[LLVMdev] Linkages handling

[Chris Lattner]

On Fri, 18 Jun 2004, Vladimir Prus wrote:

>
> Hello,
> looking at Printer::doFinalization, I have some question about support of the
> different linkages. Here's a table of how that method handles the possible
> combinations:
>
>                                  Initialized                Null
> Internal                      .data, local              .data, .comm, .local
> External                     .data, .globl             .bss, .globl,
> Appending                 ???                         ????
> LinkOnce                   llvm.linkonce.d          data, .comm
> 	                         .weak
> Weak                        ???                         ???
>
> And the question I have are:
>
> 1. Does ld really supports appending linkage?

Sorta.  Appending linkage is the LLVM way of supporting the ld
"concatenation of sections" mechanism.  I don't think any of our current
code generators support doing this, but the idea is that an appending
array could be emitted to a special section, which the linker will happily
append for you.

> 2. The difference between LLVM linkonce and weak is that linkone symbols can
> be discared, if not used.

That is correct.

> I don't think there's anything in ld which matches LLVM linkonce -- the
> symbols are never discarded. Am I wrong?

I'm not sure, but it's safe to codegen linkonce the same as weak in a
backend.  linkonce is primarily useful for LLVM->LLVM optimizers, such as
the dead global elimination pass.

> 3. I've tries to figure out how linkonce works with ld. It appears that it
> special cases sections starting with ".gnu.linkonce". In other words, naming
> a spection "llvm.linkonce.d" has no effect. Is it true?

I dunno.  :) I vagely recall reading something like that, and I'm pretty
sure there is SOME support for discarding unused objects, and I *think*
it's by section, but I don't really know. :)

In any case, the LLVM optimizer should have done its job already, so I
wouldn't worry too much about this.

> 4. Can internal null-initialized data be placed in the ".bss" section, instead
> of '.data' with '.comm' attribute, as is done now?

Yes, I think the X86 backend puts zero initialized globals in the .bss
section.  The conditions are:

  if (GV->hasInitializer()) // if the global is not external
    if (GV->getInitializer()->isNullValue())   // zero initailized
      // it can go in .bss

If you look in X86/Printer.cpp:Printer::doFinalization you can see the
logic it is using.  This was patterned mostly after what GCC was doing for
various cases, though it has a HUGE number of strange linkage types that
interact in bizarre ways so it's hard to tell.

> 5. Can linkonce null-initialized data be placed in the ".data" section, with
> the ".weak" attribute, instead of ".comm" attribute.

I'm not sure, sorry.

-Chris

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