Re: Exploit fully packed array/hash property

I've committed the safe part of the patch (almost your original idea). http://git.php.net/?p=php-src.git;a=commitdiff;h=9ded1b4edbb140520e060de597267b3cb439f4c4 The part related to HASH_FLAG_LONG_KEYS/HASH_FLAG_STRING_KEYS is here https://gist.github.com/dstogov/059c73e7c732fe55110ebf0ca18b4859 as I said, in current state, it makes reduction instead of improvement. Probably, if we find more use cases for these flags, it may make improvement. Thanks. Dmitry. ________________________________ From: Benjamin Coutu <ben.coutu@zeyos.com> Sent: Wednesday, October 19, 2016 9:58:01 PM To: Dmitry Stogov; Xinchen Hui; Nikita Popov; Joe Watkins; Bob Weinand; Andrea Faulds Cc: PHP Internals Subject: Re: [PHP-DEV] Exploit fully packed array/hash property Hi Dmitry, On second thought, I might have dismissed your HASH_FLAG_*_KEYS idea prematurely. Of course we will have to set/unset the flag in parts of the code that are very hot and naturally that will lead to a regression in terms of CPU instructions. But in regards to your idea of possibly eliminating redundant checks on every loop iteration, I think it could be interesting. It's a tradeoff between maintaining the necessary flags and avoiding repeatedly iterating over branches. Given that bitwise operations on these flags are just about the cheapest operations the CPU can perform (no pipeline stalls and flags usually already in L1 cache at that point), and branching inside a loop being extremely costly (with the risk of pipeline stalls caused by branch miss prediction, which might even happen repeatedly, and EXPECTED/UNEXPECTED macros undesirable in most of these cases), choosing to maintain those flags on write operations in order to speed up read operations suddenly looks much more appealing. In many operations that deal with the hash table in bulk we might even be able to set the flag just once upfront. So maybe we should investigate further. Your quick test is not conclusive because you have effectively only benchmarked the negatives without settling it with the positives. I don't know wether the additional code complexity is worth the effort though. Personally I'd like to focus on getting more out of the packed hash table case, meaning trying to preserve the packed characteristics and utilizing dedicated code paths as much as possible throughout the entire code base while using the existing infrastructure to do so without additional overhead. Packed arrays are very ubiquitous after all, and as HT_IS_PACKED always implicates HASH_FLAG_LONG_KEYS, there is not that great of a need for the latter if we can ensure to use packed arrays wherever possible. On another note, and this comes without any caveats, we could at least use HT_IS_WITHOUT_HOLES to get rid of repeated checks for Z_TYPE(p->val) == IS_UNDEF inside loops. This would not entail any overhead on write. What do you think? Cheers, Benjamin ========== Original ========== From: Dmitry Stogov <dmitry@zend.com> To: Benjamin Coutu <ben.coutu@zeyos.com>, Xinchen Hui <xinchen.h@zend.com>, Nikita Popov <nikita.ppv@gmail.com>, Joe Watkins <pthreads@pthreads.org>, "Bob Weinand" <bobwei9@hotmail.com>, Andrea Faulds <ajf@php.net> Date: Wed, 19 Oct 2016 18:02:53 +0200 Subject: Re: [PHP-DEV] Exploit fully packed array/hash property This is an option. If nobody propose a better solution, I'll prepare the patch tomorrow (this solution won't make BC breaks at all). BTW: I think, HASH_FLAG_*_KEYS may be used to eliminate redundant checks on every loop iteration in some functions. Thanks. Dmitry. -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: http://www.php.net/unsub.php