Adjust opcache.jit_hot_loop to a prime number

[arnaud-lb]

Loops whose number of iterations + 1 is a factor of opcache.jit_hot_loop will always be traced at the exact moment the loop condition will evaluate to false. As a result, these loops can never be JIT'ed successfully.

Example:

function f() {
    for ($i = 0; $i < 3; $i++) {
        var_dump($i);
    }
}

while (1) { f(); }

---- TRACE 1 start (loop) f() test.php:4
0006 T1 = IS_SMALLER CV0($i) int(3) ; op1(int)
0007 ;JMPNZ T1 0002
0008 RETURN null
---- TRACE 1 abort (exit from loop)

---- TRACE 1 start (loop) f() test.php:4
0006 T1 = IS_SMALLER CV0($i) int(3) ; op1(int)
0007 ;JMPNZ T1 0002
0008 RETURN null
---- TRACE 1 abort (exit from loop)

---- TRACE 1 start (loop) f() test.php:4
0006 T1 = IS_SMALLER CV0($i) int(3) ; op1(int)
0007 ;JMPNZ T1 0002
0008 RETURN null
---- TRACE 1 abort (exit from loop)

...

Here I adjust the default value of opcache.jit_hot_loop to a prime number, so this can not happen.

This results in 0.15% wall time improvement on the symfony demo benchmark.

[staabm]

Maybe add a comment that this needs to be a prime number?

[dstogov]

Here I adjust the default value of opcache.jit_hot_loop to a prime number, so this can not happen.

This will still happen in case when number of iterations is equal to opcache.jit_hot_loop.
Anyway, this makes sense.

The recommendation should also be reflected in documentation.