PHP OPcache optimization levels - what are they?

Question

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.

Answer 1

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:
  • Convert constant operands to expected types
  • Convert conditional JMP with constant operands
  • Optimize static BRK and CONT
• 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