[LLVMdev] Implement implicit TLS on Windows - need advice

[kai]

Hi Michael!

Thanks for your answer.

I got a step further - I can generate some code which looks not too bad. 
And yes - I am aware of the fact that test cases are still missing.

Thanks for pointing out that the 32bit code is a bit different from the 
64bit code. I have a real use case for the 64bit code, so this is my 
first target. I added an assert for the 32bit case. I also changed the 
name of the section to .tls$.

I still have some questions:

1) In WinCOFFObjectWriter::RecordRelocation I check for the new 
MCSymbolRefExpr::VK_SECREL. Is this the right approach or should I 
better create a new fixup kind?

2) Is there a way to lower the code so that an expression like rax+8*rbx 
is generated by default?

Thank you!

Kai

On 06.12.2011 01:22, Michael Spencer wrote:
> On Sun, Dec 4, 2011 at 9:18 AM, Kai<kai at redstar.de>  wrote:
>> Hi!
>>
>> LLVM currently does not implement the implicit TLS model on Windows. This
>> model is easy:
>>
>> - a thread local variable ends up in the .tls section
>> - to access a thread local variable, you have to do
>>   (1) load pointer to thread local storage from TEB
>>       On x86_64, this is gs:0x58, on x86 it is fs:0x2C.
>>   (2) load pointer to thread local state. In general, the index is stored in
>> variable _tls_index. For a .exe, _tls_index is always 0.
>>   (3) load offset of variable to start of .tls section.
>>   (4) the thread local variable can now accessed with the add of step 2 and
>> 3.
>>
>> For x86_64, something like the following should be generated for the tls1.ll
>> test case:
>>
>> (1)     mov     rdx, qword [gs:abs 58H]
>> (2)     mov     ecx, dword [rel _tls_index]
>>         mov     rcx, qword [rdx+rcx*8]
>> (3)     mov     eax, .tls$:i
>> (4)     mov     eax, dword [rax+rcx]
>>         ret
>>
>> (See the PECOFF spec, chapter 5.7 and http://www.nynaeve.net/?p=185 for
>> reference.)
>>
>> I tried to implement this. With the attached patch tls1.patch, a thread
>> local variable ends up in the .tls section. This looks fine.
>> With the second patch tls2.patch I try to implement the code sequence. Here
>> I have a couple of questions:
>>
>> - To get the offset to start of .tls section, I have created a new
>> MachineOperand flag. Is this the right approach? If yes then I need a hint
>> where to implement this in the WinCOFFObjectWriter.
>> - How can I code the load of variable _tls_index in SelectionDAG? I have
>> some trouble using DAG.getExternalSymbol and then loading the value.
>>
>> Thanks for your help.
>>
>> Kai
>
> Thanks for working on this!
>
> The first patch looks fine except that it's emitting to .tls when it
> should be .tls$. Also, you need to add tests.
>
> As for the second patch, that's not how MSVC 2010 emits code (and it
> needs tests).
>
> thread_local.c:
>
> #ifdef _MSC_VER
> #define __thread __declspec(thread)
> #endif
> __thread int i = 0;
>
> int foo() {
>    return i++;
> }
>
> thread_local.asm:
>
> PUBLIC	_i
> _TLS	SEGMENT
> _i	DD	00H
> _TLS	ENDS
> PUBLIC	_foo
> EXTRN	__tls_array:DWORD
> EXTRN	__tls_index:DWORD
> ; Function compile flags: /Ogtpy
> _TEXT	SEGMENT
> _foo	PROC
> ; File c:\users\mspencer\projects\llvm-project\test\thread_local.c
> ; Line 7
> 	mov	eax, DWORD PTR __tls_index
> 	mov	ecx, DWORD PTR fs:__tls_array
> 	mov	ecx, DWORD PTR [ecx+eax*4]
> 	mov	eax, DWORD PTR _i[ecx]
> 	lea	edx, DWORD PTR [eax+1]
> 	mov	DWORD PTR _i[ecx], edx
> ; Line 8
> 	ret	0
> _foo	ENDP
> _TEXT	ENDS
> END
>
> llvm-objdump -d -r -s thread_local.obj:
>
> Disassembly of section .text:
> _foo:
>         0:       a1 00 00 00 00                                  movl    0, %eax
>                                 1: IMAGE_REL_I386_DIR32  __tls_index
>         5:       64 8b 0d 00 00 00 00                            movl
>   %fs:0, %ecx
>                                 8: IMAGE_REL_I386_DIR32  __tls_array
>         c:       8b 0c 81                                        movl
>   (%ecx,%eax,4), %ecx
>         f:       8b 81 00 00 00 00                               movl
>   (%ecx), %eax
>                                11: IMAGE_REL_I386_SECREL _i
>        15:       8d 50 01                                        leal
>   1(%eax), %edx
>        18:       89 91 00 00 00 00                               movl
>   %edx, (%ecx)
>                                1a: IMAGE_REL_I386_SECREL _i
>        1e:       c3                                              ret
>
> Contents of section .tls$:
>   0000 00000000                             ....
> Contents of section .text:
>   0000 a1000000 00648b0d 00000000 8b0c818b  .....d..........
>   0010 81000000 008d5001 89910000 0000c3    ......P........
>
> - Michael Spencer
>

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