[LLVMdev] Need to create symbols only once

Nick Kledzik
Fri, 07 Dec 2012 22:59:18 -0000

On Dec 7, 2012, at 11:51 AM, Shankar Easwaran wrote:
> We have few symbols like __bss_start, __bss_end, which are Undefined
symbols in the code.
>
> I want a way in the Reader to create specific atoms before the linker
bootstraps.
>
> I didnt find a way to do that with the existing interfaces.
>
> The way it needs to work is as below :-
>
> 1) ReaderELF creates Absolute symbols (for __bss_start, __bss_end etc)
> 2) ReaderELF reads each file and adds Atoms to the list
> 3) If the atoms the linker defined were Global, the atoms that the Reader
created should get overridden with the linker created ones.
>
> This may also be needed to pul in specific symbols from archive libraries,
too.
>
> I was thinking to add an interface to ReaderELF which would be called by
the driver, but the problem is the DefinedAtom/AbsoluteAtom have which file owns
the atom.
>
> I was discussing with Michael on this, and he was proposing to add a
Pre-Read file.
>
> Do you have any other opinions too ?

We have a similar requirement in darwin's ld64 linker, but even more
general.  Any binary can do the following to introspect itself:

struct stuff { int a; int b; };

extern struct stuff*  stuff_start  __asm("section$start$__DATA$__my");
extern struct stuff*  stuff_end   __asm("section$end$__DATA$__my");

void examineSection() {
	const struct stuff* p;
	for (p = stuff_start; p < stuff_end; ++p) {
		// do stuff with p
	}
}

That is, there are magic symbol names which reference the beginning or ending of
any particular section.   To support this, the linker lazily creates atoms when
references to these magic symbols are discovered during resolving.

I have some hooks for this already in place in lld:

1) There is Writer::addFiles().  This method gives any writer a change to add
files/atoms to the set of atoms the Resolver works on.   The Writer::addFiles()
method is called after all input files are added.  If you want to add something
lazily (like darwin linker does for section$start$ symbols), the writer returns
a File object akin to a static library.  That it, it provides no initial atoms,
but can provide atoms as a last resort (so an .o files would override it).  The
WriterMachO already uses the addFiles() method to add CRuntime symbols.

2) DefinedAtom::ContentType already has typeFirstInSection and
typeLastInSection.  These are intended to be used for the content type of the
atoms which represent the magic symbols for the start and end of a section.  The
key here is that the Pass (not written yet) which sorts atoms, knows to sort
these atoms to the start or end of their respective sections.

If you don't want this full general, lazy approach, you could have your
WriteELF::addFiles() return a regular object file that has atoms named
__bss_start and __bss_end, but they are marked mergeAsWeak so that any user
defined atoms will override them.

-Nick