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

[Andrew Pinski]

Sent from my iPhone

On Jul 23, 2009, at 10:22 PM, "jim at bodwin dot us" <gcc-bugzi...@gcc.gnu.org 
> wrote:

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);
}

I think you are violating c/c++ aliasing rules here.



============================ 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