[Bug c/118758] New: [15 regression] ok code with -O2, but wrong code with -O3

[dcb314 at hotmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118758

            Bug ID: 118758
           Summary: [15 regression] ok code with -O2, but wrong code with
                    -O3
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dcb314 at hotmail dot com
  Target Milestone: ---

Created attachment 60383
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=60383&action=edit
C source code

For the attached C code, with recent gcc:

foundBugs $ rm -f ./a.out ; ~/gcc/results/bin/gcc -w -O2 bug1086.c && ./a.out
checksum = E0BB38EE
foundBugs $ rm -f ./a.out ; ~/gcc/results/bin/gcc -w -O3 bug1086.c && ./a.out
checksum = 3489BDAF
foundBugs $ rm -f ./a.out ; ~/gcc/results/bin/gcc -w -O3 -fno-strict-aliasing
bug1086.c && ./a.out
checksum = 3489BDAF
foundBugs $ rm -f ./a.out ; ~/gcc/results/bin/gcc -w -O3 -fno-strict-aliasing
-fno-loop-interchange bug1086.c && ./a.out
checksum = 3489BDAF
foundBugs $ 

For stock gcc 14.2.0:

foundBugs $ rm -f ./a.out ; ~/gcc/results.14.2.0/bin/gcc -w -O2 bug1086.c &&
./a.out
checksum = E0BB38EE
foundBugs $ rm -f ./a.out ; ~/gcc/results.14.2.0/bin/gcc -w -O3 bug1086.c &&
./a.out
checksum = E0BB38EE
foundBugs $ 

So this looks like a 15 regression.
For the earliest gcc trunk I have:

foundBugs $ rm -f ./a.out ; ~/gcc/results.20250107.asan.ubsan/bin/gcc -w -O2
bug1086.c && ./a.out
checksum = E0BB38EE
foundBugs $ rm -f ./a.out ; ~/gcc/results.20250107.asan.ubsan/bin/gcc -w -O3
bug1086.c && ./a.out
checksum = 3489BDAF
foundBugs $ 

so it looks like the bug appears sometime before g:aa688dd6302fd9fd