[Bug middle-end/35188] Missing a warning: inherently false expressions

["manu at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #6 from manu at gcc dot gnu dot org  2008-02-16 17:50 -------
> Please do not expect a compilable testcase from me. 1) I don't know whether
> this is a bug, 2) have no experience in gcc testcases.

The above is a testcase. See http://gcc.gnu.org/bugs.html#report on how to make
a good report.

A more self-contained and complete testcase follows:

extern void abort(void);

void foo(int var2)
{
  int var1 = 0;

  if (0 && 1)
    abort ();

  if (0 && var1 == 10)
    abort ();

  if (var1 == 10 && var1 == 20)
    abort ();

  if (var1 == 10)
    if (var1 == 20)
      abort ();

  if (var2 == 10 && var2 == 20)
    abort ();

  if (var2 == 10)
    if (var2 == 20)
      abort ();
}

The above gives with -Wunreachable-code -O0

/home/manuel/src/pr35188.c:8: warning: will never be executed
/home/manuel/src/pr35188.c:11: warning: will never be executed

And with -Wunreachable-code -O1

/home/manuel/src/pr35188.c: In function ?foo?:
/home/manuel/src/pr35188.c:8: warning: will never be executed
/home/manuel/src/pr35188.c:11: warning: will never be executed
/home/manuel/src/pr35188.c:14: warning: will never be executed
/home/manuel/src/pr35188.c:21: warning: will never be executed
Performing interprocedural optimizations
 <visibility> <early_local_cleanups>
/home/manuel/src/pr35188.c:17: warning: will never be executed
/home/manuel/src/pr35188.c:18: warning: will never be executed
 <inline> <static-var> <pure-const>Assembling functions:
 foo
/home/manuel/src/pr35188.c: In function ?foo?:
/home/manuel/src/pr35188.c:25: warning: will never be executed

So, it is jut that without optimizations enabled, GCC does not know that (var
== 1 && var == 2) is always false and hence it does not consider the code
unreachable.

I am inclined to close this as WONTFIX.


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org
             Status|UNCONFIRMED                 |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35188