gh-128563: A new tail-calling interpreter by Fidget-Spinner · Pull Request #128718 · python/cpython

When compiling the computed-goto interpreter, every opcode
implementation ends with an identical chunk of code, generated by the
`DISPATCH()` macro. In some cases, the compiler is able to notice
this, and replaces the code in one or more opcodes with a jump into
the tail portion of a different opcode.

However, we specifically **don't** want that to happen; the entire
premise of using computed gotos is to lift more information into the
instruction pointer in order to give the hardware branch-target-
predictor more information to work with! In my preliminary tests, this
tail-merging of opcode implementations explains most of the
performance improvement of the new tail-call interpreter (python#128718) --
compilers are much less willing to merge code across functions, and so
the tail-call interpreter preserves all (or at least more) of the
individual `DISPATCH` sites.

This change attempts to prevent the merging of `DISPATCH` calls, by
adding an (empty) `__asm__ volatile`, which acts as an opaque barrier
to the optimizer, preventing it from considering all of these
sequences as identical.