https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81310
Bug ID: 81310
Summary: Is __attribute__ alias on variables valid?
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: palves at redhat dot com
Target Milestone: ---
I'm improving GDB's support for finding symbols resulting from __attribute__
alias, and while writing a test noticed something that I'd like clarification
on.
While __attribute__ alias is documented as a function attribute:
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
I noticed that GCC lets you use it with variables too. That seems odd to me,
since that makes it possible to have two different objects with the same
address:
$ cat symbol-alias.c
extern int global1;
extern int global2;
int
main (void)
{
if (&global1 == &global2)
__builtin_abort ();
return 0;
}
$ cat symbol-alias1.c
int global1 = 1;
extern int global2 __attribute__ ((alias ("global1")));
$ /opt/gcc/bin/gcc symbol-alias.c symbol-alias1.c -O2 -Wall
$ nm ./a.out | grep " global"
0000000000601040 D global1
0000000000601040 D global2
$ ./a.out
Aborted (core dumped)
I also noticed that clang optimizes out the abort above, even at -O0, most
likely assuming that the objects can't have both the same address:
$ clang symbol-alias.c symbol-alias1.c -O0 -Wall
$ nm ./a.out | grep " global"
000000000060102c D global1
000000000060102c D global2
$ ./a.out
$ (no abort)
$ objdump -d a.out
...
0000000000400500 <main>:
400500: 55 push %rbp
400501: 48 89 e5 mov %rsp,%rbp
400504: 31 c0 xor %eax,%eax
400506: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%rbp)
40050d: 5d pop %rbp
40050e: c3 retq
40050f: 90 nop
...
So, hence the questions:
- Is __attribute__ alias valid on variables?
- If valid, ISTM the documentation needs improvement.
- If invalid, shouldn't GCC reject it?
- And if invalid, is "(&global1 == &global2) => true" a missed optimization?
