[LLVMdev] Heuristic for choosing between MCJIT and Interpreter

[Philip Reames]

On 08/11/2014 04:21 AM, David Chisnall wrote:
> Hi Josh,
>
> On 9 Aug 2014, at 21:33, Josh Klontz <josh.klontz at gmail.com> wrote:
>
>> I'm facing a situation where I have generated IR that only needs to be executed once. I've noticed for simple IR it's faster to run the interpreter on it, but for complex IR it's much better to JIT compile and execute it. I'm seeking suggestions for a good heuristic to decide which approach to take for any given IR. I'm leaning in favor of deciding based on the presence/absence of loops.
> What are you generating IR from?  You may find that an AST interpreter, although slow, will be faster than going to the effort of generating LLVM IR and then interpreting it.  LLVM IR is much slower than bytecodes for high-level languages because you have the overhead of interpreting for things that often map to a single machine instruction, whereas high-level bytecodes tend to amortise the interpretation cost by having complex operations.
>
> I've found having a working AST interpreter to be good for testing an LLVM-based JIT, as you can run the same code with both check that the same actions externally visible happen in the same order.
David makes a good set of points here.  The only reason not to take the 
approach he is suggesting is that it does involve writing and 
maintaining additional code.  If you don't really care about the 
performance of your cold code, using the LLVM interpreter is definite an 
option.  (Keep in mind that there's little active development happening 
on the interpreter and it may have started to bitrot.)

Given your original question, I'm guessing you do care about the 
performance of your cold code.  Given that, David has suggested the best 
approach.
>
> You can probably find some heuristics at the AST layer that will work well.  For example, does it contain any loops with more than N iterations (where N is a number you determine experimentally)?  Does it operate on data over a certain size?
>
> David
>
>
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