[Bug rtl-optimization/67028] New: combine bug. Different assumptions about subreg in different places.

["renlin at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67028

            Bug ID: 67028
           Summary: combine bug. Different assumptions about subreg in
                    different places.
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: renlin at gcc dot gnu.org
  Target Milestone: ---

Created attachment 36067
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36067&action=edit
test case

This is a combine bug manifest on arm target. A test case is attached.

The expected output of the test case should be:
checksum = 1

However, with the following command line:

arm-none-eabi-gcc -march=armv7-a -O3 test.c -specs=rdimon.specs -o a.out

the output is:
checksum = 0

It generates wrong code when the optimization level is: -O2, -O3, -Os
-O0, -O1 works fine.