[LLVMdev] problem with __thread on linux/x86_64

[Jay Foad]

On 9 November 2010 16:42, Jay Foad <jay.foad at gmail.com> wrote:
> Hi,
>
> On Ubuntu Linux/x86_64, I get:
>
> $ cat t.c
> #include <stdio.h>
> __thread int i = 7;
> int main() { printf("%d\n", i); }
> $ clang -o t t.c
> $ ./t
> Segmentation fault
>
> (gdb) disas
> Dump of assembler code for function main:
>   0x0000000000400560 <+0>:     push   %rbp
>   0x0000000000400561 <+1>:     mov    %rsp,%rbp
>   0x0000000000400564 <+4>:     sub    $0x10,%rsp
>   0x0000000000400568 <+8>:     movl   $0x0,-0x4(%rbp)
> => 0x000000000040056f <+15>:    mov    %fs:0x402034,%esi
>   0x0000000000400577 <+23>:    xor    %al,%al
>   0x0000000000400579 <+25>:    mov    $0x40066c,%edi
>   0x000000000040057e <+30>:    callq  0x4006e0 <printf at plt>
>   0x0000000000400583 <+35>:    mov    %eax,-0x8(%rbp)
>   0x0000000000400586 <+38>:    mov    -0x4(%rbp),%eax
>   0x0000000000400589 <+41>:    add    $0x10,%rsp
>   0x000000000040058d <+45>:    pop    %rbp
>   0x000000000040058e <+46>:    retq
> End of assembler dump.
> (gdb) p $fs
> $1 = 0
> (gdb) p *(int*)0x402034
> $2 = 0
>
> I don't really understand why it's segfaulting here, but presumably
> it's something to do with thread-local storage not being set up
> properly?
>
> I'm using Clang and LLVM from svn trunk, updated a few minutes ago.

The problem seems to be with Clang's assembler:

$ clang -S -o t.s t.c
$ clang -c -o bad.o t.s
$ gcc -o bad bad.o
$ ./bad
Segmentation fault
$ gcc -c -o good.o t.s
$ gcc -o good good.o
$ ./good
7

The only difference here is whether t.s was assembled by clang or by gcc.

Delving a bit deeper, the difference seems to be that symbol "i" in
the assembler source:

        .type   i, at object               # @i
        .section        .tdata,"awT", at progbits
        .globl  i
        .align  4
i:
        .long   7                       # 0x7
        .size   i, 4

... is given ELF symbol type STT_TLS by gcc's assembler, but
STT_OBJECT by clang's assembler.

I think this used to work, a few weeks ago. Has anything changed in
this are recently?

Thanks,
Jay.