Why does this code execute more slowly after strength-reducing multiplications to loop-carried additions?

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I was reading Agner Fog‘s optimization manuals, and I came across this example:

double data[LEN];

void compute()
{
    const double A = 1.1, B = 2.2, C = 3.3;

    int i;
    for(i=0; i<LEN; i++) {
        data[i] = A*i*i + B*i + C;
    }
}

Agner indicates that there’s a way to optimize this code – by realizing that the loop can avoid using costly multiplications, and instead use the "deltas" that are applied per iteration.

I use a piece of paper to confirm the theory, first…

Theory

…and of course, he is right – in each loop iteration we can compute the new result based on the old one, by adding a "delta". This delta starts at value "A+B", and is then incremented by "2*A" on each step.

So we update the code to look like this:

void compute()
{
    const double A = 1.1, B = 2.2, C = 3.3;
    const double A2 = A+A;
    double Z = A+B;
    double Y = C;

    int i;
    for(i=0; i<LEN; i++) {
        data[i] = Y;
        Y += Z;
        Z += A2;
    }
}

In terms of operational complexity, the difference in these two versions of the function is indeed, striking. Multiplications have a reputation for being significantly slower in our CPUs, compared to additions. And we have replaced 3 multiplications and 2 additions… with just 2 additions!

So I go ahead and add a loop to execute compute a lot of times – and then keep the minimum time it took to execute:

unsigned long long ts2ns(const struct timespec *ts)
{
    return ts->tv_sec * 1e9 + ts->tv_nsec;
}

int main(int argc, char *argv[])
{
    unsigned long long mini = 1e9;
    for (int i=0; i<1000; i++) {
        struct timespec t1, t2;
        clock_gettime(CLOCK_MONOTONIC_RAW, &t1);
        compute();
        clock_gettime(CLOCK_MONOTONIC_RAW, &t2);
        unsigned long long diff = ts2ns(&t2) - ts2ns(&t1);
        if (mini > diff) mini = diff;
    }
    printf("[-] Took: %lld ns.n", mini);
}

I compile the two versions, run them… and see this:

gcc -O3 -o 1 ./code1.c

gcc -O3 -o 2 ./code2.c

./1

[-] Took: 405858 ns.

./2

[-] Took: 791652 ns.

Well, that’s unexpected. Since we report the minimum time of execution, we are throwing away the "noise" caused by various parts of the OS. We also took care to run in a machine that does absolutely nothing. And the results are more or less repeatable – rerunning the two binaries shows this is a consistent result:

for i in {1..10} ; do ./1 ; done

[-] Took: 406886 ns.
[-] Took: 413798 ns.
[-] Took: 405856 ns.
[-] Took: 405848 ns.
[-] Took: 406839 ns.
[-] Took: 405841 ns.
[-] Took: 405853 ns.
[-] Took: 405844 ns.
[-] Took: 405837 ns.
[-] Took: 406854 ns.

for i in {1..10} ; do ./2 ; done

[-] Took: 791797 ns.
[-] Took: 791643 ns.
[-] Took: 791640 ns.
[-] Took: 791636 ns.
[-] Took: 791631 ns.
[-] Took: 791642 ns.
[-] Took: 791642 ns.
[-] Took: 791640 ns.
[-] Took: 791647 ns.
[-] Took: 791639 ns.

The only thing to do next, is to see what kind of code the compiler created for each one of the two versions.

objdump -d -S shows that the first version of compute – the "dumb", yet somehow fast code – has a loop that looks like this:

objdump naive

What about the second, optimized version – that does just two additions?

objdump optimized

Now I don’t know about you, but speaking for myself, I am… puzzled. The second version has approximately 4 times fewer instructions, with the two major ones being just SSE-based additions (addsd). The first version, not only has 4 times more instructions… it’s also full (as expected) of multiplications (mulpd).

I confess I did not expect that result. Not because I am a fan of Agner (I am, but that’s irrelevant).

Any idea what I am missing? Did I make any mistake here, that can explain the difference in speed? Note that I have done the test on a Xeon W5580 and a Xeon E5-1620 – in both, the first (dumb) version is much faster than the second one.

For easy reproduction of the results, there are two gists with the two versions of the code: Dumb yet somehow faster and optimized, yet somehow slower.

P.S. Please don’t comment on floating point accuracy issues; that’s not the point of this question.


Solution

The key to understanding the performance differences you’re seeing is in vectorization. Yes, the addition-based solution has a mere two instructions in its inner loop, but the important difference is not in how many instructions there are in the loop, but in how much work each instruction is performing.

In the first version, the output is purely dependent on the input: Each data[i] is a function just of i itself, which means that each data[i] can be computed in any order: The compiler can do them forwards, backwards, sideways, whatever, and you’ll still get the same result — unless you’re observing that memory from another thread, you’ll never notice which way the data is being crunched.

In the second version, the output isn’t dependent on i — it’s dependent on the A and Z from the last time around the loop.

If we were to represent the bodies of these loops as little mathematical functions, they’d have very different overall forms:

  • f(i) -> di
  • f(Y, Z) -> (di, Y’, Z’)

In the latter form, there’s no actual dependency on i — the only way you can compute the value of the function is by knowing the previous Y and Z from the last invocation of the function, which means that the functions form a chain — you can’t do the next one until you’ve done the previous one.

Why does that matter? Because the CPU has vector parallel instructions that each can perform two, four, or even eight arithmetic operations at the same time! (AVX CPUs can do even more in parallel.) That’s four multiplies, four adds, four subtracts, four comparisons — four whatevers! So if the output you’re trying to compute is only dependent on the input, then you can safely do two, four, or even eight at a time — it doesn’t matter if they’re forward or backward, since the result is the same. But if the output is dependent on previous computation, then you’re stuck doing it in serial form — one at a time.

That’s why the "longer" code wins for performance. Even though it has a lot more setup, and it’s actually doing a lot more work, most of that work is being done in parallel: It’s not computing just data[i] in each iteration of the loop — it’s computing data[i], data[i+1], data[i+2], and data[i+3] at the same time, and then jumping to the next set of four.

To expand out a little what I mean here, the compiler first turned the original code into something like this:

int i;
for (i = 0; i < LEN; i += 4) {
    data[i+0] = A*(i+0)*(i+0) + B*(i+0) + C;
    data[i+1] = A*(i+1)*(i+1) + B*(i+1) + C;
    data[i+2] = A*(i+2)*(i+2) + B*(i+2) + C;
    data[i+3] = A*(i+3)*(i+3) + B*(i+3) + C;
}

You can convince yourself that’ll do the same thing as the original, if you squint at it. It did that because of all of those identical vertical lines of operators: All of those * and + operations are the same operation, just being performed on different data — and the CPU has special built-in instructions that can perform multiple * or multiple + operations on different data at the same time, in a mere single clock cycle each.

Notice the letter p in the instructions in the faster solution — addpd and mulpd — and the letter s in the instructions in the slower solution — addsd. That’s "Add Packed Doubles" and "Multiply Packed Doubles," versus "Add Single Double."

Not only that, it looks like the compiler partially unrolled the loop too — the loop doesn’t just do two values each iteration, but actually four, and interleaved the operations to avoid dependencies and stalls, all of which cuts down on the number of times that the assembly code has to test i < 1000 as well.

All of this only works, though, if there are no dependencies between iterations of the loop: If the only thing that determines what happens for each data[i] is i itself. If there are dependencies, if data from the last iteration influences the next one, then the compiler may be so constrained by them that it can’t alter the code at all — instead of the compiler being able to use fancy parallel instructions or clever optimizations (CSE, strength reduction, loop unrolling, reordering, et al.), you get out code that’s exactly what you put in — add Y, then add Z, then repeat.

But here, in the first version of the code, the compiler correctly recognized that there were no dependencies in the data, and figured out that it could do the work in parallel, and so it did, and that’s what makes all the difference.

Source: StackOverflow.com

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