C - efficiently changing a function pointer based on command line input

Question

I have several similar functions, say A, B, C. I want to choose one of them with command line options. Also, I'm calling that function billion times because of that instead of checking a variable inside a function billion times, I'm defining a function pointer Phi and set it to desired function just one time. But when I set, Phi = A, (so no user input considered) my code runs in ~24 secs, when I add an if-else and set Phi to desired function, my code runs in ~30 secs with exact same parameters. (Of course command line option sets Phi to A) What is the efficient way to handle this case?

My functions:

double funcA(double r)
{
    return 0;
}

double funcB(double r)
{
    return 1;
}

double funcC(double r)
{
    return r;
}

void computationFunctionFast(Context *userInputs) {
    double (*Phi)(double) = funcA;
    
    /* computation codes */
}

void computationFunctionSlow(Context *userInputs) {
    double (*Phi)(double);
    switch (userInputs->funcEnum) {
        case A:
        Phi = funcA;
        break;

        case B:
        Phi = funcB;
        break;

        case C:
        Phi = funcC;
    }
    
    /* computation codes */
}

I've tried gcc, clang, icx with -O2 and -O3 optimizations. (gcc has no performance difference in mentioned cases but has the worst performance) Although I'm using C, I've tried std::function too. I've tried defining Phi function in different scopes etc.

Answer 1

Generally, there are a few things here that are slightly bad for performance:

• Branches/comparisons lead to inefficient use of branch prediction/instruction cache and might affect pipelining too.
• Function pointers are notoriously inefficient since they generally block inlining and generally the compiler can't do much about them.

Here's an example based on your code:

double computationFunctionSlow (int input, double val) {
    double (*Phi)(double);
    switch (input) {
        case 0:  Phi = funcA; break;
        case 1:  Phi = funcB; break;
        case 2:  Phi = funcC; break;
    }
    double res = Phi(val);
    return res;
}

clang 15.0.0 x86_64 -O3 gives:

computationFunctionSlow:                # @computationFunctionSlow
        cmp     edi, 2
        ja      .LBB3_1
        movsxd  rax, edi
        lea     rcx, [rip + .Lswitch.table.computationFunctionSlow]
        jmp     qword ptr [rcx + 8*rax]         # TAILCALL
.LBB3_1:
        xorps   xmm0, xmm0
        ret
.Lswitch.table.computationFunctionSlow:
        .quad   funcA
        .quad   funcB
        .quad   funcC

Even though the numbers I picked are adjacent, the usual compilers fail to optimize out the comparison cmp. Even when I include a default: return 0; it is still there. You can quite easily manually optimize any switch with contiguous indices like this into a function pointer jump table:

double computationFunctionSlow (int input, double val) {
    double (*Phi[3])(double) = {funcA, funcB, funcC};
    double res = Phi[input](val);
    return res;
}

clang 15.0.0 x86_64 -O3 gives:

computationFunctionSlow:                # @computationFunctionSlow
        movsxd  rax, edi
        lea     rcx, [rip + .L__const.computationFunctionSlow.Phi]
        jmp     qword ptr [rcx + 8*rax]         # TAILCALL
.L__const.computationFunctionSlow.Phi:
        .quad   funcA
        .quad   funcB
        .quad   funcC

This leads to slightly better code here as the comparison instruction/branch is now removed. However, this is really a micro optimization that shouldn't have that much impact of performance. You have to benchmark it for sure to see if there's any improvement.

(Also gcc 12.2 didn't optimize this code as good, why I went with clang for this example.)

Godbolt link: https://godbolt.org/z/ja4zerj7o

Answer 2

There isn't a more "efficient" way to handle this case, you are already doing what you should.

The difference in timing you observe is because:

1. In the first case (Phi = funcA) the compiler knows the function will always be the same and is therefore able to optimize its calls. Depending on what your "computation code" does, this could mean inlining the function and simplifying a lot of calculations for you.

2. In the second case (Phi = <choice from user>) the compiler cannot know which function will be selected, and therefore cannot optimize any of the calls made to it by the rest of the code. It also cannot propagate optimizations to other parts of your "computation code" like in the first case.

In general, there isn't much you can do. Dynamic function pointers inherently add a bit of runtime overhead and make optimizations harder (or impossible).

---

What you could try is duplicating the "computation code" inside different functions or different branches that you only enter after asserting that Phi is equal to a constant, like so:

void computationFunctionSlow(Context *userInputs) {
    if (userInputs->funcEnum == A) {
        const double (*Phi)(double) = funcA;
        // computation code
    } else if (...) {
        // ...
    } 
}

In the above piece of code, the compiler knows that inside any of those if blocks the value of Phi can only have one value, and could therefore be able to perform the same optimizations discussed in point 1 above.

Answer 3

There's no need to put an enum in your userInputs when all you do with it is use it to select a function pointer. Just add the function pointer in the structure directly and eliminate the branching done on every call.

Instead of

struct Context
{
    .
    .
    .
    enum funcType funcEnum;
};

use

struct Context
{
    .
    .
    .
    double (*phi)(double);
};

You'd wind up with something like this:

void computationFunctionSlow(Context *userInputs) {

    /* computation codes */
    double result = userInputs->phi( data );
}