gcc and clang generate SSE2 instructions for scalar (non-SIMD) arithmetic, depending on the target architecture and compiler flags. For x86_64 targets, the call

gcc and clang generate SSE2 instructions for scalar (non-SIMD) arithmetic, depending on the target architecture and compiler flags. For x86_64 targets, the calling convention puts floating point arguments in xmm registers, so the compilers must use SSE2 (and it's faster than x87 anyway). That's what you're seeing. If you look at the instructions in libpng.so that mention xmm registers, you'll see that they aren't SIMD instructions.