GCC problem with raw double type comparisons

Question

I have the following bit of code, however when compiling it with GCC 4.4 with various optimization flags I get some unexpected results when its run.

#include         


        

Answer 1

The problem is likely the result of losing some precision when storing the result of an expression vs. the compiler not doing so in a local as an optimization:

double d = v[i % 4] * i;  // the result, `d`, might be kept in a register 
                          //   instead of storing it in a memory location, 
                          //   keeping full precision

if(lst[i] != d) {         // the value stored in lst[i] may have lost some 
                          //   precision since it had to be stored in memory,
                          //   which might not be able to hold the full 
                          //   precision that the expression generated

The C99 standard says in 6.3.1.8/2 "Usual arithmetic conversions":

The values of floating operands and of the results of floating expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby.