I’m not sure it’s register allocation. VC++ is indeed less than ideal, but 2x performance difference is IMO too much to explain by register allocation alone. F

I’m not sure it’s register allocation. VC++ is indeed less than ideal, but 2x performance difference is IMO too much to explain by register allocation alone. First check that you’re passing correct flags to VC++ compiler and linker: optimized release configuration, correct /arch switch, and ideally LTCG. BTW if you’re using cmake build system it’s rather hard to do, cmake support of VC++ compiler is not great. Another thing, VC++ doesn’t like emitting giant functions. A possible reason for 2x difference VC++ failed to inline stuff, and your function has calls to other functions instead of a single large one. Note the compiler supports __forceinline keyword to work around that.