[Bug rtl-optimization/49169] New: ARM: optimisations strip the Thumb/ARM mode bit off function pointers

[michael.hope at linaro dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49169

           Summary: ARM: optimisations strip the Thumb/ARM mode bit off
                    function pointers
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: michael.hope@linaro.org


ARM devices encode the instruction set mode in the LSB of the function address.
 Functions are word aligned on ARM.  If you try to test the LSB of a function
pointer then GCC assumes that the two least significant bits are zero and
optimises away the test.

This problem is seen in Mono and was originally reported at:
 https://bugs.launchpad.net/ubuntu/+source/gcc-4.5/+bug/721531

A reduced test case is:

void main() {
        void *p = main;
        if ((int)p & 1) printf ("HIT!\n");
}

When compiled with -march=armv7-a -mthumb -O0 then the word 'HIT!' will show. 
When compiled with -O2, the branch is not taken.

The problem does not occur in 4.4.5.  It does occur in 4.5.2, 4.6.0, and trunk
r174044.