[Bug tree-optimization/40844] New: O2 optimizes out assignment to bitfield

[jim at bodwin dot us]

Incorrect code is produced for the following source with the O2 option.  In
particular, the assignment to the bitfield field2 is optimized out of the code
entirely and regImage is left all zero.  Correct code is produced with the O1
option and (at least) with gcc version 4.3.2.

====================  Source Code

typedef struct hwreg {
    short field1 : 4;
    short field2 : 2;
    short field3 : 10;
} hwreg_t;

void foo(volatile short *hwPtr, short f2) {
    hwreg_t regImage;           // Place to build reg image bit-by-bit
    *(short *) &regImage = 0;

    regImage.field2 = f2;

    *hwPtr = *((short *) &regImage);
}

============================ Object dump


/tmp/foo.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <foo>:
   0:   e3a03000        mov     r3, #0  ; 0x0
   4:   e24dd004        sub     sp, sp, #4      ; 0x4
   8:   e1c030b0        strh    r3, [r0]
   c:   e28dd004        add     sp, sp, #4      ; 0x4
  10:   e12fff1e        bx      lr

============================  gcc -v output

Configured with: ../gcc-4.4.1/configure --target=arm-elf --disable-__cxa_atexit
--with-newlib --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
--with-gxx-include-dir=/home/jmb/arm-elf/include --prefix=/home/jmb/arm-elf :
(reconfigured) ../gcc-4.4.1/configure --target=arm-elf --disable-__cxa_atexit
--with-newlib --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
--with-gxx-include-dir=/home/jmb/arm-elf/include --prefix=/home/jmb/arm-elf
Thread model: single
gcc version 4.4.1 (GCC)


-- 
           Summary: O2 optimizes out assignment to bitfield
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jim at bodwin dot us
 GCC build triplet: i486-pc-linux-gnu
  GCC host triplet: i486-pc-linux-gnu
GCC target triplet: arm-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40844