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> the equivalent code looks to be straightforwardly expressible in C with Intel intrinsics if that’s more your jam. (Granted, that’s essentially assembly except

> the equivalent code looks to be straightforwardly expressible in C with Intel intrinsics if that’s more your jam. (Granted, that’s essentially assembly except with a register allocator, so the practical difference is limited.) The vectorization is most of the speedup, not the assembly. At my day job I have a small pile of code I'm responsible for which is a giant pile of intrinsics. We compile to GCC and MSVC. We have one function that is just a straight function. There are no loops, there is one branch. There is nothing that isn't either a vector intrinsic or an address calculation which I'm pretty sure is simple enough that it can be included in x86's fancy inline memory address calculation thingie. There's basically nothing for the compiler to do except translate vector intrinsics and address calculation from C into assembly. The code when run in GCC is approximately twice as fast as MSVC. The compiler is also responsible for register allocation, and MSVC is criminally insane at it. One of these days I'll have to rewrite this function in assembly, just to get MSVC up to GCC speeds, but frankly I don't want to.

Const-me 1y ago
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.

Const-me 1y ago

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.