Creating an implementation for a dynamic language using just in time compilation (JIT) techniques involves a lot of compromises mainly between complexity of the implementation, speed, warm-up time and memory usage.
Especially speed is a difficult trade-off because it’s very easy to end-up spending more time optimizing a piece of code and emitting the assembly than we will ever be able to save by executing faster than executing it in a less optimized way.
This causes most JIT language implementations to use an approach of different tiers – approaches to running the code and different amount of optimizations done depending on how often the specific piece of code gets executed. Thereby reducing the chance that more time will get spend transforming the code in to a more efficient representation than it would take to execute it in a less efficient representation.
baseline just in time compiler
We noticed that our interpreter is interpreting code quite slowly while the LLVM tier takes a lot of time to JIT (even with the object cache which made it much faster) so it was obvious that we either have to speed the interpreter up or introduce a new tier in between.
There are well-known problems with our interpreter, mainly it’s slow because it does not represent the code in a contiguous block of memory (bytecode) but instead it involves a lot of pointer chasing because we reuse our AST nodes. Fixing this would be comparable easy but we still thought that this will only improve the performance a little bit but will not give us the performance we want.
About a year ago we introduced a new execution tier instead, the baseline jit (bjit). It is used for python code which is executed a medium number of times and therefore lives between the interpreter and the LLVM JIT tier. In practice this means most code which executes more than 25 times will currently end-up in the bjit and if it gets executed more than about 2500 times we will recompile it using the LLVM tier.
The main goal of the bjit is to generate reasonable machine code very fast and making heavy use of inline caches to get good performance (more on this further down).
It involved a number of design decisions (some may change in the future) but what we currently ended up with:
- reuse our inline cache mechanism
- it transform the bjit from only being able to remove the interpretation overhead (which is quite low for python – it depends on the workload but probably not more than 20%) to a JIT which actually is able to improve the performance by a much larger factor
- generate machine code for a basic block at a time
- only generating code for blocks which actually get executed reduces the time to generate code and memory usage at the expense of not being able to do optimizations across blocks (at the moment)
- highly coupled to the interpreter and using the same frame format
- making it very easy and fast to switch between the interpreter and bjit at every basic block start
- we can fallback to the interpreter for blocks which contain operations which we are unable to JIT or for blocks which are unreasonable to JIT because the may be very large and generating code for them would cost too much memory
- makes it easy to tier up to the bjit when we interpret a function which contains a loop with a large amount of iterations
- does not use type analysis and all code it generates makes no assumptions about types
- this makes it always safe to execute code in the bjit
- type specific code is only inside the ICs and always contains a call to a generic implementation in case the assumptions don’t hold
- all types are boxed / real python objects
- it collects type information which we will use in LLVM tier to generate more optimized code later on if the function turns out to be hot
- if an assumption in the LLVM tier turns out to be wrong we will deoptimize to the interpreter/bjit
Inline Cache
the inline cache mechanism is used in the LLVM tier and in the baseline JIT and is currently responsible for most of the performance improvements over the cpython interpreter (which does not use this technique). It removes most of the dynamic dictionary lookups and additional branching which a “normal” python interpreter often has to do. For every operation where we can use ICs we will provide a block of memory and fill it with a lot of nops and a call to the generic implementation of the operation. Therefore the first time we execute the code we will call into the generic implementation but it will trace the execution of the operation using the arguments supplied. It then fills in the block of memory a more optimized type specific version of the operation which we can use the next time we will hit this IC slot if the assumptions the trace made still hold.
Here is a simple diagram of how a IC with two slots could look like:
A simple example will make it easier to understand what we are doing.
For the python function:
def f(a, b):
return a + b
The CFG will look like this:
Block 0 'entry'; Predecessors: Successors:
#0 = a
#1 = b
#2 = #0+#1
return #2
We will now look at the IC for #2 = #0+#1
For example if we call f(1, 1) for the first time the C++ function binop() will trace the execution and fill in the memory block with the code to do an addition between two python int objects (it uses a C++ helper function called intAddInt()):
Notice the guard comparisons inside the first IC slot, they make sure that we will only use the more optimized implementation of the operation if it’s safe to do so (in this case the arguments have the same types and the types did not get modified since the trace got created) and otherwise jump to the next IC slot. Or if there is no optimized version call the generic implementation which is always safe to execute.
Most code is not very dynamic which means filling in one or two slots with optimized versions of a operation is enough to catch all encountered cases.
For example if we later on call f("hello ", "world") we will add a new slot in the IC:
We use ICs for nearly all operations not only for binary ones like the example showed. We also use them for stuff like global scope variable lookup, retrieving and setting attributes and much more (we also support more than two slots). Not all traces call helper functions like we have seen in the example some are inlined in the slot.
Pyston will overwrite slots if they already generated slots turn out to be invalid or unused because they assumption of the trace don’t hold anymore. Some code (luckily this is uncommon) is highly dynamic in this cases we will try to fill in the slot with a less aggressive version if possible – one which makes less assumption. If not possible we will just always call the generic version (like cpython always does).
The code we emit inside the ICs has similar trade offs to the bjit code – mainly it needs to get emitted very fast. We prefer generating smaller code instead of faster one because of the fixed size of the inline cache. It’s better to generate a smaller version which allows us to embed more slots if necessary and trashes the instruction cache less.
lots of ideas for improvements
Both the inline cache mechanism and the bjit have a lot of room for improvements. Some of the ideas we have are:
- directly emit the content of some of the IC slots of the bjit in the LLVM tier as LLVM IR which makes it accessible to a powerful optimization pipeline which emits much better code with sophisticated inlining and much more
- generating better representation for highly polymorphic sides
- smarter (less) guards
- introducing a simple IR which allows us to do some optimizations
- better register allocation
- allow tracing of additional operations
- removal of unnecessary reference counting operations
- the whole trace generation requires writing manual c++ code (called ‘rewriter’ inside the code base) which makes them quite hard to write but with the benefit of giving us total control of how a slot looks like. In the future we could try find a better trade-off by automatically generating them from the c++ code or LLVM IR when possible
We’ve already made a lot of improvements in this area, stay tuned for a 0.5.1 blog post talking about them 🙂
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