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