[Bug c++/44780] New: [4.6 regression] Bogus set-but-not-used variable warning

[martin at mpa-garching dot mpg dot de]

Current trunk g++ warns about an set-but-unused variable in the attached
testcase:

~/tmp>g++ -v -W -Wall -c -O2 test.cc
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/afs/mpa/data/martin/ugcc/libexec/gcc/x86_64-unknown-linux-gnu/4.6.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /scratch/martin/gcc/configure --disable-bootstrap
--enable-gold --enable-plugins --prefix=/afs/mpa/data/martin/ugcc
--with-ppl=/afs/mpa/data/martin/numlibs64
--with-cloog=/afs/mpa/data/martin/numlibs64
--with-libelf=/afs/mpa/data/martin/numlibs64 --enable-languages=c++,fortran
--enable-target=all --enable-checking=release
Thread model: posix
gcc version 4.6.0 20100701 (experimental) [trunk revision 161665] (GCC) 
COLLECT_GCC_OPTIONS='-v' '-W' '-Wall' '-c' '-O2' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 /afs/mpa/data/martin/ugcc/libexec/gcc/x86_64-unknown-linux-gnu/4.6.0/cc1plus
-quiet -v -D_GNU_SOURCE test.cc -quiet -dumpbase test.cc -mtune=generic
-march=x86-64 -auxbase test -O2 -W -Wall -version -o /tmp/ccmz9mpH.s
GNU C++ (GCC) version 4.6.0 20100701 (experimental) [trunk revision 161665]
(x86_64-unknown-linux-gnu)
        compiled by GNU C version 4.6.0 20100628 (experimental) [trunk revision
161485], GMP version 4.3.2, MPFR version 2.4.2, MPC version 0.8.1
warning: GMP header version 4.3.2 differs from library version 4.3.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory
"/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../x86_64-unknown-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:

/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0

/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/x86_64-unknown-linux-gnu

/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/backward
 /usr/local/include
 /afs/mpa/data/martin/ugcc/include
 /afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/include
 /afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/include-fixed
 /usr/include
End of search list.
GNU C++ (GCC) version 4.6.0 20100701 (experimental) [trunk revision 161665]
(x86_64-unknown-linux-gnu)
        compiled by GNU C version 4.6.0 20100628 (experimental) [trunk revision
161485], GMP version 4.3.2, MPFR version 2.4.2, MPC version 0.8.1
warning: GMP header version 4.3.2 differs from library version 4.3.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 5f71a12a0438e83fd7b8caa75cf5702d
test.cc: In function '__m128d build_V2DF(long long unsigned int, long long
unsigned int)':
test.cc:10:8: warning: variable 'tmp' set but not used
[-Wunused-but-set-variable]
COLLECT_GCC_OPTIONS='-v' '-W' '-Wall' '-c' '-O2' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 as -V -Qy --64 -o test.o /tmp/ccmz9mpH.s
GNU assembler version 2.19.1 (x86_64-unknown-linux-gnu) using BFD version (GNU
Binutils) 2.19.1
COMPILER_PATH=/afs/mpa/data/martin/ugcc/libexec/gcc/x86_64-unknown-linux-gnu/4.6.0/:/afs/mpa/data/martin/ugcc/libexec/gcc/x86_64-unknown-linux-gnu/4.6.0/:/afs/mpa/data/martin/ugcc/libexec/gcc/x86_64-unknown-linux-gnu/:/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/:/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/
LIBRARY_PATH=/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/:/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/afs/mpa/data/martin/ugcc/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-W' '-Wall' '-c' '-O2' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'

This seems bogus. Other interesting observations:
- current trunk gcc (opposed to g++) does not produce the warning
- the warning is only given for the case with the SSE2 type, not for the other
case with the more "conventional" types.

gcc/g++ 4.4.2 and 4.5.* do not issue warnings.


-- 
           Summary: [4.6 regression] Bogus set-but-not-used variable warning
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: martin at mpa-garching dot mpg dot de
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44780