Wednesday 19 June 2019
floating point - Avoiding denormal values in C++
Answer
Answer
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?
Answer
You can test whether a float is denormal using
#include
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
#include
if ( flt != 0 && std::fabsf( flt ) < std::numeric_limits::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.
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