Wwise SDK 2018.1.11

Adds the lower single-precision, floating-point values of a and b; the upper three single-precision, floating-point values are passed through from a. r0 := a0 + b0; r1 := a1; r2 := a2; r3 := a3 (see _mm_add_ss)

AkSimd.h の 187 行目に定義があります。