CODEWITHSUNDEEP
ccompilationconstantfolding
user1446072
user1446072

Reputation: 93

How effective is constant folding in a C compiler?

My question may be too easy, but I have not found the answer, sorry for that.

if I have some code like this:

...
#define N 6
...
float a, b;
...
a = 2.0 * 3 * N * b;
...

then, after compilation, will this code become something like this?

...
a = 36.0 * b;
...

In other words, the constant part will be calculated at compile time, right?

Thank you in advance.

Upvotes: 3

Views: 1602

Answers (2)

Jeyaram
Jeyaram

Reputation: 9494

Here is sample code, 

#include <stdio.h>

#define val 10

int main()
{

int b = 100;
int k = (5 * val) + b;
//int k = (5 * 25) + b; 

return 0;
}

val is replaced 10 during pre-processing,

cpp ss.c gives me

int main()
{    
         int b = 100;
         int k = (5 * 10) + b;    
         return 0;
}

If we look at assembly(gcc -S ss.c), 

 .file   "ss.c"
        .text
        .globl  main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movl    $100, -8(%rbp)
        movl    -8(%rbp), %eax
        addl    $50, %eax            //5 * 10 calculated !!!
        movl    %eax, -4(%rbp)
        movl    $0, %eax
        popq    %rbp
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
        .section        .note.GNU-stack,"",@progbits

so constants are calculated at compile time itself in gcc.

Upvotes: 0

Alexey Frunze
Alexey Frunze

Reputation: 62106

Most likely, but not guaranteed.

You can try and look at the disassembly of your program (either in a debugger, or in a disassembler or use a compiler switch (if available) to produce assembly code from your C code).

Upvotes: 4

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