c,macros , Do macros run faster than global variables in C? How to change macros between runs?


Do macros run faster than global variables in C? How to change macros between runs?

Question:

Tag: c,macros

I'm writing a program in C in which there are several constants I would like all of my functions to use. So far, I have used macros. A simplified version of the program looks as follows.

#define CONSTANT 10 //int
int multiplication_by_constant(int a){ return a*CONSTANT;}
int main(){
   for(int i = 1; i< 10; i++)
       printf("%d\n",multiplication_by_constant(i));
}

Now I want to run experiments on the program by running it several times with different values of the constant. I want to automate this, and not re-compile each time I change CONSTANT. I used a simple solution by changing the macros to global variables, putting the original main function into a new 'program()' function, then to run the experiments in the main file, i.e.

int CONSTANT =  10; //int
int multiplication_by_constant(int a){ return a*CONSTANT;}
void program(){
   for(int i = 1; i< 10; i++)
       printf("%d\n",multiplication_by_constant(i));
}
int main(){
   while(CONSTANT < 100){
       program();
   }
   return 0;
}

I discovered that this implementation lead to quite a substantial loss of performance of the loop

for(int i = 1; i< 10; i++)
       printf("%d\n",multiplication_by_constant(i));

which now requires around 50% more run time than when I was using the macro. Is this normal?

Can anyone suggest a more efficient way for me to run my experiment?

PS In C++, I would do this by defining a 'class program', setting CONSTANT to be a class member and 'multiplication_by_constant' a member function. I could then easily run the experiments through 'main' using the class definition, and I think I wouldn't have the efficiency loss... I have to use C in this implementation which is why I've resorted to the global variables and functions.


Answer:

Well, here's a potential source of the performance difference. With a macro, the value of CONSTANT is known at compile time, so the compiler can take advantage of that knowledge and structure the machine code a little differently. I took a variation of your code and used gcc -Wa,-aldh to get an assembly listing1 for both the macro and global variable versions, with an interesting difference.

First is the macro version:


 3                    .globl mul_by_const
 5                    mul_by_const:
 6                    .LFB2:
 7 0000 55                    pushq   %rbp
 8                    .LCFI0:
 9 0001 4889E5                movq    %rsp, %rbp
10                    .LCFI1:
11 0004 897DFC                movl    %edi, -4(%rbp)
12 0007 8B55FC                movl    -4(%rbp), %edx
13 000a 89D0                  movl    %edx, %eax 14 000c C1E002 sall $2, %eax 15 000f 01D0 addl %edx, %eax 16 0011 01C0 addl %eax, %eax
17 0013 C9                    leave
18 0014 C3                    ret

The highlighted lines are the meat of the function; instead of multiplying the value in %eax by 10, the function does an arithmetic shift left by 2 places followed by an addition (effectively multiplying by 5), and then adds the result to itself. For example, given an i value of 3:

 3 << 2 == 12
 3 + 12 == 15
15 + 15 == 30

If you change the value of CONSTANT, the compiler will generate different machine code (a CONSTANT value of 7 results in an arithmetic shift left by 3 followed by a subtraction, while a value of 19 gives a left shift of 3 followed by 3 additions, etc).

Compare that with the global variable version:


 2                    .globl CONSTANT
 3                            .data
 4                            .align 4
 7                    CONSTANT:
 8 0000 0A000000              .long   10
 9                            .text
10                    .globl mul_by_const
12                    mul_by_const:
13                    .LFB2:
14 0000 55                    pushq   %rbp
15                    .LCFI0:
16 0001 4889E5                movq    %rsp, %rbp
17                    .LCFI1:
18 0004 897DFC                movl    %edi, -4(%rbp)
19 0007 8B050000              movl    CONSTANT(%rip), %eax
19      0000 20 000d 0FAF45FC imull -4(%rbp), %eax
21 0011 C9                    leave
22 0012 C3                    ret

This version uses the imull opcode to do the multiplication. Since the value of CONSTANT isn't known at compile time, the compiler can't do any special optimizations based on that value.

Now, this is how things shake out on my particular platform; I don't know what compiler you're using or what OS you're running on, so it's likely that the machine code you're getting is different from what I have above. I'm just pointing out that knowing CONSTANT at compile time allows the compiler to do some extra optimization.

Edit

If you change the value of a macro, you have to recompile. You could put the macro in a header file, and then write a script to regenerate the header and recompile, something like

#!/bin/bash

let CONSTANT=1
while [ $CONSTANT -lt 20 ]
do
  cat > const.h << EOF
#ifndef CONST_H
#define CONST_H
#define CONSTANT $CONSTANT
#endif
EOF
  # build and run your test code, assuming profiling is captured
  # automatically
  gcc -o test test.c
  ./test
  let CONSTANT=CONSTANT+1
done

and your C code would #include the const.h file:

#include "const.h"
int multiplication_by_constant(int a){ return a*CONSTANT;}
...


1. Platform is SUSE Linux Enterprise Server 10 (x86_64), compiler is gcc 4.1.2


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