Visualization of JITted code

What does the jitviewer really do?

At the top of the page, you will see the list of pieces of code which has been compiled by the JIT. You will see entries for both normal loops and for "entry bridges". This is not the right place to discuss the difference between those, but you most probably want to look at loops, because usually it's where most of the time is spent.

Note that for each loop, you will see the name of the function which contains the first instruction of the loop. However, thanks to the inlining done by the JIT, it will contain also the code for other functions.

Once you select a loop, the jitviewer shows how the JIT has compiled the Python source code into assembler in a hierarchical way. It displays four levels:

• Python source code: only the lines shown in azure have been compiled for this particular loop, the ones in gray have not.

• Python bytecode, the one you would get by doing:

  def f(a, b):
     return a + b
  
  import dis
  dis.dis(f)
  

  The opcodes are e.g. LOAD_FAST, LOAD_GLOBAL etc. The opcodes which are not in bold have been completely optimized aways by the JIT.

• Intermediate representation of jit code (IR). This is a combination of operations (like integer addition, reading fields out of structures) and guards (which check that the assumptions we made are actually true). Guards are in red. These operations are "at the same level as C": so, for example, + takes two unboxed integers which can be stored into the register of the CPU.

• Assembler: you can see it by clicking on "Show assembler" in the menu on the right.

Sometimes you'll find that a guard fails often enough that a new piece of assembler is required to be compiled. This is an alternative path through the code and it's called a bridge. You can see bridges in the jitviewer when there is a link next to a guard. For more information about purpose look up the jit documentation.

I'm still confused

Jitviewer is not perfect when it comes to explaining what's going on. Feel free to pop up on IRC or send us a mail to the mailing list, we'll try to explain and/or improve the situation. Consult the contact page for details.

Cheers,
fijal & antocuni