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Date: Sun, 30 Jan 2011 21:01:00 -0000
Message-ID: <AANLkTi=wMQx1xxDGuRgkJQ+FvR0e6UKUwKWQT1npQ_z3@mail.gmail.com>
Subject: Re: GCC Internals: built-in functions?
From: Jonathan Wakely <jwakely.gcc@gmail.com>
To: Amittai Aviram <amittai.aviram@yale.edu>
Cc: gcc-help@gcc.gnu.org
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On 30 January 2011 20:45, Amittai Aviram wrote:
> On this GCC Internals page
>
> http://en.wikibooks.org/wiki/GNU_C_Compiler_Internals/GNU_C_Compiler_Arch=
itecture_3_4
>
> I found the following in the "GCC Initialization" section:
>
> "GCC built-in functions are the functions that are evaluated at compile t=
ime. For example, if the size argument of a strcpy() function is a constant=
 then GCC replaces the function call with the required number of assignment=
s."
>
> I was curious about this, so I tried compiling to assembly (-S) a very si=
mple program:
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
>
> int main(void) {
>
> =A0 =A0 =A0 =A0char s1[0x10];
> =A0 =A0 =A0 =A0char * s0 =3D "HELLO";
> =A0 =A0 =A0 =A0strcpy(s1, s0);
> =A0 =A0 =A0 =A0printf("%s %s\n", s0, s1);
> =A0 =A0 =A0 =A0return EXIT_SUCCESS;
> }
>
> But the resulting assembly code simply calls strcpy with the two argument=
s, just as I would have expected had I not read the above sentence:
>
> =A0 =A0 =A0 =A0movq =A0 =A0$.LC0, -40(%rbp)
> =A0 =A0 =A0 =A0movq =A0 =A0-40(%rbp), %rdx
> =A0 =A0 =A0 =A0leaq =A0 =A0-32(%rbp), %rax
> =A0 =A0 =A0 =A0movq =A0 =A0%rdx, %rsi
> =A0 =A0 =A0 =A0movq =A0 =A0%rax, %rdi
> =A0 =A0 =A0 =A0call =A0 =A0strcpy
>
> (Here, .LC0 labels the string "HELLO".)
>
> So what does that sentence actually mean and what am I missing? =A0Thanks!

strcpy has no 'size' parameter, I assume it's meant to say strncpy