Hi,
Consider the following C source:
$ cat globaltest.c
int MyGlobal;
void set_global() {
MyGlobal = 5;
}
I generated object files using clang and gcc as follows:
$ clang -o globaltest_clang.o -c globaltest.c
$ gcc -o globaltest_gcc.o -c globaltest.c
I notice that gcc generated encoding of the instruction that moves 5
to global is pc-relative location but clang generated encoding of the
same instruction is 0-reg relative (See instruction at location 4 in
the objdumps below).
I understand upon linking this object (given the relocation record)
with other objects to get a final executable (or DSO) results in a
fully functional binary. But, as far as the correctness of generated
object code, is this a bug?
Additional information:
objdump of the generated object files:
$ objdump -d globaltest_clang.o
globaltest_clang.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <set_global>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: c7 04 25 00 00 00 00 movl $0x5,0x0
b: 05 00 00 00
f: 5d pop %rbp
10: c3 retq
$ objdump -d globaltest_gcc.o
globaltest_gcc.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <set_global>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: c7 05 00 00 00 00 05 movl $0x5,0x0(%rip) # e
<set_global+0xe>
b: 00 00 00
e: 90 nop
f: 5d pop %rbp
10: c3 retq
Thanks,
Bharadwaj