[Bug c/84121] New: optimizer breaks code without -frwapv, but -Wall -Wextra show no warnings

[noreply at nullhomotopie dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84121

            Bug ID: 84121
           Summary: optimizer breaks code without -frwapv, but -Wall
                    -Wextra show no warnings
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: noreply at nullhomotopie dot de
  Target Milestone: ---

Created attachment 43285
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43285&action=edit
test code

The attached program runs into an endless loop when compiled with

   gcc -O3 -o itest ./itest.c -Wall -Wextra 

Removing "-O3" or adding "-fwrapv" or changing the "int fac" in the code to
"unsigned int fac" fixes the issue.

A debug statement shows that the loop of the form 

   while(fac) {
       printf("fac=%d\n",fac);
       ...
   }

is not left, even though "fac" is eventually zero.  (If this is a bug in the
code I would expect at least a compiler warning.)

The output of gcc -v is

  COLLECT_GCC=/home/cn/mygcc73/bin/gcc
 
COLLECT_LTO_WRAPPER=/home/cn/mygcc73/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/lto-wrapper
  Target: x86_64-pc-linux-gnu
  Configured with: ./configure --prefix=/home/cn/mygcc73 --disable-multilib
  Thread model: posix
  gcc version 7.3.0 (GCC)