Quoting Arjan van de Ven <[email protected]>:
>
> >
> > As long as nobody takes the address of them (which wouldn't compile today
> > anyway) then the compiler should be able to not allocate store for these.
> > That they're const might help too.
>
> are you really sure?
I've run some tests, and the compiler seems to do the right thing -I must admit,
that I trusted the compiler to do it blindly on my first attempt :)- , see
below:
> cat flag.h
static const int __attribute__((__deprecated__)) SA_INTERRUPT = 0x123456;
> cat use_flag.c
#include <stdio.h>
#include "flag.h"
int main()
{
int flags = SA_INTERRUPT;
printf("%d", flags);
}
> cat dont_use_flag.c
#include <stdio.h>
#include "flag.h"
int main()
{
int flags = 0;
printf("%d", flags);
}
> gcc --version
gcc (GCC) 4.1.1 (Gentoo 4.1.1-r3)
> for i in *use_flag.c; do gcc -o $(echo $i | sed 's/.c//') -O2 -g $i; done
use_flag.c: In function 'main':
use_flag.c:6: warning: 'SA_INTERRUPT' is deprecated (declared at flag.h:1)
> size *use_flag
text data bss dec hex filename
897 260 4 1161 489 dont_use_flag
897 260 4 1161 489 use_flag
# The relevant parts of the compiled code, see how the flag is replaced with
# the const value in the code. It does not result in a code size increment.
> objdump -d {dont_,}use_flag | grep -A11 '<main>'
080483b0 <main>:
80483b0: 8d 4c 24 04 lea 0x4(%esp),%ecx
80483b4: 83 e4 f0 and $0xfffffff0,%esp
80483b7: ff 71 fc pushl 0xfffffffc(%ecx)
80483ba: 31 c0 xor %eax,%eax
80483bc: 55 push %ebp
80483bd: 89 e5 mov %esp,%ebp
80483bf: 51 push %ecx
80483c0: 83 ec 14 sub $0x14,%esp
80483c3: 89 44 24 04 mov %eax,0x4(%esp)
80483c7: c7 04 24 b8 84 04 08 movl $0x80484b8,(%esp)
80483ce: e8 05 ff ff ff call 80482d8 <printf@plt>
--
080483b0 <main>:
80483b0: 8d 4c 24 04 lea 0x4(%esp),%ecx
80483b4: 83 e4 f0 and $0xfffffff0,%esp
80483b7: ff 71 fc pushl 0xfffffffc(%ecx)
80483ba: b8 56 34 12 00 mov $0x123456,%eax
80483bf: 55 push %ebp
80483c0: 89 e5 mov %esp,%ebp
80483c2: 51 push %ecx
80483c3: 83 ec 14 sub $0x14,%esp
80483c6: 89 44 24 04 mov %eax,0x4(%esp)
80483ca: c7 04 24 b8 84 04 08 movl $0x80484b8,(%esp)
80483d1: e8 02 ff ff ff call 80482d8 <printf@plt>
======== With 3.4.3 ==========
$ gcc --version
gcc (GCC) 3.4.3 20050227 (Red Hat 3.4.3-22.fc3)
$ size {dont_,}use_flag
text data bss dec hex filename
851 256 4 1111 457 dont_use_flag
855 256 4 1115 45b use_flag
[def@bigboss ~]$ objdump -d {dont_,}use_flag | grep -A11 '<main>'
08048368 <main>:
8048368: 55 push %ebp
8048369: 89 e5 mov %esp,%ebp
804836b: 83 ec 08 sub $0x8,%esp
804836e: 83 e4 f0 and $0xfffffff0,%esp
8048371: 83 ec 18 sub $0x18,%esp
8048374: 6a 00 push $0x0
8048376: 68 64 84 04 08 push $0x8048464
804837b: e8 30 ff ff ff call 80482b0 <printf@plt>
--
08048368 <main>:
8048368: 55 push %ebp
8048369: 89 e5 mov %esp,%ebp
804836b: 83 ec 08 sub $0x8,%esp
804836e: 83 e4 f0 and $0xfffffff0,%esp
8048371: 83 ec 18 sub $0x18,%esp
8048374: 68 56 34 12 00 push $0x123456
8048379: 68 68 84 04 08 push $0x8048468
804837e: e8 2d ff ff ff call 80482b0 <printf@plt>
So I'd say that both in 3.4.3 and 4.1.1, no extra space is needed for the "const
static int" flag.
Regards,
Frederik
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