hi,
configuration:
[mathieu@mathieu-laptop test]$ gcc -dumpmachine
x86_64-redhat-linux
[mathieu@mathieu-laptop test]$ gcc --version
gcc (GCC) 4.4.0 20090506 (Red Hat 4.4.0-4)
[mathieu@mathieu-laptop test]$ uname -a
Linux mathieu-laptop 2.6.29.5-191.fc11.x86_64 #1 SMP Tue Jun 16
23:23:21 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux
testcase:
[mathieu@mathieu-laptop test]$ cat >test.c
__thread int g_a;
int main (int argc, char *argv[])
{
g_a = 0;
return 0;
}
[mathieu@mathieu-laptop test]$ gcc -c -fpie ./test.c -o test.o
[mathieu@mathieu-laptop test]$ objdump -d ./test.o
0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 89 7d fc mov %edi,-0x4(%rbp)
7: 48 89 75 f0 mov %rsi,-0x10(%rbp)
b: 64 c7 04 25 00 00 00 movl $0x0,%fs:0x0
12: 00 00 00 00 00
17: b8 00 00 00 00 mov $0x0,%eax
1c: c9 leaveq
1d: c3 retq
hrm, this code appears to be use a static tls model
and, now:
[mathieu@mathieu-laptop test]$ gcc -c -fpic ./test.c -o test.o
[mathieu@mathieu-laptop test]$ objdump -d ./test.o
0000000000000000 <main>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 83 ec 10 sub $0x10,%rsp
8: 89 7d fc mov %edi,-0x4(%rbp)
b: 48 89 75 f0 mov %rsi,-0x10(%rbp)
f: 66 48 8d 3d 00 00 00 lea 0x0(%rip),%rdi # 17 <main+0x17>
16: 00
17: 66 66 48 e8 00 00 00 callq 1f <main+0x1f>
1e: 00
1f: c7 00 00 00 00 00 movl $0x0,(%rax)
25: b8 00 00 00 00 mov $0x0,%eax
2a: c9 leaveq
2b: c3 retq
This code is using a dynamic tls model (the callq instruction is
calling __tls_get_addr)
I am pretty happy because I think that this is the first time I
actually see a difference between -fpic and -fpie so, I feel I
understand better what -fpie really does but what I get is not really
what I was hoping for. i.e., I was hoping that -fpie would not require
the use of the static tls model. I tried passing
-ftls-model=global-dynamic with -fpie but that did not help in any
way. Hence, my questions:
1) is this (which tls model to use by default) the gist of the
difference between -fpie and -fpic ? Are there other subtle
differences between the two ?
2) is there a way to make -fpie still use a dynamic tls model (short
of hacking the code to add a tls_model("global-dynamic") attribute to
my __thread variable declaration ?
I tried some more experimentation:
[mathieu@mathieu-laptop test]$ gcc -fpie -pie ./test.c -o test
/usr/bin/ld: /tmp/ccsIE4YR.o: relocation R_X86_64_TPOFF32 against
`g_a' can not be used when making a shared object; recompile with
-fPIC
/tmp/ccsIE4YR.o: could not read symbols: Bad value
collect2: ld returned 1 exit status
[mathieu@mathieu-laptop test]$ gcc -fpie ./test.c -o test
[mathieu@mathieu-laptop test]$ readelf -h ./test |grep Type
Type: EXEC (Executable file)
[mathieu@mathieu-laptop test]$ gcc -pie -fpic ./test.c -o test
[mathieu@mathieu-laptop test]$ readelf -h ./test |grep Type
Type: DYN (Shared object file)
Which brings me to more questions:
1) why does "gcc -fpie -pie" fail to work ? Is this not the
recommended way to use -fpie and -pie ? Could it be that -fpie is
using the static tls model when it should not ?
2) is "gcc -fpic -pie" expected to do anything sensible ? (as it
appears to be doing)
3) is "gcc -fpie" really unsupported ?
--
Mathieu Lacage <mathieu.lacage@xxxxxxxxx>
