The compiler cannot eliminate the addition of a floating-point positive zero because it is not an identity operation. By IEEE 754 rules, the result of adding +0. to −0. is not −0.; it is +0.
The compiler may eliminate the subtraction of +0. or the addition of −0. because those are identity operations.
For example, when I compile this:
double foo(double x) { return x + 0.; }
with Apple GNU C 4.2.1 using -O3 on an Intel Mac, the resulting assembly code contains addsd LC0(%rip), %xmm0. When I compile this:
double foo(double x) { return x - 0.; }
there is no add instruction; the assembly merely returns its input.
So, it is likely the code in the original question contained an add instruction for this statement:
y[i] = y[i] + 0;
but contained no instruction for this statement:
y[i] = y[i] - 0;
However, the first statement involved arithmetic with subnormal values in y[i], so it was sufficient to slow down the program.