Bug #81090 [Ana->Csd]: Typed property performance degradation with .= operator

Edit report at https://bugs.php.net/bug.php?id=81090&edit=1 ID: 81090 Updated by: git@php.net Reported by: dktapps at pmmp dot io Summary: Typed property performance degradation with .= operator -Status: Analyzed +Status: Closed Type: Bug Package: Performance problem Operating System: Windows PHP Version: 7.4.19 Block user comment: N Private report: N New Comment: Automatic comment on behalf of nikic Revision: https://github.com/php/php-src/commit/82f6f6da67a265170fe533d97edb59520da0024f Log: Fixed bug #81090 Previous Comments: ------------------------------------------------------------------------ [2021-05-31 10:17:11] nikic@php.net This happens because typed properties perform binary operations into a temporary first, so that the original value is retained if a TypeError occurs. ------------------------------------------------------------------------ [2021-05-30 13:28:20] dktapps at pmmp dot io Profiling shows that the vast majority of the CPU time is spent in concat_function(), but only for the typed property. This bug also persists on PHP 8.0 with function JIT enabled. The performance degradation is exponential with higher loop count, which leads me to believe that every .= on a typed property is triggering a full string copy for some reason, despite the string's refcount being 1. With type (7m 30s, 1,000,000 loops): https://user-images.githubusercontent.com/14214667/120105960-05484200-c153-11eb-8540-00e8ea39e0a7.png Without type (<1s, 1,000,000 loops): https://user-images.githubusercontent.com/14214667/120105988-29a41e80-c153-11eb-8337-ed91a41fe1fc.png ------------------------------------------------------------------------ [2021-05-30 12:41:01] dktapps at pmmp dot io Description: ------------ There appears to be a change in the behaviour of .= with typed properties, as compared to typeless properties. The below script demonstrates a significant performance degradation with typed properties (in the ballpark of 2 orders of magnitude). Test script: --------------- https://gist.github.com/dktapps/8a5d839f4c884f92d0b56f04c3b7d7d4 Expected result: ---------------- The time per operation should only be slightly slower for typed properties than typeless properties (like it is for other things, like plain assignments). Actual result: -------------- The time per operation is 2 orders of magnitude worse on the first loop than the second one. On my machine, the script produces the following: Typed property time: 8701.32 ns/op Typeless property time: 211.446 ns/op ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=81090&edit=1