[Bug c/58698] New: Spurious "may be used unitialized" warning when compiling with -Os

[Bug c/58698] New: Spurious "may be used unitialized" warning when compiling with -Os

[arequipeno at gmail dot com]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

            Bug ID: 58698
           Summary: Spurious "may be used unitialized" warning when
                    compiling with -Os
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arequipeno at gmail dot com

When compiling the (soon to be) attached file with -Os, I receive the following
warning:

[pilcher@ian freecusd2]$ gcc -Os -W -Wall -Wextra -lpthread -c -o /dev/null
raid.c
raid.c: In function ‘fcd_raid_fn’:
raid.c:694:20: warning: ‘array’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
  array->ideal_devs = atoi(c + matches[4].rm_so);
                    ^
raid.c:642:25: note: ‘array’ was declared here
  struct fcd_raid_array *array;
                         ^

I don't get this warning with -O0, -O1, -O2, or -O3.

(array is initialized by the call to fcd_find_array at line 651; that function
can be found at line 371.)

This is gcc 4.8.1 on Fedora 19 (x86_64):

[pilcher@ian ~]$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.1/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id --with-linker-hash-style=gnu
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin
--enable-initfini-array --enable-java-awt=gtk --disable-dssi
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-isl=/builddir/build/BUILD/gcc-4.8.1-20130603/obj-x86_64-redhat-linux/isl-install
--with-cloog=/builddir/build/BUILD/gcc-4.8.1-20130603/obj-x86_64-redhat-linux/cloog-install
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.8.1 20130603 (Red Hat 4.8.1-1) (GCC)
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From: "arequipeno at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/58698] Spurious "may be used unitialized" warning when compiling with -Os
Date: Sat, 12 Oct 2013 03:09:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idX698

--- Comment #1 from Ian Pilcher <arequipeno at gmail dot com> ---
Created attachment 30987
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id0987&actioníit
C source file

[Bug c/58698] Spurious "may be used unitialized" warning when compiling with -Os

[arequipeno at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

--- Comment #2 from Ian Pilcher <arequipeno at gmail dot com> ---
Created attachment 30988
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30988&action=edit
Header file (required to compile C source)

[Bug c/58698] Spurious "may be used unitialized" warning when compiling with -Os

[arequipeno at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

--- Comment #3 from Ian Pilcher <arequipeno at gmail dot com> ---
Created attachment 30989
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30989&action=edit
Preprocessed file

[Bug c/58698] Spurious "may be used unitialized" warning when compiling with -Os

[arequipeno at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

--- Comment #4 from Ian Pilcher <arequipeno at gmail dot com> ---
Created attachment 30990
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30990&action=edit
Assembler file

[Bug tree-optimization/58698] Spurious "may be used unitialized" warning when compiling with -Os

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
          Component|c                           |tree-optimization
      Known to fail|                            |4.7.0, 4.8.1

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It happens with 4.7.0 also.

[Bug tree-optimization/58698] [4.7/4.8/4.9 Regression] Spurious "may be used unitialized" warning when compiling with -Os

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-10-14
      Known to work|                            |4.6.3
   Target Milestone|---                         |4.7.4
            Summary|Spurious "may be used       |[4.7/4.8/4.9 Regression]
                   |unitialized" warning when   |Spurious "may be used
                   |compiling with -Os          |unitialized" warning when
                   |                            |compiling with -Os
     Ever confirmed|0                           |1
      Known to fail|                            |4.9.0

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed also for trunk.  Does not happen with 4.6.

[Bug tree-optimization/58698] [4.7/4.8/4.9 Regression] Spurious "may be used unitialized" warning when compiling with -Os

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

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

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

--- Comment #7 from Jeffrey A. Law <law at redhat dot com> ---
This is a result of jump threading being throttled at -Os.

When optimizing for size, we will not thread through a block with side effects
that has more than one predecessor.  Such blocks have to be duplicated to
isolate the optimizable path through the CFG.  The duplication can obviously
contribute to code size increases.

In this particular example we have a CFG path we can  optimize. 
bb52->bb95->bb198

(gdb) p debug_bb_n (52)
<bb 52>:
_173 = (sizetype) _170;
_174 = buf_111 + _173;
fcd_err (4, "raid.c:382: WARNING: RAID device name \'%.*s\' too long\n", _171,
_174);
goto <bb 95>;

$63 = (basic_block_def *) 0x7ffff7d7f208
(gdb) p debug_bb_n (95)
<bb 95>:
# _200 = PHI <-1(52), -1(68), 0(69), 1(71), -1(72), 1(92), ret_201(93), -1(94)>
# array_380 = PHI <array_431(52), _209(68), _209(69), _209(71), _209(72),
array_427(92), array_416(93), array_416(94)>
uuid ={v} {CLOBBER};
if (_200 < 0)
  goto <bb 198>;
else
  goto <bb 96>;

$64 = (basic_block_def *) 0x7ffff7d7f888
(gdb) p debug_bb_n (198)
<bb 198>:
goto <bb 160>;


So normally we'd duplicate bb95 so that we can optimize the path 52->95->198
since on that path we know where bb95 will go.

With jump threading throttled because bb95 has > 1 pred and statements with
side effects (uuid = ...), the path is not optimized as stays as shown above.

Thus, the uninitialized warning analysis has to consider that the path
52->95->96 may be traversed and if that path is followed, it'll use "array".

[Bug tree-optimization/58698] [4.7/4.8/4.9 Regression] Spurious "may be used unitialized" warning when compiling with -Os

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug tree-optimization/58698] [4.7/4.8/4.9 Regression] Spurious "may be used unitialized" warning when compiling with -Os

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58698

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #8 from Jeffrey A. Law <law at redhat dot com> ---
Confirmed duplicate of 42145.

*** This bug has been marked as a duplicate of bug 42145 ***