[Bug c++/40825] New: mis-optimization of libstdc++'s atomic.cc:__atomic_flag_wait_explicit() at -O2

[Bug c++/40825] New: mis-optimization of libstdc++'s atomic.cc:__atomic_flag_wait_explicit() at -O2

[jbeulich at novell dot com]

Compiling this file for NetWare, or, after pre-processing it and stripping down
to a reasonably small one, natively for Linux produces "'d' is used
uninitialized in this function".

While the original code in libstdc++ looks broken (which will be reported as a
separate bug), the warning and the generated code are incorrect, since 'd'
definitely gets constructed at source level.

Neither the waring nor the incorrect code generation can be observed at -O1.

I will attach the stripped-down, usable on plain Linux, example in a second.


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           Summary: mis-optimization of libstdc++'s
                    atomic.cc:__atomic_flag_wait_explicit() at -O2
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jbeulich at novell dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40825

[Bug c++/40825] mis-optimization of libstdc++'s atomic.cc:__atomic_flag_wait_explicit() at -O2

[jbeulich at novell dot com]

------- Comment #1 from jbeulich at novell dot com  2009-07-22 14:06 -------
Created an attachment (id=18238)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18238&action=view)
example source


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40825

[Bug c++/40825] mis-optimization of libstdc++'s atomic.cc:__atomic_flag_wait_explicit() at -O2

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2009-07-22 14:08 -------
Can you give a hint on what exactly is miscompiled?  (the testcase isn't an
executable one that fails if miscompiled...)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40825

[Bug c++/40825] mis-optimization of libstdc++'s atomic.cc:__atomic_flag_wait_explicit() at -O2

[paolo dot carlini at oracle dot com]

------- Comment #3 from paolo dot carlini at oracle dot com  2009-07-22 14:09 -------
The library issue is PR40654.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40825

[Bug c++/40825] mis-optimization of libstdc++'s atomic.cc:__atomic_flag_wait_explicit() at -O2

[jbeulich at novell dot com]

------- Comment #4 from jbeulich at novell dot com  2009-07-22 14:17 -------
Just as the warning says - construction of 'd' is missing. The warning lead me
to look at the generated code, just to see that they are in sync (and hence
perhaps have the same root cause).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40825