Some Assembly Required

The feedback I got on yesterday’s article on float-to-int conversion prompted me to look more closely into all the different options MSVC actually gives you for rounding on the x86 architecture. It turns out that with /fp:fast set it can do one of three things (in addition to the magic-number rounding you can write yourself):

• By default it will call a function _ftol2_sse, which tests the CPU to see if it has SSE2 functionality. If so, it uses the native SSE2 instruction cvttsd2si. If not, it calls _ftol(). This is quite slow because it has to perform that CPU test for every single conversion, and because there is that overhead of a function call.
• With /QIfist specified, the compiler simply emits a fistp opcode to convert the x87 floating point register to an integer in memory directly. It uses whatever rounding mode happens to be set in the CPU at the moment.
• With /arch:SSE2 specified, the compiler assumes that the program will only run on CPUs with SSE2, so it emits the cvttsd2si opcode directly instead of calling _ftol2_sse. Like /QIfist, this replaces a function call with a single instruction, but it’s even faster and not deprecated. As commenter cb points out, the intrinsics also let you specify truncation or rounding without having to fool around with CPU modes.

I raced the different techniques against each other and the clear winner was the function compiled with /arch:SSE2 set. Thus, if you can assume that your customer will have a CPU with SSE2 enabled, setting that simple compiler switch will provide you with superior performance for basically no work. The only caveat is that the SSE scalar operations operate at a maximum of double-precision floats, whereas the old x87 FPU instructions are internally 80-bit — but I’ve never seen a game application where that level of precision makes a difference.

According to the Steam Hardware Survey, 95% of our customers have SSE2-capable CPUs. The rest are probably not playing your most recent releases anyway.