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Reason for ZEND_JIT_COUNTER_INIT set to 32531?

php.internals

Su, Tao

4 years ago

Hi Internal,
I am not sure this is a correct mail list for asking source code related question.
If not, I am sorry. Appreciate you guys' help.

I am reading PHP8 JIT related source code and found that zend_jit.h has a magic definition
ZEND_JIT_COUNTER_INIT as 32531 which I do not quite understand how this number was chosen and why?
Anybody can provide some historic information?
#define ZEND_JIT_COUNTER_INIT         32531

A code example using this definition (looks like the 2nd parameter computes into a fixed number if hot_func is set by php.ini)
ZEND_OPCODE_TAIL_CALL_EX(zend_jit_trace_counter_helper,
((ZEND_JIT_COUNTER_INIT + JIT_G(hot_func) - 1) / JIT_G(hot_func)));


=======================================
Tony Su (Su, Tao)
make a 'lazy' programmer diligently with efficiency

Dmitry Stogov

4 years ago

This it's my choice.
This number is common for all JIT counters (hot_loop, hot_func, hot_return).
It's value was selected to provide the best precision when different
counters have different thresholds.
Unfortunately, I can't remember the exact equations or logic I used at that
time.

Thanks. Dmitry.

On Thu, Jan 13, 2022 at 1:01 PM Su, Tao <tao.su@intel.com> wrote:

Su, Tao

4 years ago

Thanks Dmitry for the information.

With a few debug sessions, I found that 32531 is working well
for any thresholds, and is also a big prime number :-).

-----Original Message-----
From: Dmitry Stogov <dmitrystogov@gmail.com>
Sent: Monday, January 17, 2022 1:37 PM
To: Su, Tao <tao.su@intel.com>
Cc: PHP internals <internals@lists.php.net>
Subject: Re: [PHP-DEV] Reason for ZEND_JIT_COUNTER_INIT set to 32531?

This it's my choice.
This number is common for all JIT counters (hot_loop, hot_func, hot_return).
It's value was selected to provide the best precision when different counters have different thresholds.
Unfortunately, I can't remember the exact equations or logic I used at that time.

Thanks. Dmitry.

On Thu, Jan 13, 2022 at 1:01 PM Su, Tao <tao.su@intel.com> wrote:

Dmitry Stogov

4 years ago

Oh, probably this is just the biggest prime number that fits into a signed
16-bit number :)

On Mon, Jan 17, 2022 at 10:49 AM Su, Tao <tao.su@intel.com> wrote:

Larry Garfield

4 years ago

On Mon, Jan 17, 2022, at 6:12 AM, Dmitry Stogov wrote:

> Oh, probably this is just the biggest prime number that fits into a signed
> 16-bit number :)

Seems like the sort of thing that should be documented inline.  (Also, why it needs to be a prime number.)

--Larry Garfield

Su, Tao

4 years ago

Interesting! I would like to dig deeper or ask google for more information.
I had an impression that I once read a paper about this kind of magic number, but not sure where it was from.

@Dmitry Stogov<mailto:dmitrystogov@gmail.com> With your permission, if I found anything, I would like to add a few comments into the source code file for records.

From: Dmitry Stogov <dmitrystogov@gmail.com>
Sent: Monday, January 17, 2022 8:12 PM
To: Su, Tao <tao.su@intel.com>
Cc: PHP internals <internals@lists.php.net>
Subject: Re: [PHP-DEV] Reason for ZEND_JIT_COUNTER_INIT set to 32531?

Oh, probably this is just the biggest prime number that fits into a signed 16-bit number :)

On Mon, Jan 17, 2022 at 10:49 AM Su, Tao <tao.su@intel.com<mailto:tao.su@intel.com>> wrote:
Thanks Dmitry for the information.

With a few debug sessions, I found that 32531 is working well
for any thresholds, and is also a big prime number :-).

-----Original Message-----
From: Dmitry Stogov <dmitrystogov@gmail.com<mailto:dmitrystogov@gmail.com>>
Sent: Monday, January 17, 2022 1:37 PM
To: Su, Tao <tao.su@intel.com<mailto:tao.su@intel.com>>
Cc: PHP internals <internals@lists.php.net<mailto:internals@lists.php.net>>
Subject: Re: [PHP-DEV] Reason for ZEND_JIT_COUNTER_INIT set to 32531?

This it's my choice.
This number is common for all JIT counters (hot_loop, hot_func, hot_return).
It's value was selected to provide the best precision when different counters have different thresholds.
Unfortunately, I can't remember the exact equations or logic I used at that time.

Thanks. Dmitry.

On Thu, Jan 13, 2022 at 1:01 PM Su, Tao <tao.su@intel.com<mailto:tao.su@intel.com>> wrote: