TLS details on Linux for x86 and x64

[Liu Hao]

Dear x86 and x64 developers,

I and JonY are fumbling for implementation of native TLS support for x86
and x64 on Windows. Similarly to that on Linux, the address of a thread
local object on Windows is calculated via indirection from the FS or GS
segment register, but requires more than one steps. We have trouble
comprehending how TLS works on Linux at the moment:

Compiling this program:
```
int a = 12345;
int get_a(void){ return a; }

__thread int b = 67890;
int get_b(void){ return b; }
```

with `gcc -O2 -S` on x64 results in:
```
.file   "test.c"
.text
.p2align 4,,15
.globl  get_a
.type   get_a, @function
get_a:
.LFB0:
.cfi_startproc
movla(%rip), %eax
ret
.cfi_endproc
.LFE0:
.size   get_a, .-get_a
.p2align 4,,15
.globl  get_b
.type   get_b, @function
get_b:
.LFB1:
.cfi_startproc
movl%fs:b@tpoff, %eax
ret
.cfi_endproc
.LFE1:
.size   get_b, .-get_b
.globl  b
.section.tdata,"awT",@progbits
.align 4
.type   b, @object
.size   b, 4
b:
.long   67890
.globl  a
.data
.align 4
.type   a, @object
.size   a, 4
a:
.long   12345
.ident  "GCC: (Debian 6.3.0-18) 6.3.0 20170516"
.section.note.GNU-stack,"",@progbits

```

The questions are:

0) What is the magical `@tpoff` suffix supposed to do? The `@ntpoff` and
`@dtpoff` things are documented in System V ABI but there doesn't seem
to be anything about `@tpoff`.
1) How does LD tell that `b` (a thread-local integer) is different from
`a` (a static integer)? `a` is apparently offset from RIP, but what
thing is `b` offset from?
2) TLS initializers are placed into specially named sections. The
sections will have the names like `.tls$XXX` where `$XXX` is used to
sort these sections and discarded thereafter. How is LD supposed to
associate the section containing the initializer with the symbol of
object being initialized, without disordering?

Any help will be appreciated.

-- 
Best regards,
LH_Mouse

Re: TLS details on Linux for x86 and x64

[Andrew Haley]

On 02/12/17 14:04, Liu Hao wrote:
> 
> 0) What is the magical `@tpoff` suffix supposed to do? The `@ntpoff` and
> `@dtpoff` things are documented in System V ABI but there doesn't seem
> to be anything about `@tpoff`.
> 1) How does LD tell that `b` (a thread-local integer) is different from
> `a` (a static integer)? `a` is apparently offset from RIP, but what
> thing is `b` offset from?
> 2) TLS initializers are placed into specially named sections. The
> sections will have the names like `.tls$XXX` where `$XXX` is used to
> sort these sections and discarded thereafter. How is LD supposed to
> associate the section containing the initializer with the symbol of
> object being initialized, without disordering?
> 
> Any help will be appreciated.

Have you read

https://www.fsfla.org/~lxoliva/writeups/TLS/RFC-TLSDESC-x86.txt

?

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. 
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671