What is the difference between while(true) and for(;;) in PHP?

Question

Is there any difference in PHP between while(true) and for(;;) besides syntax and readability?

Nethermost

@Touchpad I am curious to know the answer specific to PHP – Nethermost

Is there preoptizations in php? :) – Attire

codepad.viper-7.com/awq3Mh generally I am seeing a tiny bit faster performance from the while(true) – Lapse

PHP is based on C++, so this would be a duplicate of the above question. There is no difference between the two. – Perfumery

...And it scales. The while loop seems consistently measurably faster on large loops codepad.viper-7.com/19nv5D Internally, I can't tell you why. – Lapse

@AzizSaleh so PHP is compiled to C++ and then executed? I though PHP was written in C and compiled to its own bytecode. – Nethermost

My bad, yes I meant C not C++ (I confuse them most of the time). – Perfumery

If you want to write less, you can just use while(1) – Puccoon

@user3132718: for(;;) is even shorter (by one character). The length doesn't matter, but readability does. – Wentz

Are people actually choosing whether to use while(true) or for(;;) based on how many opcodes are emitted? In PHP? Isn't that somewhat, just a little bit of a micro-optimization? – Raybin

If you are saving 1 opcode inside a tight loop that only contains 10-20 opcodes, that's potentially a 5-10% speed increase, which could be significant. Before the question was asked, it was possible the answer was more than 1 opcode difference. – Trichome

@Raybin the question was to understand the difference as far as the engine is concerned. The rational decision is still made based on readability, i.e. just use while(true) – Nethermost

Why was this put on hold as opinion-based? Maybe if the question asked about readability, but it explicitly excluded that. Crazy. – Conn

@IanGoldby It's reopened now. – Adam

Answer 1

Ok, so first off, let me say this: Use while(true), as it gives the most semantic meaning. You need to parse for (;;) as it's not something you see often.

With that said, let's analyze:

Opcodes

The code

while(true) {
    break;
}
echo "hi!";

Compiles down to the opcodes:

0: JMPZ(true, 3)
1: BRK(1, 3)
2: JMP(0)
3: ECHO("hi!")

So basically, it does a check if "true", and if not, jumps to the 4th opcode which is the echo opcode). Then it breaks (which is really just a static jump to the 4th opcode). Then the end of the loop would be an unconditional jump back to the original check

Compare that to:

for (;;) {
    break;
}
echo "hi!";

Compiles down to:

0: JMPZNZ(true, 2, 4)
1: JMP(0)
2: BRK(1, 4)
3: JMP(1)
4: ECHO("hi!")

So we can immediately see that there's an extra opcode in the for(;;) version.

Opcode Definitions

JMPZ(condition, position)

This opcode jumps if the condition is false. If it is true, it does nothing but advance one opcode.

JMPZNZ(condition, pos1, pos2)

This opcode jumps to pos1 if the condition is true, and pos2 if the condition is false.

JMP(position)

This opcode always jumps to the opcode at the specified position.

BRK(level, position)

This breaks level levels to the opcode at position

ECHO(string)

Outputs the string

Are They The Same

Well, looking at the opcodes, it's clear that they are not identical. They are ==, but not ===. The while(true) loop does a conditional jump followed by code followed by an unconditional jump. The for(;;) loop does a conditional jump, followed by code, followed by an unconditional jump, followed by another unconditional jump. So it does an extra jump.

Opcache

In 5.5, the Optimizer portion of opcache will optimize static conditional jumps.

So that means the while(true) code will optimize down to:

0: BRK(1, 2)
1: JMP(0)
2: ECHO("hi!")

And for(;;) loop becomes:

0: BRK(1, 2)
1: JMP(0)
2: ECHO("hi!")

This is because the optimizer will find and optimize out jump-chains. So if you're using 5.5's built-in opcache, they will be identical...

Caution

This is a complete and utter micro-optimization to base a decision on. Use the readable one. Don't use one based on performance. The difference is there, but it's trivial.

Unesco

What’s the point of the JMP(1) as the first instruction in the optimized while version? Isn’t that effectively a no-op? Why doesn’t it get optimized away? – Oxidimetry

@bdesham: it does appear to get optimized away: lxr.php.net/xref/PHP_TRUNK/ext/opcache/Optimizer/pass3.c#134 It also looks like JMP(2); JMP(0); will get optimized down to JMP(2);, but I can't tell immediately if it will be optimized further to a NOP... – Unesco

Excellent analysis! The identical behavior results in identical optimization (as it should, ideally), and the more semantic construction has fewer opcodes pre-optimization, making it the "better" choice for such micro-optimization, reinforcing the fact that while(true) should be preferred. – Whereof

"Use while(true), as it gives the most semantic meaning. You need to parse for (;;) as it's not something you see often." Personally, I parse (;;) as a crying face, because why are you writing infinite loops? That makes me sad. – Incongruous

@mikeTheLiar Any particular reason as to why? – Aberdare

@Dan it's usually indicative that there's something smelly going on. Not always, but usually. – Incongruous

@mikeTheLiar Any event loop is, technically, a simple infinite loop polling for input. As for the "you need to parse for (;;)" part -- it's actually quite often used in C instead of while (1) because linters complain less about the for variant [citation needed] – Chrysoprase

@ST3 I used codepad.viper-7.com to do it online. But check out a blog I wrote on the topic and Another blog on the subject... – Unesco

@akled no. Because "semantic meaning" is context free. You just need to see the structure itself to understand what it does. As opposed to "pragmatic meaning" which requires understanding of the context (surrounding content) to understand the meaning implied. foreach ($a as $b) {} requires understanding of what $a may be (and its behaviors). Which is not semantic, but pragmatic meaning. while(true) is purely semantic, as nothing but the grammar presented in front of you is needed to decipher the meaning. – Unesco