GH-116017: Get rid of _COLD_EXIT

[brandtbucher]

The cold exits use quite a bit of memory on JIT builds, and complicate memory allocation improvements (since they are the only immortal, shared-by-all-interpreters, unlinked, un-invalidatable executors). They also introduce a very subtle back-and-forth dance between executors when warming up side exits, and represent a significant proportion of the uops executed.

This:

• Removes them and puts all of their logic into the _EXIT_TRACE instruction (previously this logic was shared between the exit_to_trace label (only used by _EXIT_TRACE) and _COLD_EXIT...
• ...and also changes _EXIT_TRACE to use an oparg instead of an exit_index...
• ...which allows us to simplify the tier two instruction format.

Separately, I added some missing stats to _DYNAMIC_EXIT.

This results in a 34.7% reduction in traces executed, and a 1.6% reduction in uops executed (both corresponding to 3.7 billion fewer _COLD_EXITS). Per platform:

• aarch64-apple-darwin: 3% faster, 9% less memory
• aarch64-unknown-linux-gnu: 9% faster, 2% less memory
• i686-pc-windows-msvc: 3% faster
• x86_64-pc-windows-msvc: 3% faster
• x86_64-unknown-linux-gnu: 1% faster, 2% less memory

• Issue: Compiling tiny traces wastes lots of memory #116017

[Fidget-Spinner]

Maybe a question for @mdboom instead of you: why are the figures on the time plot different than the figures given in geometric mean? E.g. darwin arm64 says "Geometric mean: 1.00x faster" but the figure shows 1.027x" or roughly 3%?

[brandtbucher]

I think the number in the README overview of the results is the "HPT" calculation, which (I believe) weights by "noise". You can see that those particular results were pulled upwards by the startup benchmarks, which have a higher standard deviation than most of the others.

But yeah, @mdboom can probably explain better.

[mdboom]

The plot takes the mean across all of the distributions of all of the benchmarks. So it takes into account that the distributions of all of the benchmarks aren't necessarily normally distributed.

The geometric mean value (from pyperf) is the nth root of multiplying all of the means of all the benchmarks together.

[markshannon]

One minor nit, otherwise it looks good.