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assemblygcclinkerrelocationcompiler-explorer

Relocation error happening locally but not in godbolt

Consider this program on godbolt:

#include <cassert>
#include <cstdint>

int64_t const x[] = { 42, 2, 3, 4 };

int64_t f() {
  int64_t i;
  asm volatile (
    "xor %[i], %[i]\n\t"
    "movq _ZL1x(,%[i],8), %[i]"
  : [i]"=r"(i));
  return i;
}

int64_t g() {
  int64_t i;
  asm volatile (
    "xor %[i], %[i]"
  : [i]"=r"(i));
  i = x[i];
  return i;
}

int main() {
  assert(f() == 42);
}

It compiles, links and runs fine for gcc 13.1 but clang 16.0 gives a linker error:

[...]: relocation R_X86_64_32S against `.rodata.cst32' can not be used when making a PIE object; recompile with -fPIE

If I try the same code locally and compile it with g++ -O3 main.cpp (same gcc version) I get the same error above. (Recompiling with -fPIE doesn't fix.)

It's worth noticing that the code generated by gcc for f() and g() are identical:

_Z1fv:
  xor %rax, %rax
  movq _ZL1x(,%rax,8), %rax
  ret
_Z1gv:
  xor %rax, %rax
  movq _ZL1x(,%rax,8), %rax
  ret

and if I remove f from the source file and use g in main, then all compilers are happy, locally and on godbolt.

I understand that directly writing _ZL1x in my inline assembly is weird and very likely to be a bad practice but that's not the point here. I would like to know why gcc is happy on godbolt and not locally and, more importantly, how could I make it to work locally and (if possible) for clang as well.

Solution

  • Most Linux distros configure GCC and Clang with -fPIE -pie -fstack-protector-strong on by default, and perhaps -fno-plt. So they're making Position-Independent Executables that have to be relocatable to anywhere in 64-bit address-space.

    Matt Godbolt's Compiler Explorer has compilers installed with their vanilla configuration, with none of those options on by default, so they're making traditional executables that get linked to a specific address in the low 31 bits of virtual address space. (Except x86-64 Clang 16.0 which has at least -fPIE enabled. This is purely up to the sysadmins of https://godbolt.org/ using the default config options when building compilers from source.)

    Any addressing mode other than symbol(%rip) uses at most a 32-bit sign-extended displacement1, so an absolute address has to fit into that. That only works in a Linux non-PIE executable, and I think a Windows non-LargeAddressAware executable or dll. Not in MachO64 no matter how you link it.

    If you locally did gcc -O3 -S, you could see the PIE-compatible asm it made for your g() function, using a RIP-relative LEA first.

    See

    (Recompiling with -fPIE doesn't fix.)

    That error message is assuming the machine code was generated by a compiler from a .c, not just assembled from hand-written asm. With hand-written asm, there is no compilation step, only assembling. Or, you the human are the "compiler", so you need to position-independent asm to implement the algorithm that exists in your brain. (64-bit absolute addresses are also allowed, with the dynamic linker applying a text-relocation fixup on load, but RIP-relative addressing is efficient.)

    Footnote 1: Except the special 64-bit absolute addressing mode for load/store of the accumulator, like movabs foo, %eax, which is less compact than RIP-relative so you don't want it if you're building normal code where static data is within +-2GiB of code. There's a reason compilers don't use it.