Bug with MT_UNROLL_MORE

[mrmbernardi]

The following code is bugged when MT_UNROLL_MORE is set.

    // i = [0 ... 226]
    while (i < DIFF)
    {
        /*
         * We're doing 226 = 113*2, an even number of steps, so we can safely
         * unroll one more step here for speed:
         */
        UNROLL(i + PERIOD);

#ifdef MT_UNROLL_MORE
        UNROLL(i + PERIOD);
#endif
    }

With MT_UNROLL_MORE, the last iteration begins with i set to 226. The loop enters and completes two more iterations, ending with i = 228, completing one more iteration than it should've. Interestingly, G++ is smart enough to produce a warning about this.

With MT_UNROLL_MORE, the following code fixes the bug and produces the same output as without MT_UNROLL_MORE:

    while (i < DIFF - 2)
    {
        /*
         * We're doing 226 = 113*2, an even number of steps, so we can safely
         * unroll one more step here for speed:
         */
        UNROLL(i + PERIOD);

#ifdef MT_UNROLL_MORE
        UNROLL(i + PERIOD);
#endif
    }
    UNROLL(i + PERIOD);

[zhao-shihan]

I agree - subscripts from 0 to 226 actually iterate 227 times.

[macvip]

Please excuse my shallow understanding if I were wrong. Literally, the total execution times of UNROLL is determined by SIZE (see the following loop), so the above modifications will not affect the final execution result.

[mrmbernardi]

The expression passed to UNROLL is different. I discovered this bug when I was comparing the output against std::mt19937 and it was different. Implementing the fix I mentioned brings this code back in line with the output of std::mt19937.