[Bug tree-optimization/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[Bug tree-optimization/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |NEW
      Known to work|                            |
           Keywords|                            |diagnostic
   Last reconfirmed|                            |2013-11-19
          Component|regression                  |tree-optimization
             Blocks|                            |24639
         Resolution|DUPLICATE                   |---
     Ever confirmed|0                           |1
      Known to fail|                            |

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
PR20968 was closed, but this one is still valid in some cases. In particular
this testcase with -Os produces a warning.

int translate(int in);

int main(int argc, char* argv[])
{
  int ret;

  if (argc)
    ret = translate(5);

  if (!argc || !ret)
    ret = 1;

  return ret;
}

The reason is that with -Os, the check "if (!argc || !ret)" is transformed
into:

  # ret_1 = PHI <ret_5(D)(2), [pr19085.c : 8:9] ret_8(3)>
  # .MEM_3 = PHI <.MEM_6(D)(2), .MEM_7(3)>
  [pr19085.c : 10:7] # RANGE [0, 1]
  _9 = argc_4(D) == 0;
  [pr19085.c : 10:16] # RANGE [0, 1]
  _10 = ret_1 == 0;
  [pr19085.c : 10:13] # RANGE [0, 1]
  _11 = _10 | _9;
  [pr19085.c : 10:6] if (_11 != 0)
>From gcc-bugs-return-434973-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Tue Nov 19 09:15:06 2013
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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug lto/59183] configure of pdsh successfully checks for function shl_load when lto is enabled
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY183

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, configure tests are usually not prepared for compilers applying a lot of
optimization - especially not assuming that it sees the whole program.

Thus, run configure tests with -O0, not with optimization.

This is hardly a GCC bug, it just optimized away the test which boils down to

char (*f) () = shl_load;
int
main ()
{
return f != shl_load;
}

where obviously the test is always false.  LTO then can eliminate the global
variable 'f' because with -flto it knows it cannot be refered to from
elsewhere and whoops - the reference to shl_load is gone.

[Bug tree-optimization/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at redhat dot com

--- Comment #5 from Jeffrey A. Law <law at redhat dot com> ---
-Os disables jump threading except for very specific circumstances because the
block duplication generally increases code size.  As a result of restricting
the optimizer, certain paths through the CFG which are unexecutable have to be
left in the CFG which leads to the false positive.

[Bug tree-optimization/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arequipeno at gmail dot com

--- Comment #6 from Jeffrey A. Law <law at redhat dot com> ---
*** Bug 58698 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rbd at debian dot org

--- Comment #7 from Jeffrey A. Law <law at redhat dot com> ---
*** Bug 58455 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppluzhnikov at google dot com

--- Comment #8 from Jeffrey A. Law <law at redhat dot com> ---
*** Bug 57072 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bp at alien8 dot de

--- Comment #9 from Jeffrey A. Law <law at redhat dot com> ---
*** Bug 55759 has been marked as a duplicate of this bug. ***

[Bug tree-optimization/42145] Incorrect "may be used uninitialized warning" for a very specific test case

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lacombar at gmail dot com

--- Comment #10 from Jeffrey A. Law <law at redhat dot com> ---
*** Bug 49848 has been marked as a duplicate of this bug. ***