Some projects rely on SIMD to perform computations. Sometimes, utilizing
specific SIMD instructions can result in 10 times better performance, so
it is important for the ports system to properly support SIMD. There are
some projects that do automatic run-time SIMD detection, like Embree,
but there are many that do not.
I proposed the solution for this using flavors, see the proposed port
science/g2o: https://reviews.freebsd.org/D13610
It maps SIMD levels into port flavors. So that g2o-sse42 is for SSE42,
and g2o-nosimd is not using SIMD.
Later, the user will to install the flavor corresponding to his
machine's SIMD support.
Further, ports framework and tools (pkg) should automatically detect
machine's SIMD, and install the ports with the correct SIMD flavor for it.
It seems to me that this is the right solution, and that flavors fit the
purpose very well. Flavors are defined as needed for situations when the
whole project needs to be rebuilt, and in this case it needs to be
rebuilt because SIMD instructions inside are different.
Sadly, Mathieu disagreed, and asked to "Always build with the lowest
available instructions."
How can FreeBSD support SSE then? There should be some consistent path
forward on SIMD support. Just always using the lowest SIMD isn't an answer.
Yuri
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