Why -fno-pic and -fPIC gives the same results?

Clang Frontend
1

Hi,

I got this. Why -fno-pic and -fPIC are the same?

$ cat f.c
void f() { }
$ clang -fno-pic f.c -c -o nopic.o
$ clang -fPIC f.c -c -o pic.o
$ cmp nopic.o pic.o
$ echo "$?"
0

2

Hello,

On many targets the instructions needed for an empty function won't
need to refer to an address at all, so will always be position
independent, hence no difference in code-generation. If you make the
example a bit more complex, such as using a global variable then
you'll see some code-generation differences between -fPIC and
-fno-pic.

Hope this helps.

Peter

3

On many targets the instructions needed for an empty function won't
need to refer to an address at all, so will always be position
independent, hence no difference in code-generation. If you make the
example a bit more complex, such as using a global variable then
you'll see some code-generation differences between -fPIC and
-fno-pic.

Could you help make a minimal example? I used some global variables.
But the results are still the same. Thanks.

4

A caveat, this is on X86_64. Your clang may have a target where all
code is PIC. Let me know if I've misunderstood your question?

int global;

int f() { return global; }

I've edited the assembler output to remove directives.
clang f.c -fno-pic -S -o -
f:
pushq %rbp
movq %rsp, %rbp
movl global, %eax
popq %rbp
retq

clang f.c -fPIC -S -o -
f:
pushq %rbp
movq %rsp, %rbp
movq global@GOTPCREL(%rip), %rax
movl (%rax), %eax
popq %rbp
retq

5

Here is what I have. So Mac OS X does not support -fno-pic? I think
that clang should show some messages when -fno-pic is not supported.

$ uname
Darwin
$ cat main.sh
#!/usr/bin/env bash
# vim: set noexpandtab tabstop=2:

set -v
function cmd {
diff <($CC -S -fno-pic f.c -c -o -) <($CC -S -fPIC f.c -c -o -)
}

CC=clang cmd
CC=gcc cmd
$ ./main.sh
function cmd {
diff <($CC -S -fno-pic f.c -c -o -) <($CC -S -fPIC f.c -c -o -)
}

CC=clang cmd
CC=gcc cmd
$ cat f.c
/* vim: set noexpandtab tabstop=2: */
#include <stdio.h>

int global;
int f() { return global; }

$ uname
Linux
$ ./main.sh
function cmd {
diff <("$CC" -S -fno-pic f.c -c -o -) <("$CC" -S -fPIC f.c -c -o -)
}

CC=clang cmd
"$CC" -S -fPIC f.c -c -o -
"$CC" -S -fno-pic f.c -c -o -
14c14,15
< movl global, %eax

6

64-bit mac binaries (both x86 and arm) always force PIC on and it can’t be disabled there. (cf http://llvm-cs.pcc.me.uk/tools/clang/lib/Driver/ToolChains/Darwin.cpp#2222)