Explore the idea of peeking SIMD data

[ebassi]

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It might be possible to return a pointer to the start of a graphene_simd4_t (or graphene_simd4x4f_t) to functions that expect an array of floating point values; this would allow removing a stack allocation when all we care about is passing a bunch of floats to, say, GL.

Experiments on x86_64 seem to yield positive results, but it could be a combination of recent compilers and specific SIMD types, so this would require further investigation:

• does passing the reference to an __m128 or a float32x4 type actually lead to a SIMD register read?
• if the read happens, is it dependent on the OS?
• if the read happens, is it dependent on the type or version of the compiler?