[Bug c++/17544] New: incorrect -Wunreachable-code warning for reachable code following a do statement

[Bug c++/17544] New: incorrect -Wunreachable-code warning for reachable code following a do statement

[sebor at roguewave dot com]

The warning below is incorrect -- the program always reaches the flagged line.
Note that replacing the do statement with a for or while statement eliminates
the warning.

$ cat -n u.cpp && g++ -O -Wunreachable-code u.cpp && ./a.out a; echo $?
     1  int main (int argc, char *argv[])
     2  {
     3      const char* const s = argc < 2 ? "" : argv [1];
     4
     5      int i = 0;
     6
     7      do {
     8          ++i;
     9      } while (i < s [0]);
    10
    11      return i;
    12  }
    13
u.cpp: In function `int main(int, char**)':
u.cpp:11: warning: will never be executed
97

-- 
           Summary: incorrect -Wunreachable-code warning for reachable code
                    following a do statement
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sebor at roguewave dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: all
  GCC host triplet: all
GCC target triplet: all


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug rtl-optimization/17544] incorrect -Wunreachable-code warning for reachable code following a do statement

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |rtl-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug rtl-optimization/17544] incorrect -Wunreachable-code warning for reachable code following a do statement

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug rtl-optimization/17544] [4.0 Regression] incorrect -Wunreachable-code warning for reachable code following a do statement

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-04 12:28 -------
Confirmed, a regression on mainline. Simplified testcase:

----------------------------
int main (void)    // 1
{                  // 2
  return 0;        // 3
}                  // 4
----------------------------
17544.cc: In function `int main()':
17544.cc:4: warning: will never be executed

The warning is probably on the implicit return which is put at the end of the 
definition of main by the compiler (main is allowed to not have an explicit 
return statement).


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
      Known to fail|                            |4.0.0
      Known to work|                            |3.4.2
   Last reconfirmed|0000-00-00 00:00:00         |2004-10-04 12:29:00
               date|                            |
            Summary|incorrect -Wunreachable-code|[4.0 Regression] incorrect -
                   |warning for reachable code  |Wunreachable-code warning
                   |following a do statement    |for reachable code following
                   |                            |a do statement
   Target Milestone|---                         |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug rtl-optimization/17544] [4.0 Regression] incorrect -Wunreachable-code warning for reachable code following a do statement

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-04 12:37 -------
The implicit return statement is added within cp/decl.c:finish_function:

      if (DECL_MAIN_P (current_function_decl))
        {
          /* Make it so that `main' always returns 0 by default.  */
#if VMS_TARGET
          finish_return_stmt (integer_one_node);
#else
          finish_return_stmt (integer_zero_node);
#endif
        }




-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug c++/17544] [4.0 Regression] incorrect -Wunreachable-code warning for reachable code following a do statement

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-04 12:38 -------
Setting this as C++ bug, the C++ frontend should mark the statement in some 
special way that middle-end can later detect to avoid the warning.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |giovannibajo at libero dot
                   |                            |it
          Component|rtl-optimization            |c++


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug c++/17544] [4.0 Regression] incorrect -Wunreachable-code warning for reachable code following a do statement

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-05 02:41 -------
I think -std=c99 will also cause it too, maybe we should mark this as compiler generated.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug c++/17544] [4.0 Regression] incorrect -Wunreachable-code warning for reachable code following a do statement

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-06 03:50 -------
Yes this shows up in the C front-end also with -std=c99.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug c++/17544] [4.0 Regression] incorrect -Wunreachable-code warning for reachable code following a do statement

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-06 13:49 -------
One thing to due or to try is to add a new function which finish the return expression with a location 
which is unkown, tried the obvious push_srcloc/pop_srcloc but that would ICE.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug c++/17544] [4.0 Regression] incorrect -Wunreachable-code warning for reachable code following a do statement

[mmitchel at gcc dot gnu dot org]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2004-10-09 22:59 -------
This is another example of why it is a mistake to use the optimizers to issue
warnings about unreachable code, uninitialized variables, etc.  

There is no easy fix for the general case.  The front end cannot avoid emitting
the return statement in all cases in which the back end might determine it is
unnecessary.  We do not have an equivalent of DECL_ARTIFICIAL for statements or
expressions so there is no good way to flag the compiler-generated return for
the middle end.

The short-term fix is probably to special-case the situation where the last
statement in main is a return, either in the middle end or in the front end. 
The longer-term fix is to move this warning out of the back end.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug middle-end/17544] [4.0 Regression] incorrect -Wunreachable-code warning for mains with a return statement

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-12-20 01:11 -------
Reproduces for C with -std=c99. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug middle-end/17544] [4.0 Regression] incorrect -Wunreachable-code warning for mains with a return statement

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-12-20 01:12 -------
>From pure annoyance with such a bug being unfixed for such a long 
time, I'll just take this on.  Probably putting a flag on the 
compiler generated return statement is enough to make this silly 
warning go away.... 
 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |steven at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug middle-end/17544] [4.0 Regression] incorrect -Wunreachable-code warning for mains with a return statement

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-12-22 22:34 -------
Spot the bug: 
 
main () 
{ 
  intD.0 D.1384; 
 
  # BLOCK 0, starting at line 3 
  # PRED: ENTRY 
  [t.c : 3] D.1384 = 0; 
  [t.c : 3] goto <bb 2> (<L0>); 
  # SUCC: 2 
 
  # BLOCK 1, starting at line 4 
  # PRED: 
  [t.c : 4] D.1384 = 0; 
  # SUCC: 2 
 
  # BLOCK 2 
  # PRED: 0 1 
<L0>:; 
  return D.1384; 
  # SUCC: EXIT 
 
} 
 
Note how the compiler generated "return 0;" has a locus.  Not giving 
it one should fix it. 
 
Testing a patch. 
 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug middle-end/17544] [4.0 Regression] incorrect -Wunreachable-code warning for mains with a return statement

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-12-23 01:19 -------
Look, no hands (or testing): 
 
Index: cp/decl.c 
=================================================================== 
RCS file: /cvs/gcc/gcc/gcc/cp/decl.c,v 
retrieving revision 1.1346 
diff -u -3 -p -r1.1346 decl.c 
--- cp/decl.c   22 Dec 2004 03:34:53 -0000      1.1346 
+++ cp/decl.c   23 Dec 2004 01:19:29 -0000 
@@ -10631,12 +10631,19 @@ finish_function (int flags) 
     { 
       if (DECL_MAIN_P (current_function_decl)) 
        { 
-         /* Make it so that `main' always returns 0 by default.  */ 
+         tree stmt; 
+ 
+         /* Make it so that `main' always returns 0 by default (or 
+            1 for VMS).  */ 
 #if VMS_TARGET 
          finish_return_stmt (integer_one_node); 
 #else 
          finish_return_stmt (integer_zero_node); 
 #endif 
+         /* Hack.  We don't want the middle-end to warn that this 
+            return is unreachable, so put the statement on the 
+            special line 0.  */ 
+         annotate_with_file_line (stmt, input_filename, 0); 
        } 
 
       /* Finish dealing with exception specifiers.  */ 
 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug middle-end/17544] [4.0 Regression] incorrect -Wunreachable-code warning for mains with a return statement

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-12-31 10:38 -------
Testing a patch. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug middle-end/17544] [4.0 Regression] incorrect -Wunreachable-code warning for mains with a return statement

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-12-31 12:02 -------
(From update of attachment 7808)
Patch here: http://gcc.gnu.org/ml/gcc-patches/2004-12/msg02140.html


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
Attachment #7808 is|0                           |1
           obsolete|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug middle-end/17544] [4.0 Regression] incorrect -Wunreachable-code warning for mains with a return statement

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-01-01 16:15 -------
Subject: Bug 17544

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	steven@gcc.gnu.org	2005-01-01 16:15:24

Modified files:
	gcc            : ChangeLog c-decl.c emit-rtl.c tree-cfg.c 
	gcc/cp         : ChangeLog decl.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/g++.dg/warn: Wunreachable-code-1.C 
	gcc/testsuite/gcc.dg: 20041231-1.c 

Log message:
	* emit-rtl.c (add_insn_before): Fix comment typo.
	
	PR middle-end/17544
	* c-decl.c (finish_function): If compiling C99, annotate the
	compiler generated return with the current file name and line 0.
	* tree-cfg.c (remove_useless_stmts_warn_notreached): Only warn if
	the source line is greater than 0.
	(remove_bb): Likewise.
	
	cp/
	PR middle-end/17544
	* decl.c (finish_function): Fix comment.  Annotate the compiler
	generated return with the current file name and line 0.
	
	testsuite/
	* gcc.dg/20041231-1.C: New test.
	* g++.dg/warn/Wunreachable-code-1.C: New test.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.6994&r2=2.6995
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/c-decl.c.diff?cvsroot=gcc&r1=1.620&r2=1.621
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/emit-rtl.c.diff?cvsroot=gcc&r1=1.432&r2=1.433
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-cfg.c.diff?cvsroot=gcc&r1=2.136&r2=2.137
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/ChangeLog.diff?cvsroot=gcc&r1=1.4565&r2=1.4566
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/decl.c.diff?cvsroot=gcc&r1=1.1349&r2=1.1350
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.4830&r2=1.4831
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/warn/Wunreachable-code-1.C.diff?cvsroot=gcc&r1=NONE&r2=1.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/20041231-1.c.diff?cvsroot=gcc&r1=NONE&r2=1.1



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544

[Bug middle-end/17544] [4.0 Regression] incorrect -Wunreachable-code warning for mains with a return statement

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2005-01-01 16:26 -------
. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17544