[Bug target/56550] New: cortex-m3: incorrect write to member of volatile packed structure

["jblackarty at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56550

             Bug #: 56550
           Summary: cortex-m3: incorrect write to member of volatile
                    packed structure
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jblackarty@gmail.com


GCC generates invalid write access instructions for unaligned member of
volatile packed structure on ARM Cortex-M3 target.

Simple testcase compiled with "arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -g3
-gdwarf-2 -nostartfiles testcase.c -o testcase.elf":

#include <stdint.h>
struct _test_struct_t {
    uint8_t m1;
    uint8_t m2;
    uint32_t m3;
    uint8_t m4;
    uint32_t m5;
} __attribute__((packed));
volatile struct _test_struct_t test_struct;
int main(void) {
    int test_var = 0xFFFFFF01;
    test_struct.m5 = test_var;
    // here test_struct.m5 == 0x01
    return 0;
}

When I remove m1,m2,m3, result is 0xFFFF01.
When I remove m1, m2, m3, m4, result become correct.

I tested it in Yagarto ARM toolchain (win32 build) with gcc versions 4.7.2 and
4.6.0 both.

This situation looks similar to #48784 and #20102.