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

[pinskia at gmail dot com]

------- Comment #1 from pinskia at gmail dot com  2009-07-24 05:54 -------
Subject: Re:   New: O2 optimizes out assignment to bitfield



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
>


-- 


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