On Fri, May 28, 2021 at 2:44 AM Shreyan Avigyan
<pythonshreya...@gmail.com> wrote:
>
> My proposal is somewhat the sum of all of your ideas. Well I propose there
> should a STORE_STATIC_FAST opcode that stores a static variable. Static
> variable will be declared only once and will be initialized to None
> (statement syntax will be similar to that of global). It will be initialized
> in MAKE_FUNCTION. Now it will be set by STORE_STATIC_FAST. Where will the
> variables be stored? It will have references in locals and __statics__.
> Therefore LOAD_FAST can find it. So I don't hope there will be performance
> decrease but performance increase is also not guaranteed. :-)
>
The duplicated store fixes half the problem, but it still fails on the
recursion example that I posted in reply to Steve. It would be a nice
optimization, but it may or may not be sufficient.
> And if these are thread unsafe then is __defaults__ also thread unsafe?
>
Thread safety isn't a problem with constants. Python guarantees that
internal details (like CPython's reference counts) aren't going to be
trampled on, and inside your code, nothing is going to change
__defaults__ (unless you're doing something bizarre, in which case it
isn't about __defaults__ any more). Thread safety only becomes an
issue when you have something like this:
counter = 0
def get_next():
global counter
counter += 1
return counter
This disassembles to:
6 0 LOAD_GLOBAL 0 (counter)
2 LOAD_CONST 1 (1)
4 INPLACE_ADD
6 STORE_GLOBAL 0 (counter)
7 8 LOAD_GLOBAL 0 (counter)
10 RETURN_VALUE
A context switch can happen between any two of those instructions.
That means one thread could load the global, then another thread could
load the same value, resulting in both of them writing back the same
incremented value. Or, between opcodes 6 and 8 (between the lines of
Python code), you could store the value, then fetch back a different
value.
None of this is a problem if you're using constants. The only reason
to use statics instead of global constants is performance - the
"len=len" trick is specific to this performance advantage - but you
don't have to worry about thread safety.
ChrisA
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