[Bug c++/24522] New: htonl in optimized template function generates compiler warning

[Bug c++/24522] New: htonl in optimized template function generates compiler warning

[ckillian at cs dot ucsd dot edu]

Overview Description: More detailed expansion of summary.

        When compiling template functions in g++ 4.0.2 (Debian 4.0.2-2) (Debian
testing/unstable) with -O2 and -Wall, the compiler generates a warning about a
statement with no effect.  This seems to be due to the inlining with __v; as a
statement, but the warning does not occur in non-template functions or with
other compiler versions tried (3.4).

    Steps to Reproduce: Minimized, easy-to-follow steps that will trigger the
bug. Include any special setup steps.

        1) Create the following .cc file (as test.cc):
#include <netinet/in.h>

template<typename S>
void serialize(const uint32_t* pitem, const S& item) {
  uint32_t t2 = htonl(item);
}


        2) Compile "test.cc"
$ g++ -O2 -Wall -c -o test.o test.cc

    Actual Results: What the application did after performing the above steps.

        g++ generated a warning that the htonl statement has no effect.
$ g++ -O2 -Wall -c -o test.o test.cc
test.cc: In function 'void serialize(const uint32_t*, const S&)':
test.cc:5: warning: statement has no effect

    Expected Results: What the application should have done, were the bug not
present.

        test.o should have been created without warning

    Build Date & Platform: Date and platform of the build that you first
encountered the bug in.

>Release:       4.0.2 (Debian 4.0.2-2) (Debian testing/unstable)
configured with: ../src/configure -v
--enable-languages=c,c++,java,f95,objc,ada,treelang --prefix=/usr
--enable-shared --with-system-zlib
--libexecdir=/usr/lib--without-included-gettext --enable-threads=posix
--enable-nls --program-suffix=-4.0 --enable-__cxa_atexit
--enable-libstdcxx-allocator=mt --enable-clocale=gnu--enable-libstdcxx-debug
--enable-java-gc=boehm --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-4.0-1.4.2.0/jre --enable-mpfr
--disable-werror --enable-checking=release i486-linux-gnu
>Environment:
System: Linux killian.ucsd.edu 2.6.12.20051019-ck1 #1 SMP Wed Oct 19 09:24:47
PDT 2005 i686 GNU/Linux
Architecture: i686

    Additional Builds and Platforms: Whether or not the bug takes place on
other platforms (or browsers, if applicable).

        - Also Occurs On        
        Unknown

        - Doesn't Occur On        
        g++ version 3.4.4 20050721 (Red Hat 3.4.4-2)
        Or without -O2, -Wall, or outside of a template function.


-- 
           Summary: htonl in optimized template function generates compiler
                    warning
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ckillian at cs dot ucsd dot edu
 GCC build triplet: i486-pc-linux-gnu
  GCC host triplet: i486-pc-linux-gnu
GCC target triplet: i486-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522

[Bug c++/24522] htonl in optimized template function generates compiler warning

[ckillian at cs dot ucsd dot edu]

------- Comment #2 from ckillian at cs dot ucsd dot edu  2005-10-25 16:26 -------
Created an attachment (id=10056)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=10056&action=view)
preprocessor output


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522

[Bug c++/24522] htonl in optimized template function generates compiler warning

[ckillian at cs dot ucsd dot edu]

------- Comment #1 from ckillian at cs dot ucsd dot edu  2005-10-25 16:26 -------
Created an attachment (id=10055)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=10055&action=view)
The test.cc file


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522

[Bug c++/24522] htonl in optimized template function generates compiler warning

[ian at airs dot com]

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------- Comment #3 from ian at airs dot com  2005-10-25 16:36 -------
I believe this winds up being a duplicate of PR c++/8057, which is fixed on
mainline.  With the test case in the PR, I see the warning with 4.0, but I do
not see the warning on mainline.

Note that this slightly modified test case:

#include <netinet/in.h>

template<typename S>
void serialize(const uint32_t* pitem, const S& item) {
  uint32_t t2 = htonl(item);
}

template void serialize<int> (const uint32_t*, const int&);

issues a warning on both 4.0 and mainline:

foo.cc: In function ‘void serialize(const uint32_t*, const S&) [with S = int]’:
foo.cc:8:   instantiated from here
foo.cc:5: warning: unused variable ‘t2’


-- 

ian at airs dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ian at airs dot com
      Known to work|                            |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522

[Bug c++/24522] htonl in optimized template function generates compiler warning

[ckillian at cs dot ucsd dot edu]

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------- Comment #4 from ckillian at cs dot ucsd dot edu  2005-10-25 17:09 -------
(In reply to comment #3)
> I believe this winds up being a duplicate of PR c++/8057, which is fixed on
> mainline.  With the test case in the PR, I see the warning with 4.0, but I do
> not see the warning on mainline.
> 
> Note that this slightly modified test case:
> 
> #include <netinet/in.h>
> 
> template<typename S>
> void serialize(const uint32_t* pitem, const S& item) {
>   uint32_t t2 = htonl(item);
> }
> 
> template void serialize<int> (const uint32_t*, const int&);
> 
> issues a warning on both 4.0 and mainline:
> 
> foo.cc: In function ‘void serialize(const uint32_t*, const S&) [with S = int]’:
> foo.cc:8:   instantiated from here
> foo.cc:5: warning: unused variable ‘t2’
> 

The unused variable warning of course is behavior as expected for this small
sample.  My original statement that the sample should compile without warning
should seems not to be correct.  That could be corrected by using the variable,
for example by printing it.  The motivating example appended the bytes of t2 to
a std::string which was passed in by reference.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522

[Bug c++/24522] htonl in optimized template function generates compiler warning

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2005-10-25 17:51 -------
Here is a reduced testcase:
template <int >
void f(int i)
{
  int i1 = (__extension__ ({int i2 = i; i2;}));
}


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
      Known to work|4.1.0                       |4.1.0 3.4.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522

[Bug c++/24522] [4.0 Regression] htonl in optimized template function generates compiler warning

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2005-10-25 17:59 -------
(In reply to comment #3)
> I believe this winds up being a duplicate of PR c++/8057, which is fixed on
> mainline.  With the test case in the PR, I see the warning with 4.0, but I do
> not see the warning on mainline.

I don't think this is a duplicate of PR 8057 because my reduced testcase worked
in 3.4.5.  I think this was introduced by one of the C++ patches between 4.0.1
and 4.0.2
>        Or without -O2, -Wall, or outside of a template function.
the without -O2, is because the headers change, which is why my simplified
example shows the issue at every optimization level.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
  GCC build triplet|i486-pc-linux-gnu           |
   GCC host triplet|i486-pc-linux-gnu           |
 GCC target triplet|i486-pc-linux-gnu           |
      Known to fail|                            |4.0.3
      Known to work|4.1.0 3.4.0                 |4.1.0 3.4.0 3.4.5
   Last reconfirmed|0000-00-00 00:00:00         |2005-10-25 17:59:02
               date|                            |
            Summary|htonl in optimized template |[4.0 Regression] htonl in
                   |function generates compiler |optimized template function
                   |warning                     |generates compiler warning
   Target Milestone|---                         |4.0.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522

[Bug c++/24522] [4.0 Regression] htonl in optimized template function generates compiler warning

[mmitchel at gcc dot gnu dot org]

-- 

mmitchel at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.0.3                       |4.0.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522

[Bug c++/24522] [4.0 Regression] htonl in optimized template function generates compiler warning

[gdr at gcc dot gnu dot org]

------- Comment #7 from gdr at gcc dot gnu dot org  2007-02-03 15:48 -------
Fixed in GCC-4.1.0 and higher


-- 

gdr at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|4.0.4                       |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24522