Test Suite has errors on MacOS

[fyellin]

The Test Suite is showing errors when run on MacOS running Apple M1 Pro.

It seems that on the M1 Pro's GPU, exp2(x) is returning a slightly incorrect number $x < 0$.

I have texted it on the following values:

exp2(-1) =  0.4999999403953552
exp2(-2) = 0.2499999701976776
exp2(-3) = 0.1249999850988388

This causes any shader that uses PACK_BYTES_INTO_FLOAT_CODE or PACK_FLOAT_INTO_BYTES_CODE to work incorrectly. These are used by the test suite, but are not used by Quirk itself on the Mac, since it can use the more direct float options. I looked at the shaders used by all the example circuits, and the only use of exp2 was a guaranteed non-negative exponent.

Replacing exp2(exponent) with exp2(max(exponent, 0)) / exp2(max(-exponent, 0)) does work correctly. When I replaced the two code snippets above to use this alternate version, all tests passed (except for the Painter print_line aligned, which claims it is very machine and browser specific.)

I'm not quite sure what the best way to deal with this is. Ignore it? I at least wanted there to be some documentation.