GCC Internals: built-in functions?

Amittai Aviram amittai.aviram@yale.edu
Sun Jan 30 20:50:00 GMT 2011


On this GCC Internals page

http://en.wikibooks.org/wiki/GNU_C_Compiler_Internals/GNU_C_Compiler_Architecture_3_4

I found the following in the "GCC Initialization" section:

"GCC built-in functions are the functions that are evaluated at compile time. For example, if the size argument of a strcpy() function is a constant then GCC replaces the function call with the required number of assignments."

I was curious about this, so I tried compiling to assembly (-S) a very simple program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {

        char s1[0x10];
        char * s0 = "HELLO";
        strcpy(s1, s0);
        printf("%s %s\n", s0, s1);
        return EXIT_SUCCESS;
}

But the resulting assembly code simply calls strcpy with the two arguments, just as I would have expected had I not read the above sentence:

	movq    $.LC0, -40(%rbp)
	movq    -40(%rbp), %rdx
	leaq    -32(%rbp), %rax
	movq    %rdx, %rsi
	movq    %rax, %rdi
	call    strcpy

(Here, .LC0 labels the string "HELLO".)

So what does that sentence actually mean and what am I missing?  Thanks!

Amittai Aviram
PhD Student in Computer Science
Yale University
646 483 2639
amittai.aviram@yale.edu
http://www.amittai.com



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