97a0-742f-5ae5-cbea-1e605b0c84a9@gmail.com>
In-Reply-To: <f55c97a0-742f-5ae5-cbea-1e605b0c84a9@gmail.com>
Accept-Language: en-CA, en-US
Content-Language: en-CA
x-ms-exchange-messagesentrepresentingtype: 1
x-tmn: [PGjpSs3KVJIfBiG4XHIl7aGCnStthlyk5yHmIR4sb+oGJUBK4lclkNkTp3FCysRI]
x-ms-publictraffictype: Email
x-incomingheadercount: 45
x-eopattributedmessage: 0
x-ms-office365-filtering-correlation-id: 923a2e8a-ab4c-4e41-44f9-08d793061943
x-ms-traffictypediagnostic: BN7NAM10HT125:
x-microsoft-antispam: BCL:0;
x-microsoft-antispam-message-info:
 AS7qdSPqmKArp93BvamsFni9wfQhw3Nc/A5j4/O9xoFEwmeFU0mLMqzV1NMo1mYRfWDb0T4d9aj+Kwng55TqHJLCAqpwBrW4UzLZKlWxS5Sfva3jo3zrBIFcq/7q2gomO+lLxuwlYEhKr5fDasQjrYY9InduuqJOr1MUZEWKR6DgSiSnX9ySuMubBBN9ZKW2
x-ms-exchange-transport-forked: True
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-OriginatorOrg: hotmail.com
X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000
X-MS-Exchange-CrossTenant-Network-Message-Id: 923a2e8a-ab4c-4e41-44f9-08d793061943
X-MS-Exchange-CrossTenant-rms-persistedconsumerorg: 00000000-0000-0000-0000-000000000000
X-MS-Exchange-CrossTenant-originalarrivaltime: 07 Jan 2020 00:11:09.8690
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Internet
X-MS-Exchange-CrossTenant-id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN7NAM10HT125
Subject: Re: [PHP-DEV] Initializing constants once, with code?
From: tysonandre775@hotmail.com (tyson andre)

=0A=
> const DEBUG_MODE =3D false;=0A=
> =0A=
> function foo() {=0A=
>  if (DEBUG_MODE) {=0A=
>    // Do stuff.=0A=
>  }=0A=
> }=0A=
> =0A=
> PHP will currently (AIUI) inline the DEBUG_MODE value at compile time,=0A=
> notice that the code path is unreachable, and omit it from the compiled o=
pcodes entirely.=0A=
> (I'm pretty sure it does that now, at least; if not, it certainly can be.=
)=0A=
=0A=
It actually can't do that safely for global constants.=0A=
(But it can and does do that for class constants, *within the class*)=0A=
For example, if you declare a constant twice (declare() or const DEBUG_MODE=
 =3D), then php emits a notice, but continues executing keeping the value t=
he constant previously had.=0A=
You can use opcache's debug output to confirm that.=0A=
=0A=
php -d zend_extension=3Dopcache.so -d opcache.enable_cli=3D1  \=0A=
   -d opcache.file_cache=3D -d opcache.opt_debug_level=3D0x20000 example.ph=
p=0A=
=0A=
```=0A=
<?php=0A=
=0A=
const DEBUG =3D false;=0A=
class TempClass {=0A=
    const DEBUG =3D false;=0A=
    public function main() {=0A=
        if (DEBUG) { echo "In debug for global\n"; }=0A=
        if (TempClass::DEBUG) { echo "In debug for class const\n"; }  // op=
timized out=0A=
    }=0A=
}=0A=
/*=0A=
L0 (7):     T0 =3D FETCH_CONSTANT string("DEBUG")=0A=
L1 (7):     JMPZ T0 L4=0A=
L2 (7):     ECHO string("In debug for global=0A=
")=0A=
L3 (9):     RETURN null=0A=
L4 (9):     RETURN null=0A=
 */=0A=
```=0A=
=0A=
A 'static const' syntax enforcing that constants can only be declared in on=
e place would help in making these sorts of optimizations easier=0A=
(with changes to opcache, for expressions that can be evaluated at compile =
time)=0A=
But so would preloading in the future, since php could likely recompile fun=
ctions with the constants known from the preload script=0A=
(maybe it does this already).=