Is it possible to cast floats directly to __m128 if they are 16 byte aligned?

Question

Is it safe/possible/advisable to cast floats directly to __m128 if they are 16 byte aligned?

I noticed using _mm_load_ps and _mm_stor

Answer 1

Going by http://msdn.microsoft.com/en-us/library/ayeb3ayc.aspx, it's possible but not safe or recommended.

You should not access the __m128 fields directly.

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And here's the reason why:

http://social.msdn.microsoft.com/Forums/en-US/vclanguage/thread/766c8ddc-2e83-46f0-b5a1-31acbb6ac2c5/

1. Casting float* to __m128 will not work. C++ compiler converts assignment to __m128 type to SSE instruction loading 4 float numbers to SSE register. Assuming that this casting is compiled, it doesn't create working code, because SEE loading instruction is not generated.

__m128 variable is not actually variable or array. This is placeholder for SSE register, replaced by C++ compiler to SSE Assembly instruction. To understand this better, read Intel Assembly Programming Reference.