Why does changing 0.1f to 0 slow down performance by 10x?

Question

Why does this bit of code, const float x[16] = { 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6}; const float z[16] = {1.123, 1.234, 1.345, 156.467, 1.578, 1.689, 1.790, 1.812, 1.923, 2.034, 2.145, 2.256, 2.367, 2.478, 2.589, 2.690}; float y[16]; for (int i = 0; i < 16; i++) { y[i] = x[i]; } for (int j = 0; j < 9000000; j++) { for (int i = 0; i < 16; i++) { y[i] *= x[i]; y[i] /= z[i]; y[i] = y[i] + 0.1f; // <-- y[i] = y[i] - 0.1f; // <-- } } run more than 10 times faster than the following bit (identical except where noted)? const float x[16] = { 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6}; const float z[16] = {1.123, 1.234, 1.345, 156.467, 1.578, 1.689, 1.790, 1.812, 1.923, 2.034, 2.145, 2.256, 2.367, 2.478, 2.589, 2.690}; float y[16]; for (int i = 0; i < 16; i++) { y[i] = x[i]; } for (int j = 0; j < 9000000; j++) { for (int i = 0; i < 16; i++) { y[i] *= x[i]; y[i] /= z[i]; y[i] = y[i] + 0; // <-- y[i] = y[i] - 0; // <-- } } when compiling with Visual Studio 2010 SP1. (I haven't tested with other compilers.)

Answer 1

My guess without running this and looking at disassembly (this would be the best way to determine for sure) would be that your arrays are of type float while 0 is of type int by default. Subtracting 0.1f requires no type conversion on the part of the compiler whereas subtracting an int requires the compiler to output instructions to convert the integer to an IEEE754 floating point number which is not free!