[LLVMdev] Collapse OBJC_CLASSLIST_REFERENCES globals in llvm-link?

[Nathan McCauley]

Hello list,

I'm wondering about the feasibility of collapsing the following two globals
within llvm-link:

@"OBJC_CLASSLIST_REFERENCES_$_2" = private global %struct.objc_class*
bitcast (%struct._class_t* @"OBJC_CLASS_$_NSString" to %struct.objc_class*),
section "__DATA, __objc_classrefs, regular, no_dead_strip", align 4
@"OBJC_CLASSLIST_REFERENCES_$_8" = private global %struct.objc_class*
bitcast (%struct._class_t* @"OBJC_CLASS_$_NSString" to %struct.objc_class*),
section "__DATA, __objc_classrefs, regular, no_dead_strip", align 4

The reason we'd like to do it in llvm-link is because the Mach-O linker
isn't doing it for us, and instead giving us the following error when we try
to link:

ld: in /var/folders/q7/q7f3KCNoGkOtsKthZvb6LJWOlJU/-Tmp-//ccjnT818.o,
__objc_classrefs element missing reloc (count=2) for target class in
/var/folders/q7/q7f3KCNoGkOtsKthZvb6LJWOlJU/-Tmp-//ccjnT818.o

collect2: ld returned 1 exit status

The code that generates this error (from ld.cpp) is here:

template <typename A>
void AnonymousAtom<A>::resolveName()
{
/*some code omitted*/
   else if ( (strncmp(fSection->sectname(),  "__objc_classrefs", 16) == 0)
&& (strcmp(fSection->segname(), "__DATA") == 0) ) {
                std::vector<ObjectFile::Reference*>& references =
this->getReferences();
                if ( references.size() != 1 )
                        throwf("__objc_classrefs element missing reloc
(count=%ld) for target class in %s", references.size(), fOwner.getPath());
                const char* targetName = references[0]->getTargetName();
                if ( strncmp(targetName, "_OBJC_CLASS_$_", 14) == 0 )
                        asprintf((char**)&fSynthesizedName,
"objc-class-ref-to-%s", &targetName[14]);
                else
                        asprintf((char**)&fSynthesizedName,
"objc-class-ref-to-%s", targetName);
        }
}

The Mach-O linker knows how to do the collapsing, but does not do it.  It
doesn't do it (seemingly) because we remove the .subsections_via_symbols
assembly directive from the .s file generated by llc.  We removed this
directive at the recommendation of Apple support, because the linker was
shuffling our code in a way that broke it.  (We insert labels in the middle
of instruction streams, that aren't referenced by anything else).  I can't
for the life of me find the code in the linker that does the collapsing; all
I know is that it isn't doing it.

I'm also open to other suggestions from the list on how to work around this
issue.  Basically, it seems like I need to collapse these two references
somewhere, either in llvm-link or by hand-editting the assembly file.  Does
anyone have any insight or advice on how to work around this linker
limitation?  Thanks in advance.

Nathan McCauley
Arxan Technologies
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