gcc can use builtin functions in many cases, their documentation says: Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function is emitted. so therefore gcc will not need to link against the math library when using the builtin function … Read more
Looking at <cmath> for libstdc++ shipped with gcc 4.9 you get this: constexpr bool isnan(double __x) { return __builtin_isnan(__x); } A constexpr function could be aggressively inlined and, of course, the function just delegates the work over to __builtin_isnan. The <math.h> header doesn’t use __builtin_isnan, rather it uses an __isnan implementation which is kind of … Read more
Yes, they are equivalent in behavior. Then why do people use the (1 << 7) version? I guess, they use it to document it is a power of 2. Calculating the condition every time must be an overhead! I am unable to find the reason behind this! Not really, any normal compiler will replace 1 … Read more
(Flynn’s answer is the correct answer. This one addresses the problem more generally.) You seem to be assuming, OP, that the floating-point inaccuracy in your code is deterministic and predictably wrong (in a way, your approach is the polar opposite of that of people who don’t understand floating point yet). Although (as Ben points out) … Read more