[Bug c/24097] New: x86_64 -fPIC code gen bug (extern/static linkage confusion)

[mcoiley at yahoo dot com]

With this testcase:

int foo (int (*cb)(void)) { return (*cb)(); }
static int callback(void) { return 0; }

int main (void)
{
 int callback (void);
 return foo (callback);
}

On x86_64 with -fPIC, the address of callback as passed to foo is incorrect.

If callback isn't static, it works (through the GOT).  If the declaration of
callback is removed it works (computes the address as %rip minus something).  As
written, it does a failed combination of the two.  The result is that instead of
passing callback's address, it loads the first 8 bytes at callback's address and
passes those.  (It also erroneously claims callback is unused, but I'm guessing
that's a side-effect.)

The code is questionable, but my reading of the standard says it's legal and
that callback is still static after the (implicitly extern) declaration.

% gcc-3.4.4 -O2 -fPIC -Wall -save-temps -o bug bug.c 
bug.c:2: warning: 'callback' defined but not used

% gcc-3.4.4 -v
Reading specs from /usr/local/lib/gcc/x86_64-pc-linux-gnu/3.4.4/specs
Configured with: /tmp/src/gcc-3.4.4/configure --enable-languages=c,c++
--enable-version-specific-runtime-libs --disable-shared --enable-threads
--with-gnu-as --with-gnu-ld --with-pic --host=x86_64-pc-linux-gnu
Thread model: posix
gcc version 3.4.4

[I bootstrapped this 3.4.4; it also fails with stock 3.4.3 from RedHat.]

-- 
           Summary: x86_64 -fPIC code gen bug (extern/static linkage
                    confusion)
           Product: gcc
           Version: 3.4.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mcoiley at yahoo dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: x86_64-pc-linux-gnu
  GCC host triplet: x86_64-pc-linux-gnu
GCC target triplet: x86_64-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24097