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
