[Bug middle-end/59970] New: Bogus warnings at low optimization levels

[Bug middle-end/59970] New: Bogus warnings at low optimization levels

[tschwinge at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59970

            Bug ID: 59970
           Summary: Bogus warnings at low optimization levels
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: middle-end
          Assignee: tschwinge at gcc dot gnu.org
          Reporter: tschwinge at gcc dot gnu.org
             Build: x86_64-linux-gnu

Created attachment 31970
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31970&action=edit
cp-demangle.c

The attached code is greatly reduced from libiberty/cp-demangle.c.  As
discussed in
<http://news.gmane.org/find-root.php?message_id=%3C87bnyw41hf.fsf%40kepler.schwinge.homeip.net%3E>,
with trunk r207200, it exhibits the following behavior:

    $ for t in '' -DTOGGLE1; do for a in '' -DABORT; do for o in -O0 -Og -O1
-O2 -O3 -Ofast; do (set -x && gcc/xgcc -Bgcc/ -Wall -Wextra -c
../../cp-demangle.c $t $a $o); done; done; done
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -O0
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -Og
    ../../cp-demangle.c: In function 'd_demangle_callback':
    ../../cp-demangle.c:40:3: warning: 'dc' may be used uninitialized in this
function [-Wmaybe-uninitialized]
       return dc;
       ^
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -O1
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -O2
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -O3
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -Ofast
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DABORT -O0
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DABORT -Og
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DABORT -O1
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DABORT -O2
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DABORT -O3
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DABORT -Ofast
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -O0
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -Og
    ../../cp-demangle.c: In function 'd_demangle_callback':
    ../../cp-demangle.c:40:3: warning: 'dc' may be used uninitialized in this
function [-Wmaybe-uninitialized]
       return dc;
       ^
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -O1
    ../../cp-demangle.c: In function 'd_demangle_callback':
    ../../cp-demangle.c:40:3: warning: 'dc' may be used uninitialized in this
function [-Wmaybe-uninitialized]
       return dc;
       ^
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -O2
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -O3
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -Ofast
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -DABORT
-O0
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -DABORT
-Og
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -DABORT
-O1
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -DABORT
-O2
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -DABORT
-O3
    + gcc/xgcc -Bgcc/ -Wall -Wextra -c ../../cp-demangle.c -DTOGGLE1 -DABORT
-Ofast

In certain cases, GCC fails to track that all the possible values for enum type
indeed have been covered, and so dc must have been initialized.

It does seem appropriate to me for GCC not to do such tracking at low
optimization levels, but then the uninitialized warning should also be disabled
(which I assume is happening for -O0?) to avoid false positive warnings.

The abort to libiberty/cp-demangle.c has been added in r207200.  The
libgomp/env.c change r203514 looks like another instance of this issue,
<http://news.gmane.org/find-root.php?message_id=%3C20131014082733.GF30970%40tucnak.zalov.cz%3E>.

I'm assigning this to me, tentatively, low priority, so if someone has any
clues, don't hesititate to take it over.

[Bug tree-optimization/59970] Bogus warnings at low optimization levels

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59970

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |tree-optimization

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't think this is a GCC bug except for the unitialized predicates is not
working.  The reason why it fails for -Og is because jump threading is disabled
as you want to be able to step through the code.  -O1 must not decided jump
threading is profitable either.

For the unitialized predicates issue, it might be because switch statements are
not handled.

[Bug tree-optimization/59970] Bogus warnings at low optimization levels

[matt at godbolt dot org]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset="UTF-8", Size: 4303 bytes --]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59970

Matt Godbolt <matt at godbolt dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |matt at godbolt dot org

--- Comment #2 from Matt Godbolt <matt at godbolt dot org> ---
FWIW I stumbled over what I believe is the same issue inside the boost v1.55.0
lexical_cast implementation.  That also only shows up as a "possible
uninitialized" at -Og optimization only.  I can provide preprocessed source if
necessary (though, being boost, it's 80k lines, ugh) but if you have gcc4.9.0
and boost 1.55 available:

$ cat test.cc
#include <boost/lexical_cast.hpp>

int test() { return boost::lexical_cast<int>("1"); }

$ g++ -Wall -Wextra -Og -c test.cc
test.cc: In function ‘int test()’:
test.cc:4:40: warning: ‘result’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
     return boost::lexical_cast<int>("1");


The relevant lines in boost itself appear to be
http://svn.boost.org/svn/boost/tags/release/Boost_1_55_0/boost/lexical_cast.hpp
lines 2365-2377 where the "result" is optionally initialized.

I'll also submit this to the boost developers in case they have some ideas.
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Subject: [Bug go/60931] libgo has issues when page size is not 4k
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`931

Anton Blanchard <anton at samba dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #32669|0                           |1
        is obsolete|                            |

--- Comment #7 from Anton Blanchard <anton at samba dot org> ---
Created attachment 32679
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id2679&actioníit
runtime: Fix garbage collector issue with non 4kB system page size

The go garbage collector tracks memory in terms of 4kB pages. Most of
the code checks getpagesize() at runtime and does the right thing.

On a 64kB ppc64 box I see SEGVs in long running processes which has
been diagnosed as a bug in scavengelist. scavengelist does a
madvise(MADV_DONTNEED) without rounding the arguments to the system
page size. A strace of one of the failures shows the problem:

madvise(0xc211030000, 4096, MADV_DONTNEED) = 0

The kernel rounds the length up to 64kB and we mark 60kB of valid data
as no longer needed.

Round start up to a system page and end down before calling madvise.

[Bug tree-optimization/59970] Bogus warnings at low optimization levels

[mac at mcrowe dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59970

Mike Crowe <mac at mcrowe dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mac at mcrowe dot com

--- Comment #3 from Mike Crowe <mac at mcrowe dot com> ---
Created attachment 35384
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35384&action=edit
Preprocessed source for boost::lexical_cast case

I can reproduce the same problem with Debian Jessie's gcc 4.9.2 and a
self-compiled gcc 4.9.1 when compiling:

 #include <boost/lexical_cast.hpp>

 int wibble()
 {
     return boost::lexical_cast<int>("1");
 }

Preprocessed source attached (compressed to avoid file size limit.)

 $ g++ -Wall -Werror -Og -save-temps test.cpp
 test.cpp: In function ‘int wibble()’:
 test.cpp:5:40: error: ‘result’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
      return boost::lexical_cast<int>("1");
                                        ^
 cc1plus: all warnings being treated as errors

Oddly the warning is not reported when the function is named "main"!
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From: "jay.foad at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/65847] New: SSE2 code for adding two structs is much worse at -O3 than at -O2
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide847

            Bug ID: 65847
           Summary: SSE2 code for adding two structs is much worse at -O3
                    than at -O2
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jay.foad at gmail dot com

On x86_64 I get decent code at -O2:

$ cat zplus.c
typedef struct { double a, b; } Z;
Z zplus(Z x, Z y) { return (Z){ x.a + y.a, x.b + y.b }; }
$ gcc -O2 -S -o - zplus.c
...
zplus:
.LFB0:
        .cfi_startproc
        addsd   %xmm3, %xmm1
        addsd   %xmm2, %xmm0
        ret
        .cfi_endproc
.LFE0:
...

but awful code at -O3:

$ gcc -O3 -S -o - zplus.c
...
zplus:
.LFB0:
        .cfi_startproc
        movq    %xmm0, -40(%rsp)
        movq    %xmm1, -32(%rsp)
        movq    %xmm2, -56(%rsp)
        movq    %xmm3, -48(%rsp)
        movupd  -40(%rsp), %xmm1
        movupd  -56(%rsp), %xmm0
        addpd   %xmm0, %xmm1
        movaps  %xmm1, -72(%rsp)
        movq    -72(%rsp), %rdx
        movq    -64(%rsp), %rax
        movq    %rdx, -72(%rsp)
        movsd   -72(%rsp), %xmm0
        movq    %rax, -72(%rsp)
        movsd   -72(%rsp), %xmm1
        ret
...

I see similar bad code generated by various versions of GCC, starting around
version 4.8:
gcc-4.8 (Ubuntu 4.8.3-12ubuntu3) 4.8.3
gcc (Ubuntu 4.9.1-16ubuntu6) 4.9.1
gcc (GCC) 6.0.0 20150422 (experimental)

[Bug tree-optimization/59970] Bogus warnings at low optimization levels

[mac at mcrowe dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59970

--- Comment #4 from Mike Crowe <mac at mcrowe dot com> ---
The test case in comment #3 shows the same problem in gcc-5-branch r222333.

[Bug tree-optimization/59970] Bogus Wuninitialized warnings at low optimization levels

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59970

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |7.0
                 CC|                            |msebor at gcc dot gnu.org
      Known to fail|                            |4.8.4, 4.9.4, 5.5.0, 6.4.0
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #7 from Martin Sebor <msebor at gcc dot gnu.org> ---
The test case in comment #0 is not diagnosed anymore.  Bisection points to
r240221 as the fix.  Ditto for attachment 35384 from comment #3.  Let me add
the one from attachment 31970 to the test suite and resolve this as fixed.

[Bug tree-optimization/59970] Bogus Wuninitialized warnings at low optimization levels

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59970

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Sebor <msebor@gcc.gnu.org>:

https://gcc.gnu.org/g:980b12cc81979e52f491bf0dfe961d30c07fe864

commit r11-7869-g980b12cc81979e52f491bf0dfe961d30c07fe864
Author: Martin Sebor <msebor@redhat.com>
Date:   Fri Mar 26 16:37:34 2021 -0600

    PR tree-optimization/59970 - Bogus -Wmaybe-uninitialized at low
optimization levels

            PR tree-optimization/59970
            * gcc.dg/uninit-pr59970.c: New test.