[Bug c++/59800] New: Compilation with g++ fails when -Ofast -flto is used to compile code using some <random> distribution

[Bug c++/59800] New: Compilation with g++ fails when -Ofast -flto is used to compile code using some <random> distribution

[rionda at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59800

            Bug ID: 59800
           Summary: Compilation with g++ fails when -Ofast -flto is used
                    to compile code using some <random> distribution
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rionda at gmail dot com

Created attachment 31828
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31828&action=edit
Code that fails

The attached code compiles cleanly when using 
g++ -Ofast -Wall -Wextra -Wno-unused-parameter test.cpp test.cpp
and when using
g++ -flto -Wall -Wextra -Wno-unused-parameter test.cpp
but fails when using "g++8 -std=c++11 -Ofast -flto -Wall -Wextra
-Wno-unused-parameter test.cpp" (i.e., when using both -Ofast and -flto).

The reported error is:
test.cpp: In function 'main':
/opt/local/include/gcc48/c++/bits/random.h:2404:60: warning: 'MEM[(struct
normal_distribution *)&my_lognormal + 16B]._M_saved' may be used uninitialized
in this function [-Wmaybe-uninitialized]
         { return std::exp(__p.s() * _M_nd(__urng) + __p.m()); }
                                                            ^
test.cpp:11:33: note: 'MEM[(struct normal_distribution *)&my_lognormal +
16B]._M_saved' was declared here
  lognormal_distribution<double> my_lognormal(0.0, 1.0);
                                 ^

I am running 
g++-mp-4.8 (MacPorts gcc48 4.8.2_0) 4.8.2
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
on Mac OS X 10.6.8
(uname -a : darwin xxxx.local 10.8.0 Darwin Kernel Version 10.8.0: Tue Jun  7
16:33:36 PDT 2011; root:xnu-1504.15.3~1/RELEASE_I386 i386)

[Bug c++/59800] Compilation with g++ fails when -Ofast -flto is used to compile code using some <random> distribution

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59800

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2014-01-14
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
That's not an error but a warning?!  Anyway, it's triggered by
-Wmaybe-uninitialized.

Also happens on trunk.

[Bug c++/59800] Compilation with g++ fails when -Ofast -flto is used to compile code using some <random> distribution

[rionda at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59800

--- Comment #3 from Matteo Riondato <rionda at gmail dot com> ---
What exactly does the "WAITING" status mean?

[Bug c++/59800] Compilation with g++ fails when -Ofast -flto is used to compile code using some <random> distribution

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59800

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

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

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
The meaning is explained here: https://gcc.gnu.org/bugs/management.html
but I think it was a slip in this case.

Probably the uninit pass is not powerful enough, but in this case the warning
should be suppressed for system-headers:

manuel@gcc10:~$ ~/test1/210581/install/bin/g++ -std=c++11  -O2 -Wsystem-headers
-Wmaybe-uninitialized test.cc
In file included from
/home/manuel/test1/210581/install/include/c++/4.10.0/random:51:0,
                 from test.cc:3:
/home/manuel/test1/210581/install/include/c++/4.10.0/bits/random.tcc: In
function ‘int main(int, char**)’:
/home/manuel/test1/210581/install/include/c++/4.10.0/bits/random.tcc:1964:2:
warning: ‘*((void*)(&
my_lognormal)+16).std::normal_distribution<double>::_M_saved’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
  __ret = __ret * __param.stddev() + __param.mean();
  ^
test.cc:11:33: note: ‘*((void*)(&
my_lognormal)+16).std::normal_distribution<double>::_M_saved’ was declared here
  lognormal_distribution<double> my_lognormal(0.0, 1.0);
                                 ^

and -flto bypassed the system_header pragma somehow.
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Subject: [Bug c++/46097] Switch to warn of global variables in a C++ shared object
Date: Wed, 09 Jul 2014 21:13:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idF097

Phil Miller <unmobile at gmail dot com> changed:

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

--- Comment #32 from Phil Miller <unmobile at gmail dot com> ---
A switch to generate warnings about global variables would be useful to me and
my colleagues (http://ppl.cs.illinois.edu/) for a reason totally distinct from
ODR violations. We have a parallel programming system that includes a feature
for running application code in multiple, nominally independent, user-level
threads within a process. A mechanism to call attention to code that may fail
or misbehave in this setting would be helpful.

I would definitely agree that this doesn't belong in -Wall or -Wextra, as such.
So a -fnote-global-variables might be more appropriate.