-Wall option position of Clang and GCC

fujii@ubuntu3 $ cat a.cc 

int main()
{
  int x;
  return x;
}
fujii@ubuntu3 $ clang++ --version
,clang version 7.0.0 (tags/RELEASE_700/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/fujii/work/llvm/clang+llvm-7.0.0-x86_64-linux-gnu-ubuntu-16.04/bin
fujii@ubuntu3 $ clang++ -Wall -c a.cc
a.cc:5:10: warning: variable 'x' is uninitialized when used here [-Wuninitialized]
  return x;
         ^
a.cc:4:8: note: initialize the variable 'x' to silence this warning
  int x;
       ^
        = 0
1 warning generated.
fujii@ubuntu3 $ clang++ -Wall -Wno-uninitialized -c a.cc
fujii@ubuntu3 $ clang++ -Wno-uninitialized -Wall -c a.cc
a.cc:5:10: warning: variable 'x' is uninitialized when used here [-Wuninitialized]
  return x;
         ^
a.cc:4:8: note: initialize the variable 'x' to silence this warning
  int x;
       ^
        = 0
1 warning generated.
fujii@ubuntu3 $ clang++ -Wno-uninitialized -Wuninitialized -c a.cc
a.cc:5:10: warning: variable 'x' is uninitialized when used here [-Wuninitialized]
  return x;
         ^
a.cc:4:8: note: initialize the variable 'x' to silence this warning
  int x;
       ^
        = 0
1 warning generated.
fujii@ubuntu3 $ clang++ -Wuninitialized -Wno-uninitialized -c a.cc
fujii@ubuntu3 $ g++ --version
g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0
Copyright (C) 2017 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.

fujii@ubuntu3 $ g++ -Wall -c a.cc
a.cc: In function ‘int main()’:
a.cc:5:10: warning: ‘x’ is used uninitialized in this function [-Wuninitialized]
   return x;
          ^
fujii@ubuntu3 $ g++ -Wall -Wno-uninitialized -c a.cc
fujii@ubuntu3 $ g++ -Wno-uninitialized -Wall -c a.cc
fujii@ubuntu3 $ g++ -Wno-uninitialized -Wuninitialized -c a.cc
a.cc: In function ‘int main()’:
a.cc:5:10: warning: ‘x’ is used uninitialized in this function [-Wuninitialized]
   return x;
          ^
fujii@ubuntu3 $ g++ -Wuninitialized -Wno-uninitialized -c a.cc
fujii@ubuntu3 $