[llvm-bugs] [Bug 38676] New: LTO ignores -fPIC/-fPIE when building a non PIE executable

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38676

            Bug ID: 38676
           Summary: LTO ignores -fPIC/-fPIE when building a non PIE
                    executable
           Product: new-bugs
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: mh+llvm at glandium.org
                CC: llvm-bugs at lists.llvm.org

$ cat hello.c 
#include <stdio.h>

int main() {
  printf("Hello, world\n");
  return 0;
}

$ clang-7 -o hello hello.c -O3 -fPIC -flto=thin # same result with -flto
$ objdump -d hello
(...)
0000000000401130 <main>:
  401130:       50                      push   %rax
  401131:       bf 04 20 40 00          mov    $0x402004,%edi
  401136:       e8 f5 fe ff ff          callq  401030 <puts at plt>
  40113b:       31 c0                   xor    %eax,%eax
  40113d:       59                      pop    %rcx
  40113e:       c3                      retq   
  40113f:       90                      nop
(...)

Note how the string is loaded with an absolute address, compared to:

$ clang-7 -o hello hello.c -O3 -fPIC
$ objdump -d hello
(...)
0000000000401130 <main>:
  401130:       50                      push   %rax
  401131:       48 8d 3d cc 0e 00 00    lea    0xecc(%rip),%rdi        # 402004
<_IO_stdin_used+0x4>
  401138:       e8 f3 fe ff ff          callq  401030 <puts at plt>
  40113d:       31 c0                   xor    %eax,%eax
  40113f:       59                      pop    %rcx
  401140:       c3                      retq   
  401141:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
  401148:       00 00 00 
  40114b:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
(...)

FWIW, GCC does respect -fPIC/-fPIE on LTOed non-PIE executables.

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