[Bug c/14272] New: miscompilation of __exchange_and_add (atomicity.h)

[ehrhardt at mathematik dot uni-ulm dot de]

The following piece of code (destilled from atomicity.h in
libstdc++-v3) is miscompiled by tree-ssa with -O2 on a sparc-sun-solaris2.9
platform (probably arch independant). The code outputs BUG!!! where
it clearly shouldn't. This works with C and C++.

====================== cut =============================
extern int printf (const char *, ...);

int __attribute__ ((__noinline__))
__exchange_and_add (volatile int* __mem, int __val)
{
  int __result, __tmp;
  __result = *__mem;
  *__mem += __val;
  return __result;
}

int data = 0;

int main ()
{
	if (__exchange_and_add (&data, 1) != 0) {
		printf ("BUG!!!\n");
	}
	return 0;
}
====================== cut =============================

The consequences in the context of libstdc++ on sparc-sun-solaris are
that std::cout and friends are never initialized, i.e. not even a hello
world program runs. Note that the volatile keyword is crucial, removing
it makes the bug go away. The wrong code first appears in .t35.optimized.

    regards   Christian

-- 
           Summary: miscompilation of __exchange_and_add (atomicity.h)
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: critical
          Priority: P1
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ehrhardt at mathematik dot uni-ulm dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14272