[Bug target/81490] x86: Handling of symbol ranges for __seg_fs/__seg_gs

["hjl.tools at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81490

--- Comment #27 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to H. Peter Anvin from comment #26)
> @GPREL (altough it probably should be @GPOFF by analogy with @TPOFF?) gives
> the linker an option to distinguish the relocations which need to be
> adjusted at link/load time and the ones that don't.
> 

The GP offset is fixed at link-time.

> We have our own way of doing that for the Linux kernel, but I'm guessing
> H.J. wants a more general solution.
> 

It makes __seg_fs and __seg_gs easier to use:

[hjl@gnu-6 gprel-1]$ cat x.c
int __seg_gs foo;

int
get_foo (void)
{
  return foo;
}

void
set_foo (int x)
{
  foo = x;
}

static int __seg_gs bar;

int
get_bar (void)
{
  return bar;
}

void
set_bar (int x)
{
  bar = x;
}
[hjl@gnu-6 gprel-1]$ cat foo.c
#define _GNU_SOURCE
#include <stdio.h>
#include <syscall.h>
#include <unistd.h>
#include <asm/prctl.h>

extern int __seg_gs foo;
extern int __gp;
extern int get_foo (void);
extern void set_foo (int);
extern int get_bar (void);
extern void set_bar (int);

int
setup_gp (void *p)
{
  return syscall (SYS_arch_prctl, ARCH_SET_GS, p);
}

int
main ()
{
  setup_gp (&__gp);
  printf ("foo: %d\n", foo);
  printf ("foo: %d\n", get_foo ());
  foo = 30;
  printf ("foo: %d\n", foo);
  printf ("foo: %d\n", get_foo ());
  set_foo (-30);
  printf ("foo: %d\n", foo);
  printf ("foo: %d\n", get_foo ());
  printf ("bar: %d\n", get_bar ());
  set_bar (-30);
  printf ("bar: %d\n", get_bar ());
  return 0;
}
[hjl@gnu-6 gprel-1]$ make
/export/build/gnu/gcc-x32/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-x32/build-x86_64-linux/gcc/ -mgprel -g -O2 -c -o x.o
x.c
/export/build/gnu/gcc-x32/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-x32/build-x86_64-linux/gcc/ -mgprel -g -O2 -c -o foo.o
foo.c
/export/build/gnu/gcc-x32/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-x32/build-x86_64-linux/gcc/ -mgprel -o x x.o foo.o
./x
foo: 0
foo: 0
foo: 30
foo: 30
foo: -30
foo: -30
bar: 0
bar: -30
[hjl@gnu-6 gprel-1]$