For performance sensitive tight loops, such as parsing and HTML
construction, to get the best performance it's necessary to think
about what PHP is doing on an opcode by opcode basis.
Certain flow control patterns cannot be implemented efficiently in PHP
without using "goto". The current example in Gerrit 708880
<https://gerrit.wikimedia.org/r/c/mediawiki/core/+/708880/5/includes/Html.php#545>
comes down to:
if ( $x == 1 ) {
action1();
} else {
action_not_1();
}
if ( $x == 2 ) {
action2();
} else {
action_not_2();
}
If $x==1 is true, we know that the $x==2 comparison is unnecessary and
is a waste of a couple of VM operations.
It's not feasible to just duplicate the actions, they are not as
simple as portrayed here and splitting them out to a separate function
would incur a function call overhead exceeding the proposed benefit.
I am proposing
if ( $x == 1 ) {
action1();
goto not_2; // avoid unnecessary comparison $x == 2
} else {
action_not_1();
}
if ( $x == 2 ) {
action2();
} else {
not_2:
action_not_2();
}
I'm familiar with the cultivated distaste for goto. Some people are
just parotting the textbook or their preferred authority, and others
are scarred by experience with other languages such as old BASIC
dialects. But I don't think either rationale really holds up to scrutiny.
I think goto is often easier to read than workarounds for the lack of
goto. For example, maybe you could do the current example with break:
do {
do {
if ( $x === 1 ) {
action1();
break;
} else {
action_not_1();
}
if ( $x === 2 ) {
action2();
break 2;
}
} while ( false );
action_not_2();
} while ( false );
But I don't think that's an improvement for readability.
You can certainly use goto in a way that makes things unreadable, but
that goes for a lot of things.
I am requesting that goto be considered acceptable for micro-optimisation.
When performance is not a concern, abstractions can be introduced
which restructure the code so that it flows in a more conventional
way. I understand that you might do a double-take when you see "goto"
in a function. Unfamiliarity slows down comprehension. That's why I'm
suggesting that it only be used when there is a performance justification.
-- Tim Starling