The attached patch implements an opcode cache for LOAD_GLOBAL opcode, making it 2x faster.

The idea is to use the new co_extra field of code objects & PEP 509 to attach a cache structure pointing directly to the resolved global name.  When globals or builtins are updated, the cache is invalidated.

The patch adds a new file "ceval_cache.h" which provides some infrastructure to ease the pain of implementing new caches for other opcode types.  I can strip down all macros from that file if Victor finds it too hard to review.

Here's a simple script I used to make sure that the cache is working: https://gist.github.com/1st1/a9660aabdcf6b8bc6205b9fe39a82bba

You can also set OPCACHE_COLLECT_STATS to 1 to get debug stats output for the cache.