Removing Unused Functions/Dead Codes with GCC/GNU-ld

I was suprised to know that GCC failed to remove the dead codes from my program. Here’s an example:

kunil@cerpelai:~/Temp$ vi test.c
#include <stdio.h>

void unusedFunction(void) {
printf("This is a dead code\n");
}

int main(void) {
printf("Hello world\n");
return 0;
}

kunil@cerpelai:~/Temp$ gcc -Os test.c -o test.elf

If you take a look carefully on the ELF binary, you can see that GCC still includes the “unusedFunction()”, although nothing refers to it.

kunil@cerpelai:~/Temp$ objdump -d test.elf

test.elf:     file format elf32-i386
....
080483c4 <main>:
....
080483e9 <unusedFunction>:
....

So I was wondering if there’s something wrong with GCC (or GNU-ld), or should I add something to remove it.
After googling a while, I found this link, which exactly explains how to do it. So, to repeat the link, to remove the dead/unused functions:

• Compile with “-fdata-sections” to keep the data in separate data sections and “-ffunction-sections” to keep functions in separate sections, so they (data and functions) can be discarded if unused.
• Link with “–gc-sections” to remove unused sections.

So, the above example should be compiled with:

kunil@cerpelai:~/Temp$ gcc -Os -fdata-sections -ffunction-sections test.c -o test.elf -Wl,--gc-sections

And dead codes now is really really dead.

Btw, “–gc-sections” is architecture dependent. Your architecture’s GNU-ld may or may not support it. I have tested it with avr-ld and Ubuntu Intrepid ld. Both supports “–gc-sections” option.

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