bpo-43760: Speed up check for tracing in interpreter dispatch #25276
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Misc/NEWS.d/next/Core and Builtins/2021-04-08-12-20-29.bpo-43760.tBIsD8.rst
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…#25276) * Remove redundant tracing_possible field from interpreter state. * Move 'use_tracing' from tstate onto C stack, for fastest possible checking in dispatch logic. * Add comments stressing the importance stack discipline when dealing with CFrames. * Add NEWS
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The code to check if tracing is active occurs once per opcode in the interpreter and is executed in the order of 100 million times per second. It needs to be as efficient as possible. It cannot be kept in a register as it needs to be modified outside of the interpreter, so on the C stack is the fastest possible location for it.
This PR keeps
use_tracingin the C frame of the current interpreter making the check for tracing as fast as possible.Doing so complicates access to
use_tracingfrom other code and requires us to be careful about maintaining stack discipline.For anything less performance critical than
use_tracingthis probably wouldn't be worth it, but asuse_tracingis so performance critical it is worth it.https://bugs.python.org/issue43760