Is it realistic to use -O3 or -Ofast to compile your benchmark code or will it remove code?

Question

When compiling the benchmark code below with -O3 I was impressed by the difference it made in latency so i began to wonder whether the compiler is not \"cheatin

Answer 1

It can be very difficult to benchmark what you think you are measuring. In the case of the inner loop:

for (int j = 0;  j < load;  ++j)
        if (i % 4 == 0)
                x += (i % 4) * (i % 8);
        else    x -= (i % 16) * (i % 32);

A shrewd compiler might be able to see through that and change the code to something like:

 x = load * 174;   // example only

I know that isn't equivalent, but there is some fairly simple expression which can replace that loop.

The way to be sure is to use the gcc -S compiler option and look at the assembly code it generates.