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2022-11-15

Could too much function pointers (delegates) inside many huge loops have an effect of unoptimizable code?

We have this simple c modified code from stack overflow called funcptrtest.c:

include <stdio.h>
/*   C code for program funcptrtest.c 
  => https://pastebin.com/rUtXfgSG 
*/
const int b = 23;

// A normal function with an int parameter and void return type
void fun(int a)
{
  if (a < b)
    printf("Value of a (%d) is lesser then value of b (%d)\n", a, b);
  else if (a == b)
    printf("Value of a (%d) is equal value of b (%d)\n", a, b);
  else if (a > b)
    printf("Value of a (%d) is greater than value of b (%d)\n", a, b);
}

// main => NO KNR style int main(argc, argv) int argc; char **argv 
int main(int argc, char **argv)
{
    int i = 0;
    /* fun_ptr is a pointer to function fun()
           void (*fun_ptr)(int) = &fun;
         is equivalent of following two
           void (*fun_ptr)(int);
           fun_ptr = &fun;
    */
    void (*fun_ptr)(int) = &fun;

    // call once with static int
    (*fun_ptr)(b);

    // iterate through for loop
    for (i = 1; i < 101; i+=11)  {
        // Invoking fun() using fun_ptr
        (*fun_ptr)(i);
    }

    return 0;
}

We compile it now with gnu c compiler with gcc option -S and generate an assembler file:

gcc -S funcptrtest.c -o funcptrtest.asm

Result will be something like this:

.file   "funcptrtest.c"
        .text
        .globl  b
        .section        .rodata
        .align 4
        .type   b, @object
        .size   b, 4
b:
        .long   23
        .align 8
.LC0:
        .string "Value of a (%d) is lesser then value of b (%d)\n"
        .align 8
.LC1:
        .string "Value of a (%d) is equal value of b (%d)\n"
        .align 8
.LC2:
        .string "Value of a (%d) is greater than value of b (%d)\n"
        .text
        .globl  fun
        .type   fun, @function
fun:
.LFB0:
        .cfi_startproc
        endbr64
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        subq    $16, %rsp
        movl    %edi, -4(%rbp)
        movl    $23, %eax
        cmpl    %eax, -4(%rbp)
        jge     .L2.L2:
        movl    $23, %eax
        cmpl    %eax, -4(%rbp)
        jne     .L4
        movl    $23, %edx
        movl    -4(%rbp), %eax
        movl    %eax, %esi
        leaq    .LC1(%rip), %rax
        movq    %rax, %rdi
        movl    $0, %eax
        call    printf@PLT
        jmp     .L5
        .L2:
        movl    $23, %eax
        cmpl    %eax, -4(%rbp)
        jne     .L4
        movl    $23, %edx
        movl    -4(%rbp), %eax
        movl    %eax, %esi
        leaq    .LC1(%rip), %rax
        movq    %rax, %rdi
        movl    $0, %eax
        call    printf@PLT
.L2:
        movl    $23, %eax
        cmpl    %eax, -4(%rbp)
        jne     .L4
        movl    $23, %edx
        movl    -4(%rbp), %eax
        movl    %eax, %esi
        leaq    .LC1(%rip), %rax
        movq    %rax, %rdi
        movl    $0, %eax
        call    printf@PLT
        jmp     .L5
.L4:
        movl    $23, %eax
        cmpl    %eax, -4(%rbp)
        jle     .L5
        movl    $23, %edx
        movl    -4(%rbp), %eax
        movl    %eax, %esi
        leaq    .LC2(%rip), %rax
        movq    %rax, %rdi
        movl    $0, %eax
        call    printf@PLT
.L5:
        nop
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc     
.LFE0:
        .size   fun, .-fun
        .globl  main
        .type   main, @function
main:
.LFB1:
        .cfi_startproc
        endbr64
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        subq    $32, %rsp
        movl    %edi, -20(%rbp)
        movq    %rsi, -32(%rbp)
        movl    $0, -12(%rbp)
        leaq    fun(%rip), %rax
        movq    %rax, -8(%rbp)
        movl    $23, %edx
        movq    -8(%rbp), %rax
        movl    %edx, %edi
        call    *%rax
        movl    $1, -12(%rbp)
        jmp     .L7
.L8:
        movl    -12(%rbp), %eax
        movq    -8(%rbp), %rdx
        movl    %eax, %edi
        call    *%rdx
        addl    $11, -12(%rbp)
.L7:
        cmpl    $100, -12(%rbp)
        jle     .L8
        movl    $0, %eax
        leave
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE1:
        .size   main, .-main
        .ident  "GCC: (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0"
        .section        .note.GNU-stack,"",@progbits
        .section        .note.gnu.property,"a"
        .align 8
        .long   1f - 0f
        .long   4f - 1f
        .long   5
0:
        .string "GNU"
1:
        .align 8
        .long   0xc0000002
        .long   3f - 2f
2:
        .long   0x3
3:
        .align 8
4:

Lets look, if a simple loop is faster than a function pointer delegate call:

/* program looptest.c */
include <stdio.h>

const int b = 23;

// main
int main(int argc, char **argv)
{
    int i = 0;
    int a = 23;

    /*
    if (a < b)
        printf("Value of a (%d) is lesser then value of b (%d)\n", a, b);
    else if (a == b)
       printf("Value of a (%d) is equal value of b (%d)\n", a, b);
    else if (a > b)
       printf("Value of a (%d) is greater than value of b (%d)\n", a, b);
    */

    // iterate through for loop
    for (i = 1; i < 4194304; i+=11)  {
        a = i;
        if (a < b)
           printf("Value of a (%d) is lesser then value of b (%d)\n", a, b);
        else if (a == b)
           printf("Value of a (%d) is equal value of b (%d)\n", a, b);
        else if (a > b)
           printf("Value of a (%d) is greater than value of b (%d)\n", a, b);
    }

    return 0;
}

/* program funcptrtest.c */
include <stdio.h>

const int b = 23;

// A normal function with an int parameter and void return type
void fun(int a)
{
  if (a < b)
    printf("Value of a (%d) is lesser then value of b (%d)\n", a, b);
  else if (a == b)
    printf("Value of a (%d) is equal value of b (%d)\n", a, b);
  else if (a > b)
    printf("Value of a (%d) is greater than value of b (%d)\n", a, b);
}

// main
int main(int argc, char **argv)
{
    int i = 0;
    /* fun_ptr is a pointer to function fun()

       void (*fun_ptr)(int) = &fun;
       is equivalent of following two
       void (*fun_ptr)(int);
       fun_ptr = &fun;
    */
    void (*fun_ptr)(int) = &fun;

    // call once with static int
    // (*fun_ptr)(b);

    // iterate through for loop
    for (i = 1; i < 4194304; i+=11)  {
        // Invoking fun() using fun_ptr
        (*fun_ptr)(i);
    }

    return 0;
}

Well, that's not so deterministic, of course our simple looptest.asm has a shorter assembler.

compile options

But execution time is not so huge difference, we have to simulate, that in many scenarios.

[To be continued ...]

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