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Kai

2011-Dec-04 17:18 UTC

[LLVMdev] Implement implicit TLS on Windows - need advice

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
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Michael Spencer

2011-Dec-06 00:22 UTC

head link

[LLVMdev] Implement implicit TLS on Windows - need advice

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

kai

2011-Dec-08 17:03 UTC

head link

[LLVMdev] Implement implicit TLS on Windows - need advice

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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kai

2011-Dec-08 17:06 UTC

head link

[LLVMdev] Implement implicit TLS on Windows - need advice

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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