[Bug c++/54046] New: wrong control reaches end of non-void function for switch case with throw and default

[Bug c++/54046] New: wrong control reaches end of non-void function for switch case with throw and default

[plasmahh at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

             Bug #: 54046
           Summary: wrong control reaches end of non-void function for
                    switch case with throw and default
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: plasmahh@gmx.net


Hi, somehow gcc gets confused in its control flow analysis when a default case
in a switch has a throw and break, and a variable with user defined dtor.

struct A
{
        ~A() { }
};

bool check( int x )
{
        A z;
        switch( x )
        {
                case 0:
                        return false;
                default:
                        throw "X";
                        break;
        }
}

When I either remove the break or the variable z, the warning will not issued
anymore.

[Bug c++/54046] wrong control reaches end of non-void function for switch case with throw and default

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-07-20
     Ever Confirmed|0                           |1

[Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |3.4.3
            Summary|wrong control reaches end   |[4.6/4.7/4.8 Regression]
                   |of non-void function for    |wrong control reaches end
                   |switch case with throw and  |of non-void function for
                   |default                     |switch case with throw and
                   |                            |default
      Known to fail|                            |4.1.2, 4.4.3, 4.5.2, 4.6.3,
                   |                            |4.7.1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-07-20 12:14:52 UTC ---
This is a regression, there's no warning from 3.4

[Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-07-20 12:39:30 UTC ---
void foo (void) __attribute__((noreturn));

struct A
{
  ~A () {}
};

bool
check (int x)
{
  A z;
  switch (x)
    {
    case 0:
      return false;
    default:
      foo ();
      break;
    }
}

warns the same.

[Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-07-20 12:57:42 UTC ---
Or:
void foo (void) __attribute__((noreturn));
void bar (void);

struct A
{
  ~A () {}
};

bool
check (int x)
{
  A z;
  switch (x)
    {
    case 0:
      return false;
    default:
      foo ();
      bar ();
    }
}

The problem is related to pass ordering.  EH lowering is done before CFG
cleanups, so if there is code after a noreturn spot (whether throw or noreturn
function, endless loop etc. doesn't matter), when trying to lower try ..
finally EH will see gimple_seq_may_fallthru to be true because of the extra
code after it hiding that it in fact can't fall thru.  We end up then with:
  switch (x) <default: <D.2213>, case 0: <D.2212>>
  <D.2212>:
  D.2216 = 0;
  finally_tmp.0 = 0;
  goto <D.2220>;
  <D.2213>:
  foo ();
  bar ();
  finally_tmp.0 = 1;
  <D.2220>:
  A::~A (&z);
  switch (finally_tmp.0) <default: <D.2223>, default: <D.2223>, case 1:
<D.2221>>

where D.2221 falls thru to exit without returning a value, while D.2223
doesn't.
Then cfg cleanup during cfg pass cleans that up to:
  finally_tmp.0 = 0;
  A::~A (&z);
  switch (finally_tmp.0) <default: <L12>, case 1: <L11>>
after finding out that foo is noreturn and can't fallthru.  But there is no
constant propagation and switch optimization pass that would simplify
switch (0) <default: something; case 1: somethingelse> into
goto something; scheduled before the pass_warn_function_return is scheduled.
As we aren't in SSA form at that point, doing it wouldn't be very easy.

[Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot
                   |                            |gnu.org

--- Comment #4 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-07-20 15:14:41 UTC ---
There is a machinery in gimple-low.c for this (and a pending patch at
  http://gcc.gnu.org/ml/gcc-patches/2012-07/msg00817.html
for another issue).  Maybe it can be enhanced to handle this as well.

[Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.6.4
           Priority|P3                          |P2

[Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-20 16:02:33 UTC ---
I don't see how.  The thing is, e.g. lower_stmt resets data->cannot_fallthru on
most of the statements, even if it got changed to reset it only on
GIMPLE_LABELs (or few selected others), such that say a noreturn call which
sets data->cannot_fallthru followed by assignment or another call would keep
cannot_fallthru set even when it is currently cleared, on GIMPLE_LABELs we'd
need to reset anyway, as we don't have the CFG yet and don't have info how many
gotos or other control transfer stmts to each GIMPLE_LABEL there are (and the
values of cannot_fallthru at those points).  So even just the break; after the
__cxa_throw which got gimplified into goto <somelabel>; <somelabel>: would
reset cannot_fallthru.  And the gimplifier doesn't see break; but already the
goto

[Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-20 17:27:03 UTC ---
Created attachment 28744
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28744
gcc48-pr54046.patch

Actually, with a langhook we can already at the C++ FE level get rid of the
warning for most of the cases, similarly how we don't warn for
g++.dg/warn/Wreturn-type-7.C (PR20681).

[Bug c++/54046] [4.6/4.7/4.8 Regression] wrong control reaches end of non-void function for switch case with throw and default

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-11-23 16:04:14 UTC ---
Author: jakub
Date: Fri Nov 23 16:04:03 2012
New Revision: 193762

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=193762
Log:
    PR c++/54046
    * Makefile.in (gimple-low.o): Depend on langhooks.h.
    * gimple-low.c: Include langhooks.c.
    (block_may_fallthru): Handle TARGET_EXPR and ERROR_MARK,
    by default call lang_hooks.block_may_fallthru.
    * langhooks.h (struct lang_hooks): Add block_may_fallthru
    langhook.
    * langhooks-def.h (LANG_HOOKS_BLOCK_MAY_FALLTHRU): Define.
    (LANG_HOOKS_INITIALIZER): Use it.

    * cp-objcp-common.h (LANG_HOOKS_BLOCK_MAY_FALLTHRU): Redefine.
    * cp-objcp-common.c (cxx_block_may_fallthru): New function.
    * cp-tree.h (cxx_block_may_fallthru): New prototype.

    * g++.dg/warn/Wreturn-type-8.C: New test.

Added:
    trunk/gcc/testsuite/g++.dg/warn/Wreturn-type-8.C
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/Makefile.in
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/cp-objcp-common.c
    trunk/gcc/cp/cp-objcp-common.h
    trunk/gcc/cp/cp-tree.h
    trunk/gcc/gimple-low.c
    trunk/gcc/langhooks-def.h
    trunk/gcc/langhooks.h
    trunk/gcc/testsuite/ChangeLog

[Bug c++/54046] [4.6/4.7 Regression] wrong control reaches end of non-void function for switch case with throw and default

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-02-01 14:02:41 UTC ---
Author: jakub
Date: Fri Feb  1 14:02:33 2013
New Revision: 195651

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=195651
Log:
    Backported from mainline
    2012-11-23  Jakub Jelinek  <jakub@redhat.com>

    PR c++/54046
    * Makefile.in (gimple-low.o): Depend on langhooks.h.
    * gimple-low.c: Include langhooks.c.
    (block_may_fallthru): Handle TARGET_EXPR and ERROR_MARK,
    by default call lang_hooks.block_may_fallthru.
    * langhooks.h (struct lang_hooks): Add block_may_fallthru
    langhook.
    * langhooks-def.h (LANG_HOOKS_BLOCK_MAY_FALLTHRU): Define.
    (LANG_HOOKS_INITIALIZER): Use it.

    * cp-objcp-common.h (LANG_HOOKS_BLOCK_MAY_FALLTHRU): Redefine.
    * cp-objcp-common.c (cxx_block_may_fallthru): New function.
    * cp-tree.h (cxx_block_may_fallthru): New prototype.

    * g++.dg/warn/Wreturn-type-8.C: New test.

Added:
    branches/gcc-4_7-branch/gcc/testsuite/g++.dg/warn/Wreturn-type-8.C
Modified:
    branches/gcc-4_7-branch/gcc/ChangeLog
    branches/gcc-4_7-branch/gcc/Makefile.in
    branches/gcc-4_7-branch/gcc/cp/ChangeLog
    branches/gcc-4_7-branch/gcc/cp/cp-objcp-common.c
    branches/gcc-4_7-branch/gcc/cp/cp-objcp-common.h
    branches/gcc-4_7-branch/gcc/cp/cp-tree.h
    branches/gcc-4_7-branch/gcc/gimple-low.c
    branches/gcc-4_7-branch/gcc/langhooks-def.h
    branches/gcc-4_7-branch/gcc/langhooks.h
    branches/gcc-4_7-branch/gcc/testsuite/ChangeLog

[Bug c++/54046] [4.6/4.7 Regression] wrong control reaches end of non-void function for switch case with throw and default

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54046

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
      Known to work|                            |4.7.3, 4.8.0
         Resolution|                            |FIXED

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-02-01 14:26:02 UTC ---
Fixed for 4.7.3+.