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The STORE_FAST, LOAD_FAST, and LOAD_DEREF opcodes all use fast macros for variable access. This patch harmonizes STORE_DEREF to follow the same pattern.
Both the C code and the generated assembly look nicer. Gives an approx 40% speed-up (using both Clang and GCC-6) on the "store_nonlocal" portion of the variable access benchmark at http://code.activestate.com/recipes/577834
The eliminates the nonlocal speed penalty, making cell variable updates run nearly as fast as updates to locals.

LGTM. This saves function call and INCREF/DECREF pair.
What about DELETE_DEREF? PyCell_Set() also is used in unicode_concatenate().

New changeset d78d45436753 by Raymond Hettinger in branch '3.6':
Issue #28665: Harmonize STORE_DEREF with STORE_FAST and LOAD_DEREF giving a 40% speedup.
https://hg.python.org/cpython/rev/d78d45436753 

Thanks for the quick review. I'll look at the other two cases when I get a chance.

New changeset 7ec45e7d2194 by Serhiy Storchaka in branch 'default':
Merge from 3.6 (issue #28665).
https://hg.python.org/cpython/rev/7ec45e7d2194 

You forgot to merge and created new head.

Could you please measure the performance effect of these changes?

I think the speed benefit for the last two patches is likely too modest to care about. The main reason I did the work was because you suggested it and because it seemed like a reasonable idea (the patched code looks nice, it does only work that is necessary, and it is more consistent with the other opcodes).
Let me know if you want to go forward with it or leave it in the current state (the current juxtaposition of PyCell_GET with PyCell_Set looks a little weird but it does get the job done).

The argument about "harmonizing" doesn't look strong to me. Opcodes for locals use the SETLOCAL() macro which decrefs 
old value, while opcodes for nonlocals with your patches use the PyCell_SET() macro which doesn't.
But performance arguments look more weighty. I made benchmarks. fastcell.diff speeds up STORE_FAST by 40%, delete_deref.diff speeds up DELETE_DEREF by 50%. and concat_deref.diff speeds up string concatenating up to 15%. All these operations are rare in comparison with operations with locals or LOAD_DEREF, but the cognitive cost of the optimization is pretty low. All patches LGTM.
I only have doubts that such changes could be pushed in 3.6 at this stage. This is not bug fix and isn't tweaking new 3.6 feature.

New changeset 5f3b7ceb394c by Raymond Hettinger in branch 'default':
Issue #28665: Use macro form of PyCell_GET/SET
https://hg.python.org/cpython/rev/5f3b7ceb394c 

I think small changes are fine while there is still another beta ahead but would rather just push it to 3.7 than spend more time talking about it.