GNU versus Realview versus Keil C Arm

Can someone tell me the differences or advantages/disadvantages between the 3x possible compilers. ie. Why are they offered?
GNU versus Realview versus Keil C Arm

Can someone tell me the differences or advantages/disadvantages between the 3x possible compilers. ie. Why are they offered?
Something wrong

double calc( int angel) { return sin(2*pi*(5/48000)*l); } Why do the function always returns zero?
Something wrong

double calc( int angel) { return sin(2*pi*(5/48000)*l); } Why do the function always returns zero?
something about preprocessor

when i use preprocessor #if 0 ... #endif to cross out a bunch of sentences, then after building the target. I noticed the "data" memory usage increased, why? There is no error in the result.