HHVM not compiling JWT library as expected - hhvm

Question

When I run code like this
$before = microtime(true);
$amount = 90000000;
$sum = 0;
for ($i = 0; $i < $amount; $i++) {
$sum += $i;
}
echo $sum;
$after = microtime(true);
echo '<br>'.($after-$before);
It runs much slower than if I wrapped the whole thing inside a function because HHVM does not compile for loops in the global scope.
echo "loop in function <br /><br />";
function run_loop ($amount, $sum) {
for ($i = 0; $i < $amount; $i++) {
$sum += $i;
}
return $sum;
}
echo run_loop($amount, $sum);
The last one runs 10x faster than the first one, even tho the first one still runs faster than stock PHP 5.6.
But when I try to do something like this using the JWT library from firebase
https://github.com/firebase/php-jwt
And test it like this
require_once("JWT.php");
$before = microtime(true);
$amount = 100000;
for ($i = 0; $i < $amount; $i++) {
$jwt = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ';
$decoded = JWT::decode($jwt, 'secret', array('HS256'));
}
echo $decoded->name;
then this runs just as when I wrap it inside a function
echo "loop in function <br /><br />";
function run_loop ($amount) {
$jwt = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ';
for ($i = 0; $i < $amount; $i++) {
$decoded = JWT::decode($jwt, 'secret', array('HS256'));
}
return $decoded->sub;
}
echo run_loop($amount);
This leads me to believe that HHVM does not compile the JWT library because they run just as fast even if it is for loop in the global scope or wrapped in a function. So my question is. How can I check which function or method is not supported by HHVM inside the JWT library? Is there a way for me to find all supported functions by HHVM? Do you think it is because of the JWT library?

Answer 1

HHVM can't optimize code running at the top level (i.e., outside a function) very well.
In the first example, most of the work is happening outside a function -- the "work" is the $sum += $i. Moving that into a function, your second example, allows HHVM to optimize it pretty well, since it's really trivial. (Though it's not a good benchmark because it's so unrealistic!)
In your third and fourth examples, very little work is happening in the code you showed. I suspect that JWT::decode is responsible for most of the work being done -- and it's already in a function. Moving the code calling it out of toplevel, while a good idea, doesn't help as much as before because it's doing less of the relative work. The heavy lifting was already happening in a function, namely JWT::decode.
As to how to allow HHVM to optimize JWT::decode better, that's a very complicated question. If you're interested, you can build HHVM from source yourself, use different profiling tools (such as the Linux perf command) to see where HHVM is spending its time, and optimize that.