I've heard that the 128-bit integer data-types like __int128_t provided by GCC are emulated and therefore slow. However, I understand that the various SSE instruction sets (SSE, SSE2, ..., AVX) introduced at least some instructions for 128-bit registers. I don't know very much about SSE or assembly / machine code, so I was wondering if someone could explain to me whether arithmetic with __int128_t is emulated or not using modern versions of GCC.
The reason I'm asking this is because I'm wondering if it makes sense to expect big differences in __int128_t performance between different versions of GCC, depending on what SSE instructions are taken advantage of.
So, what parts of __int128_t arithmetic are emulated by GCC, and what parts are implemented with SSE instructions (if any)?
I was confusing two different things in my question.
Firstly, as PaulR explained in the comments: "There are no 128 bit arithmetic operations in SSE or AVX (apart from bitwise operations)". Considering this, 128-bit arithmetic has to be emulated on modern x86-64 based processors (e.g. AMD Family 10 or Intel Core architecture). This has nothing to do with GCC.
The second part of the question is whether or not 128-bit arithmetic emulation in GCC benefits from SSE/AVX instructions or registers. As implied in PaulR's comments, there isn't much in SSE/AVX that's going to allow you to do 128-bit arithmetic more easily; most likely x86-64 instructions will be used for this. The code I'm interested in can't compile with -mno-sse, but it compiles fine with -mno-sse2 -mno-sse3 -mno-ssse3 -mno-sse4 -mno-sse4.1 -mno-sse4.2 -mno-avx -mno-avx2 and performance isn't affected. So my code doesn't benefit from modern SSE instructions.