On-topic: alternate Python implementations

[John Nagle]

On 8/4/2012 7:19 PM, Steven D'Aprano wrote:
> On Sat, 04 Aug 2012 18:38:33 -0700, Paul Rubin wrote:
> 
>> Steven D'Aprano <steve+comp.lang.python at pearwood.info> writes:
>>> Runtime optimizations that target the common case, but fall back to
>>> unoptimized code in the rare cases that the optimization doesn't apply,
>>> offer the opportunity of big speedups for most code at the cost of
>>> trivial slowdowns when you do something unusual.
>>
>> The problem is you can't always tell if the unusual case is being
>> exercised without an expensive dynamic check, which in some cases must
>> be repeated in every iteration of a critical inner loop, even though it
>> turns out that the program never actually uses the unusual case.

   There are other approaches. PyPy uses two interpreters and a JIT
compiler to handle the hard cases.  When code does something unexpected
to other code, the backup interpreter is used to get control out of
the trouble spot so that the JIT compiler can then recompile the
code.  (I think; I've read the paper but haven't looked at the
internals.)

   This is hard to implement and hard to get right.

				John Nagle