There is an opcache.optimization_level php.ini directive. It is a bitmask a defaults to 0xffffffff - so by default OPcache does all the optimizations.
What kind of optimizations does OPcache do? What passes on bytecode are done?
Follow-up question: is there a code pattern that OPcache can optimise very well? For example, HHVM can skip execution of files that contain only class/function declarations and it just fills class/function tables.
The bits of opcache.optimization_level correspond to:
- bit 0 - pass 1:
- CSE - constants subexpressions elimination
- Sequences of ADD_CHAR/ADD_STRING optimization
- convert CAST(IS_BOOL,x) into BOOL(x)
- convert INTI_FCALL_BY_NAME + DO_FCALL_BY_NAME into DO_FCALL
- bit 1 - pass 2:
- bit 2 - pass 3:
- Convert $a = $a + expr into $a += expr
- Convert $a++ into ++$a
- Optimize series of JMPs
- bit 3 - pass 4:
- PRINT and ECHO optimization - removed
- bit 4 - pass 5:
- block optimization (the most expensive optimization pass which perform many different optimization patterns based on CFG - control flow graph)
- bit 8 - pass 9:
- register allocation (allows re-usage of temporary variables)
- bit 9 - pass 10:
- remove NOPs
I've looked around to see if I can find anything on any code patterns that it handles better than others, but I haven't had any luck.
Information from https://github.com/zendtech/ZendOptimizerPlus/blob/master/Optimizer/zend_optimizer.c and https://gist.github.com/ck-on/4959032?ocp.php
来源：https://stackoverflow.com/questions/21181045/php-opcache-optimization-levels-what-are-they