Improve performance of instantiating exceptions/errors

[nielsdos]

The class structure is fixed, so it makes no sense to go through all the logic of looking up property info etc.
This patch introduces a local function zend_obj_prop_num_checked() to help with that.

For this benchmark:

for ($i = 0; $i < 1000000; $i++)
   new Error;

On an i7-4790:

Benchmark 1: ./sapi/cli/php  x.php
  Time (mean ± σ):     141.6 ms ±   9.3 ms    [User: 138.7 ms, System: 2.0 ms]
  Range (min … max):   135.4 ms … 177.7 ms    20 runs

Benchmark 2: ../RELx64_old/sapi/cli/php x.php
  Time (mean ± σ):     214.1 ms ±   7.0 ms    [User: 207.6 ms, System: 5.0 ms]
  Range (min … max):   206.6 ms … 230.9 ms    13 runs

Summary
  ./sapi/cli/php  x.php ran
    1.51 ± 0.11 times faster than ../RELx64_old/sapi/cli/php x.php

For this benchmark:

for ($i = 0; $i < 1000000; $i++)
    new Exception("message", 0, null);

On an i7-4790:

Benchmark 1: ./sapi/cli/php  x.php
  Time (mean ± σ):     184.3 ms ±   9.5 ms    [User: 181.2 ms, System: 1.8 ms]
  Range (min … max):   173.8 ms … 205.1 ms    15 runs

Benchmark 2: ../RELx64_old/sapi/cli/php x.php
  Time (mean ± σ):     253.7 ms ±   7.0 ms    [User: 247.6 ms, System: 4.6 ms]
  Range (min … max):   245.7 ms … 263.7 ms    11 runs

Summary
  ./sapi/cli/php  x.php ran
    1.38 ± 0.08 times faster than ../RELx64_old/sapi/cli/php x.php

For this benchmark:

for ($i = 0; $i < 1000000; $i++)
    new ErrorException("message", 0, 0, "xyz", 0, null);

On an i7-4790:

Benchmark 1: ./sapi/cli/php  x.php
  Time (mean ± σ):     223.6 ms ±   7.7 ms    [User: 220.1 ms, System: 2.4 ms]
  Range (min … max):   216.9 ms … 242.5 ms    12 runs

Benchmark 2: ../RELx64_old/sapi/cli/php x.php
  Time (mean ± σ):     343.5 ms ±   8.1 ms    [User: 337.1 ms, System: 4.6 ms]
  Range (min … max):   337.3 ms … 362.8 ms    10 runs

Summary
  ./sapi/cli/php  x.php ran
    1.54 ± 0.06 times faster than ../RELx64_old/sapi/cli/php x.php

[morrisonlevi]

I haven't measured recently but I know when people say that exceptions are expensive, they also generally factor in walking the call stack.

My employer actually has a blog article about this, written by my coworker @realFlowControl: https://www.datadoghq.com/blog/php-exception-profiling/. The blog is a sort of advertisement for the exception profiler, of course, but it has a meaningful benchmark in it that you could run that's roughly 85 call frames deep, which matched a real performance issue one of our customers had.

I'd be very curious to see your benchmarking numbers with this patch. Of course, I support any maintainable performance improvement, so in that sense, I definitely approve! I just want to understand on a deeper stack, how much this helps. And maybe adjust the "up to 50% faster" wording to be a bit more realistic :)

[nielsdos]

I haven't measured recently but I know when people say that exceptions are expensive, they also generally factor in walking the call stack.

Sure, but I'm also convinced that can be improved performance-wise 🙂

(...) but it has a meaningful benchmark in it that you could run that's roughly 85 call frames deep, which matched a real performance issue one of our customers had.

I might take a look at using that as a benchmark to improve backtrace construction, if that's something I would work on in the future.

And maybe adjust the "up to 50% faster" wording to be a bit more realistic :)

Sure

[bwoebi]

You sadly have to do the garbage-move dance as you cannot rely on the __construct being called before any object would have been assigned to these values - at least those properties which can contain objects. I.e. previous in Exception and ErrorException.

[iluuu1994]

I don't think you can depend on this since #17870. These properties are not final, so a child class may add hooks to them. A simple solution would be to check for ce->num_hooked_props == 0 and use a fallback to zend_update_property_ex() otherwise.

[bwoebi]

It's fine for the private properties though.

[iluuu1994]

Indeed, that would be fine. Not all properties are here though.

[arnaud-lb]

This is great. The same technique could possibly be used for other internal classes that have standard properties.

Just some thoughts, not meant to block this PR:

We could use stub files to generate offset macros/constants, and expose zend_obj_prop_num_checked(), or generate accessors for each prop.

Since Exceptions are almost user land classes (they have no custom handlers except clone_obj), I'm wondering how it would perform if we actually declared these classes in user land (c.f. #18204). Edit: it's slower.