[Consult] g++: About "-Wunused-variable" for constant variable initialized by function

[Chen Gang]

Hello All:

I found an issue about g++, it is OK for "-Wall -O0", but will report
-Wunused-variable for "-Wall -O1|2|3|s". The original version (e.g.
gcc 4.8.3 redhat version) does not report warning for "-Wall -O?".

The related operation:

  [root@localhost qemu_cc]# cat test.cc
  const char n() { return 1; }
  const char c =  n();
  [root@localhost qemu_cc]# /usr/local/bin/g++ -Wall -O0 -c -o test.o test.cc
  [root@localhost qemu_cc]# /usr/local/bin/g++ -Wall -O2 -c -o test.o test.cc
  test.cc:2:12: warning: 'c' defined but not used [-Wunused-variable]
   const char c =  n();
              ^

The g++ information:

  [root@localhost qemu_cc]# /usr/local/bin/g++  -v
  Using built-in specs.
  COLLECT_GCC=/usr/local/bin/g++
  
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-unknown-linux-gnu/5.0.0/lto-wrapper
  Target: x86_64-unknown-linux-gnu
  Configured with: ../gcc/configure --disable-multilibs --disable-nls
  Thread model: posix
  gcc version 5.0.0 20141003 (experimental) (GCC) 


If we are sure it is a bug, I shall try to analyze it within this month.

Thanks
-- 
Chen Gang

Open, share, and attitude like air, water, and life which God blessed