[Bug c/60725] New: [-Wreturn-type] false positive in trivial switch

[Bug c/60725] New: [-Wreturn-type] false positive in trivial switch

[skvadrik at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60725

            Bug ID: 60725
           Summary: [-Wreturn-type] false positive in trivial switch
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: skvadrik at gmail dot com

Created attachment 32510
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32510&action=edit
wreturn_false_positive.c

Given this program (see attachment):

  // wreturn_false_positive.c
  enum E { E1 };
  static inline int f1(enum E e) {
      switch (e) {
          case E1: return 1;
      }
  }
  int main () {
      f1(E1);
      return 0;
  }

gcc generates warning:

$ gcc -Wall wreturn_false_positive.c 
wreturn_false_positive.c: In function ‘f1’:
wreturn_false_positive.c:9:1: warning: control reaches end of non-void function
[-Wreturn-type]

If I compile with '-fdump-tree-all', I see that gcc adds 'default' case to
switch very early (in .gimple), and later it cannot get rid of that 'default'
path. Why does gcc add default case?
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Subject: [Bug lto/60721] xcoral fails to build with LTO: internal compiler error: verify_flow_info failed
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`721

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
IMHO we need a flag on each gimple call whether that call instance is supposed
to be 'leaf' or not, similar to how we have noreturn and nothrow flags here.

[Bug middle-end/60725] [-Wreturn-type] false positive in trivial switch

[mpolacek at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60725

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-04-01
                 CC|                            |mpolacek at gcc dot gnu.org
          Component|c                           |middle-end
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
It adds the default case at the end of the switch so we can jump over the whole
switch body.  But it should do so only if the labels cover the whole range of
the controlling expression of the switch.  Here the switch label seemingly
covers the whole range of the enum, but TYPE_MAX_VALUE of the enum E is
UINT_MAX, while the high value of the case label is 0.

[Bug middle-end/60725] [-Wreturn-type] false positive in trivial switch

[jens.auer at cgi dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60725

Jens Auer <jens.auer at cgi dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jens.auer at cgi dot com

--- Comment #2 from Jens Auer <jens.auer at cgi dot com> ---
This warnings becomes annoying when you also compile with -Wswitch-enum which
forces you to cover all enum values. When they are all covered, I do not need a
default statement, but to prevent Wreturn-type, I have to add a pseudo-default.

[Bug middle-end/60725] [-Wreturn-type] false positive in trivial switch

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60725

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

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

--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #1)
> It adds the default case at the end of the switch so we can jump over the
> whole switch body.  But it should do so only if the labels cover the whole
> range of the controlling expression of the switch.  Here the switch label
> seemingly covers the whole range of the enum, but TYPE_MAX_VALUE of the enum
> E is UINT_MAX, while the high value of the case label is 0.

But in C, e can contain any value of the underlying type (unsigned int). Thus
one can legally call f1(10), and then what happens next is probably a bug.

enum E { E1=0 };
static inline int f1(enum E e) {
  switch (e) {
  case E1: return 1;
  }
}
int main () {
  return f1(10);
}

If this is actually undefined behavior, -fsanitize=undefined does not seem to
catch it.
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Subject: [Bug middle-end/60725] [-Wreturn-type] false positive in trivial switch
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60725

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Jens Auer from comment #2)
> This warnings becomes annoying when you also compile with -Wswitch-enum
> which forces you to cover all enum values. When they are all covered, I do
> not need a default statement, but to prevent Wreturn-type, I have to add a
> pseudo-default.

Even if you do not explicitly write one, there is one for the case where the
value is outside the enum values, which is valid in C (but not in C++, as far
as I know, thus the warning would be questionable there). What is not valid is
to reach the end of a non-void function.
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Subject: [Bug middle-end/60725] [-Wreturn-type] false positive in trivial switch
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id`725

Martin Sebor <msebor at gcc dot gnu.org> changed:

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

--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
Not to complicate things too much, but reaching the closing } of a non-void
function is valid in C.  What is not valid (i.e., has undefined behavior) is
calling such a function and using its return value when none was provided (as
in the program in comment #3).  The program in the Description, however, has
well-defined behavior because f1's return value isn't used.

[Bug middle-end/60725] [-Wreturn-type] false positive in trivial switch

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60725

--- Comment #7 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #5)
> The program in the Description, however,
> has well-defined behavior because f1's return value isn't used.

It is also easy to silence the warning by adding an explicit abort() or
__builtin_unreachable() before the closing }.
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Subject: [Bug testsuite/67450] [6 Regression] ERROR: can't unset "et_cache(ia32,value)": no such element in array
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idg450

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |6.0

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
Fixed.