[Bug c/115019] New: wraparound bug with with -O3 and int128

["djnz00 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115019

            Bug ID: 115019
           Summary: wraparound bug with with -O3 and int128
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: djnz00 at gmail dot com
  Target Milestone: ---

Created attachment 58157
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58157&action=edit
reduced testcase

wraparound and condition testing of 128bit values performs incorrectly under
-O3:

  __int128_t i = ((__uint128_t)1)<<127;
  do {
    puts(itoa128(buf, i)); /* do something with i */
    i += ((__uint128_t)1)<<125;
  } while (i);

in other code, this loop runs interminably under -O3, but completes as expected
without optimization; in this reduced testcase sample, it only executes one
repeat of the loop (clang does it correctly); either way, there is a divergence
of behavior with/without -O3