Typed property assign should be optimized more

[mvorisek]

Description

Classes are very often used as a simple data holding structure which is created and destroyed millions times per script lifecycle/run.

In https://3v4l.org/CMktE/rfc#vgit.master I have discovered a "class like struct" is slower than an array. This is something not fully surprising on the first sight.

However, https://3v4l.org/hSSFJ/rfc#vgit.master, same demo with propoted properties only, with empty constructor, is not (much if any) faster.

Initially I thought the interpreted code in the constructor will contribute the most to the total run time. This seems to be not true, it seems the most of the time is spent in object allocation and release. If an object has no cycles, it should be fast. I hope there is a room to optimize the object allocation/release more and make this core part of php faster.

let's focus on typed property assign performance solely here

repro: https://3v4l.org/FGeEf

[iluuu1994]

Well, you're using multiple things that you don't for arrays:

• A constructor call
• Arguments + argument types
• Argument default values
• Parameter types
• Null handling (!== null)

Removing those gets you much closer to the array number. https://3v4l.org/JilKS/rfc#vgit.master

There's some inherent overhead for objects, (object initialization, object handlers for property assignment, one assignment per property, object store handling, a more involved cleanup, etc), but we can't just remove that. I quickly looked at a profile using Valgrind but there's nothing obvious standing out.

[mvorisek]

https://3v4l.org/FrgAY - typed property assign is one area that should be possible to be improved

final class Cl
{
    public int $start;
    public function __construct(int $start) {
        $this->start = $start;
    }
}

should have about the same performace as

final class Cl
{
    public $start; // <-- notice type was removed here
    public function __construct(int $start) {
        $this->start = $start;
    }
}

as in the code above the type of the $start is always int, ie. matching the typed property type

[bwoebi]

It's true that we don't have a type-check eliding assign-mode, but that's Optimizer territory, to infer that from the function type too.

[mvorisek]

even not for a promoted property https://3v4l.org/XBQGA/rfc#vgit.master_jit

[iluuu1994]

should have about the same performace as

This is a very simplistic view. PHP is not a compiled language. These kinds of optimizations usually require multiple specializations, i.e. multiple versions of the same function with more/less optimizations. However, introducing such functions also reduces instruction cache efficiency, and can often have the opposite effect.

Unfortunately, it's usually not possible to determine whether a change is useful without making it, and then performing benchmarks.

[mvorisek]

Maybe optimizing the current property/promoted specialization will be enough, in any case, typed property assign is very frequently used code pattern and https://3v4l.org/RFeIn/rfc#vgit.master_jit demo shows the performance can almost double.

[iluuu1994]

@bwoebi The code still contains tons of UNEXPECTED hints for typed properties. I wonder whether those should be dropped, or replaced with something more specific. In general, __builtin_expect is recommended only when you are quite sure that the given branch is taken. I thought there was an attribute that allows specifying expected percentage of branches, but I cannot seem to find it right now. Edit: It's __builtin_expect_with_probability.