Is there an official reference listing the operation of the SSE intrinsic functions for GCC, i.e. the functions in the <*mmintrin.h> header files?
As well as Intel's vol.2 PDF manual, there is also an online intrinsics guide.
The Intel® Intrinsics Guide contains reference information for Intel intrinsics, which provide access to Intel instructions such as Intel® Streaming SIMD Extensions (Intel® SSE), Intel® Advanced Vector Extensions (Intel® AVX), and Intel® Advanced Vector Extensions 2 (Intel® AVX2).
It has a full-text search, so an intrinsic can be found by its name, or by CPU instruction, CPU feature, etc. It also has a control on which ISA extension to show. This allows, for example, not searching KNC that you wouldn't likely be able to use, or MMX that is far less useful these days.
See also the tag wiki for the sse tag for links to guides and a couple tutorials, as well as this official documentation.