Avoiding denormal values in C++

 

After searching a long time for a performance bug, I read about denormal floating point values.

Apparently denormalized floating-point values can be a major performance concern as is illustrated in this question: Why does changing 0.1f to 0 slow down performance by 10x?

I have an Intel Core 2 Duo and I am compiling with gcc, using -O2 .

So what do I do? Can I somehow instruct g++ to avoid denormal values? If not, can I somehow test if a float is denormal?

You can test whether a float is denormal using 

#include <cmath>

if ( std::fpclassify( flt ) == FP_SUBNORMAL )

(Caveat: I'm not sure that this will execute at full speed in practice.)

In C++03, and this code has worked for me in practice, 

#include <cmath>
#include <limits>

if ( flt != 0 && std::fabsf( flt ) < std::numeric_limits<float>::min() ) {
    // it's denormalized
}

To decide where to apply this, you may use a sample-based analyzer like Shark, VTune, or Zoom, to highlight the instructions slowed by denormal values. Micro-optimization, even more than other optimizations, is totally hopeless without analysis both before and after.

Wait. Before you do anything, do you actually know that your code is encountering denormal values, and that they're having a measurable performance impact?

Assuming you know that, do you know if the algorithm(s) that you're using is stable if denormal support is turned off? Getting the wrong answer 10x faster is not usually a good performance optimization.

Those issues aside:

  • If you want to detect denormal values to confirm that their presence, you have a few options. If you have a C99 standard library or Boost, you can use the fpclassify macro. Alternatively, you can compare the absolute values of your data to the smallest positive normal number.

  • You can set the hardware to flush denormal values to zero (FTZ), or treat denormal inputs as zero (DAZ). The easiest way, if it is properly supported on your platform, is probably to use the fesetenv( ) function in the C header fenv.h . However, this is one of the least-widely supported features of the C standard, and is inherently platform specific anyway. You may want to just use some inline assembly to directly set the FPU state to (DAZ/FTZ).

    • Most math coprocessors have an option to truncate denormal values to zero. On x86 it is the FZ (Flush to Zero) flag in the MXCSR control register. Check your CRT implementation for a support function to set the control register. It ought to be in <float.h> , something resembling _controlfp(). The option bit usually has "FLUSH" in the #defined symbol.

    Double-check your math results after you set this. Which is something you ought to do anyway, getting denormals is a sign of health problems.

    链接地址: http://www.djcxy.com/p/85512.html

    上一篇: FLT乘法的时间很长

    下一篇: 避免C ++中的非规范值