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Looks like in with will be much in 5.9 (hopefully).

Trying it today on Linux 5.6, despite 20G of `perf.data` I get a mass of hex symbols all labelled "[JIT] tid 284196". Useless.

Meanwhile my new code (which generalizes 10s of formulas which were previously hardcoded) started at 87% slower than the hardcoded versions, and is now only 30% slower for the common cases of simple formulas on `-march=native`, and only 10-20% slower on generic `x86_64` (no architecture options). (Generic is slower than native, so that's not a win.)

Trying to figure out exactly what is slowing it down is not so easy without profiling tools. (Valgrind fails with SIGILL, so that's no use either.)

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Oh dear. My benchmarks were flawed (too small iteration count compared to overheads of other parts of the code). It's actually around 5x slower, which is pretty terrible...

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