(LLD / lto ) How to avoid GOT code generation

Hi

I’m generating a special shared object in x86_64 large memory model (-mcmodel=large).

For this object, I pass -z notext to lld to leave the text relocations alone and normally it builds and works with no problem.

But when I add link time optimization, lld generates R_X86_64_GOTOFF64 which stops itself from linking. (Note that when I objdump -r on my elf objects without lto, this relocation is not generated; but for some reason lld decides to treat the non pic lto object as a pic object and generate got/pic code)

How can I tell lld to not generate GOT / PIC code ?

Here is the example:

When I compile as follows, my shared lib is generated and works fine

clang -mcmodel=large -o sample.o -c sample.c. # -flto=thin fails

ld.lld -Bshareable -z notext -o out.so sample.o

but when I compile with -flto=thin, lld errors that R_X86_64_GOTOFF64 can’t be used against foo and foo_call and wants me to compile with -fPIC but I don’t want to use PIC and GOT

sample.c :

extern int foo;

int* bar = &foo;

int foo_call (int, int *);

int foo_call (int a, int *b) {

return a+ *b;

}

int dummy (void) {

int *fooptr = &foo;

return foo_call (1, fooptr);

}

clang -fpic -flto -mcmodel=large -o sample.o -c sample.c will work.

Your compile mode is -fno-pic (Clang default for Linux), which can
only be linked in -no-pie mode.
-fpie is compatible with -no-pie and -pie.
-fpic is compatible with -no-pie, -pie and -shared (-Bsharable).

-fno-pic + -shared can sometimes work, work in more cases with -z
notext, but still not always.

For this case: the formula for R_X86_64_GOTOFF64 is S+A-GOT where S
represents the symbol value.
Since foo is undefined, the relocation cannot be resolved at link
time. LLD does not produce a dynamic relocation
because R_X86_64_GOTOFF64 is not a generally acceptable dynamic
relocation type by ld.so implementations.

I'd say R_X86_64_GOTOFF64 is a direct access relocation type, not a
GOT indirection relocation type.

A GOT indirection relocation type should compute to a place in GOT
(e.g. G+A, G+GOT+A-P), but R_X86_64_GOTOFF64 (S+A-GOT) is different.
"GOT" refers to the address of _GLOBAL_OFFSET_TABLE_.

The large code model needs the GOT base ("GOT" in x86-64 psABI:
"Represents the address of the global offset table") as a fixed place
in the text segment to
compute the addresses of symbols. The GOT base is not used in the GOT
indirection manner.

Thank you for reply

I understand the semantics of the relocation. My point is that my system currently builds and works without -fpic nor -fpie and the R_X86_64_GOTOFF64 relocation is not generated by clang itself and without -flto, everything works and that is proof that this relocation is not essential for my application …

The problem is when I add -flto, lld’s code generation unnecessarily generates code with this relocation. I’m looking for an option to pass to lld (or the x86_64 lto plugin of lld) to keep it from doing that.

A