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I am converting SIMD code into equivalent c code. I am stuck at one of SSE instructions
__m128i _mm_packus_epi16 (__m128i a, __m128i b)
it returns
r0 := UnsignedSaturate(a0)
r1 := UnsignedSaturate(a1)
...
r7 := UnsignedSaturate(a7)
r8 := UnsignedSaturate(b0)
r9 := UnsignedSaturate(b1)
...
r15 := UnsignedSaturate(b7)
What does UnsignedSaturate mean?
Basically, “saturation” means that values beyond some “max” value get set to “max”, and values below a “min” value get set to “min”. Usually, “min” and “max” are the values appropiate for some data type.
Thus, for example, if you take arithmetic on unsigned bytes, “128+128” would have to be “256” (which is hex 0x100), which doesn’t fit into a byte. Normal integer arithmetic would create an overflow and discard the part that doesn’t fit, which means “128+128 -> 0”. With saturated arithmetic, “256 > 255” so the result is 255.
Another option would be scaling, which basically “compresses” the values to a smaller range. Saturation just cuts them off.
You can also use this to put larger types into smaller ones, like putting 16 bit values into 8 bit values. Your example most likely does exactly that, although you’ll probably know better than I do what kind of types you are dealing with there.
“UnsignedSaturation” most likely has a min of “0” and a “max” of whatever the max of the result type is. Thus, negative inputs get turned into “0”.