[Bug tree-optimization/72817] New: wrong code at -O3 on x86_64-linux-gnu (in both 32-bit and 64-bit modes)

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72817

            Bug ID: 72817
           Summary: wrong code at -O3 on x86_64-linux-gnu (in both 32-bit
                    and 64-bit modes)
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu
  Target Milestone: ---

At -O3, the generated code hangs.  At -O2, although the generated code does not
hang, there is a spurious warning of invoking undefined behavior. 

This should be a very recent regression as r238938 does not yet miscompile the
code. 


$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-source-trunk/configure --enable-languages=c,c++,lto
--prefix=/usr/local/gcc-trunk --disable-bootstrap
Thread model: posix
gcc version 7.0.0 20160805 (experimental) [trunk revision 239166] (GCC) 
$ 
$ gcc-trunk -Os small.c; ./a.out
$ gcc-6.1 -O3 small.c; ./a.out
$ 
$ /usr/local/gcc-r238938/bin/gcc -O3 small.c; ./a.out
$ 
$ gcc-trunk -O2 small.c
small.c: In function ‘main’:
small.c:6:26: warning: iteration 26 invokes undefined behavior
[-Waggressive-loop-optimizations]
   for (a = 3; a != -1; a -= 5)
                          ^~
small.c:6:3: note: within this loop
   for (a = 3; a != -1; a -= 5)
   ^~~
$ ./a.out
$ 
$ gcc-trunk -O3 small.c
$ timeout -s 9 10 ./a.out
Killed
$ 


--------------------------------


char a;
short b;

int main ()
{
  for (a = 3; a != -1; a -= 5)
    while (b)
      ;
  return 0; 
}