llvm dev - Sep 2012 - [LLVMdev] Question about ctors, dtors and sections on Windows

Hello all!

I extended the LDC2 with a pragma to register a funcion in the 
llvm.global_ctors or llvm.global_dtors list.

On Linux, references to these functions are placed in .ctors and .dtors 
sections and everything runs fine.

On Windows, functions from llvm.global_ctors are placed in section 
.CRT$XCU, which is automatically called by the MS C Runtime. However, 
functions from llvm.global_dtors are placed in section .dtors which is 
unknown to the runtime. Therefore only ctors are called. Is this 
expected behaviour?

My expectation was that the dtors placed in .CRT$XTU which are the C 
terminator functions. Maybe there is a way to customize this?

Another question is how can I control section names and attributes.
Before I found the ctors/dtors variables I tried to implement this 
functionality by placing a variable in a section with a special name. 
This works because a global variable can have an optional section name.

But:
- How can I control section attributes and alingment? E.g. the .CRT$XU 
segment is read-only. How can I specify that?
- How can I place constant data in the .rdata section?

Thanks for any hints!

Regards
Kai

Hello
> On Windows, functions from llvm.global_ctors are placed in section
.CRT$XCU,
> which is automatically called by the MS C Runtime.
Only if you link with MS runtime. Mingw follows standard .ctors / .dtors scheme.
> My expectation was that the dtors placed in .CRT$XTU which are the C
> terminator functions. Maybe there is a way to customize this?
Yes. Target (or,rather subtarget) specifies this. Looks like it was
somehow missed for dtors.
> - How can I control section attributes and alingment? E.g. the .CRT$XU
> segment is read-only. How can I specify that?
This is job of target, not user code. If you need such granularity -
write assembler
> - How can I place constant data in the .rdata section?
The global should be constant. Everything should happen automagically.
It might be some stuff which depends on relocations, etc.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

On 10.09.2012 21:53, Anton Korobeynikov wrote:
> Hello
>
>> On Windows, functions from llvm.global_ctors are placed in section
.CRT$XCU,
>> which is automatically called by the MS C Runtime.
> Only if you link with MS runtime. Mingw follows standard .ctors / .dtors
scheme.
Yes, I am targeting the MS runtime
>> My expectation was that the dtors placed in .CRT$XTU which are the C
>> terminator functions. Maybe there is a way to customize this?
> Yes. Target (or,rather subtarget) specifies this. Looks like it was
> somehow missed for dtors.
Thanks for the hint. I found it. I'll prepare a patch if I can fix it.
>> - How can I control section attributes and alingment? E.g. the .CRT$XU
>> segment is read-only. How can I specify that?
> This is job of target, not user code. If you need such granularity -
> write assembler
I am thinking about how to implement #pragma (init_seg) and 
#pragma(data_seg) from MSVC in the LDC compiler. Therefore I would need 
this control.
>> - How can I place constant data in the .rdata section?
> The global should be constant. Everything should happen automagically.
> It might be some stuff which depends on relocations, etc.
Seems to be another problem. I'll investigate this a bit more.

Thanks for your answers.

Kai

On Mon, Sep 10, 2012 at 12:05 PM, Kai <kai at redstar.de>
wrote:
> On Windows, functions from llvm.global_ctors are placed in section
.CRT$XCU,
> which is automatically called by the MS C Runtime. However, functions from
> llvm.global_dtors are placed in section .dtors which is unknown to the
> runtime. Therefore only ctors are called. Is this expected behaviour?
>
> My expectation was that the dtors placed in .CRT$XTU which are the C
> terminator functions. Maybe there is a way to customize this?
Hi Kai. I submitted the patch to use the .CRT$XCU section for ctors on
Win32; I did not do so for dtors because I was unaware of .CRT$XTU. My
apologies. If you look in lib/MC/MCObjectFileInfo.cpp:417, you can see
the following logic:

```

  if (T.getOS() == Triple::Win32) {
    StaticCtorSection       Ctx->getCOFFSection(".CRT$XCU",
                          COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
                          COFF::IMAGE_SCN_MEM_READ,
                          SectionKind::getReadOnly());
  } else {
    StaticCtorSection       Ctx->getCOFFSection(".ctors",
                          COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
                          COFF::IMAGE_SCN_MEM_READ |
                          COFF::IMAGE_SCN_MEM_WRITE,
                          SectionKind::getDataRel());
  }
```

The logic just needs to be replicated to set StaticDtorSection
appropriately for Win32 targets. Sorry again for the oversight.

-Joe