Can I temporarily enable FTZ and DAZ floating-point modes for a thread?

Question

I\'d like to enable temporarily FTZ/DAZ modes to get a performance gain for some code where strict compliance with the IEEE 754 standard is not an

Answer 1

Yes, MXCSR is part of the per-thread architectural state saved/restored by context switches, along with the xmm/ymm/zmm and x87 stack registers (using xsave/xrstor). Different threads have their own FPU state.

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Interesting idea, I'd always figured DAZ was only useful if you had denormal constants or something (or data from a file), but having other threads running without FTZ is another source of denormals.

You might also want to compile some files with -ffast-math, or a subset of those options. Note that linking with -ffast-math in gcc will include a CRT function that sets DAZ/FTZ before main(), so don't do that.

The optimizations enabled by fast-math are mostly orthogonal to whether denormals are flushed to zero. Even just -fno-math-errno lets more math functions inline (better / at-all), e.g. sqrtf, and is totally safe if you don't care about errno being set as well as getting a NaN result.