[Bug c/68963] New: O3 vs. O2 discards part of loop and terminates early

[mail+gccbugs at svenpeter dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68963

            Bug ID: 68963
           Summary: O3 vs. O2 discards part of loop and terminates early
           Product: gcc
           Version: 5.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mail+gccbugs at svenpeter dot me
  Target Milestone: ---

Created attachment 37065
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37065&action=edit
testcase to reproduce the bug

Enabling O3 runs only a part of the loop body and terminates before all
iterations. small testcase attached.

wrong result with O3:
% rm -f test123 && gcc-5.3.0 -std=c99 -W -Wall -Wextra -Wpedantic -O3 -o
test123 test123.c && ./test123
--> 3
1.000000 2.000000 3.000000 1337.000000 1337.000000 1337.000000 1337.000000
1337.000000 1337.000000 1337.000000

expected result with O2: 
% rm -f test123 && gcc-5.3.0 -std=c99 -W -Wall -Wextra -Wpedantic -O2 -o
test123 test123.c && ./test123
--> 3
--> 4
--> 5
1.000000 2.000000 3.000000 1.000000 2.000000 3.000000 1337.000000 1337.000000
1337.000000 1337.000000