floating-point-comparison

It is common knowledge that one has to be careful when comparing floating point values. Usually, instead of using == , we use some epsilon or ULP based equality testing. However, I wonder, are there any cases, when using == is perfectly fine? Look at this simple snippet, which cases are guaranteed to succeed? void fn(float a, float b) { float l1 = a/b; float l2 = a/b; if (l1==l1) { } // case a) if (l1==l2) { } // case b) if (l1==a/b) { } // case c) if (l1==5.0f/3.0f) { } // case d) } int main() { fn(5.0f, 3.0f); } Note: I've checked this and this , but they don't cover (all of) my cases. Note2

Here is the test program: void testFunc() { double maxValue = DBL_MAX; double slope = std::numeric_limits<double>::quiet_NaN(); std::cout << "slope is " << slope << std::endl; std::cout << "maxThreshold is " << maxValue << std::endl; std::cout << "the_min is " << std::min( slope, maxValue) << std::endl; std::cout << "the_min is " << std::min( DBL_MAX, std::numeric_limits<double>::quiet_NaN()) << std::endl; } int main( int argc, char* argv[] ) { testFunc(); return 0; } In Debug, I get: slope is nan maxThreshold is 1.79769e+308 the_min is nan the_min is 1.79769e+308 In Release, I get: slope is