Re: [avr-gcc-list] WinAVR 20050214 (gcc 3.4.3) and optimizer bug.

[Vesa Jääskeläinen]

Dmitry K. wrote:
> On Monday 09 May 2005 08:26, Vesa Jääskeläinen wrote:
> ...
>
> > --- begin of code snippet ---
> > #define u_long unsigned long
> > #define u_char unsigned char
> >
> > void test(int req, void *conf)
> > {
> >     void **ppv = (void **) conf;
> >     u_long *lvp = (u_long *) conf;
> >     u_long lv = *lvp;
> >     u_char bv = (u_char) lv;
> >
> >     switch (req) {
> >     case 1:
> >         if (bv)
> >         {
> >             asm ( "nop ; bv needs to be defined and used");
> >         }
> >         break;
> >
> >     case 2:
> >         asm ( "nop ; entry to case 2");
> >
> >         if (ppv && (*ppv != 0)) {
> >             asm ( "nop ; ppv and *ppv are not zeros");
> >         } else {
> >             asm ( "nop ; either ppv or *ppv is zero");
> >         }
> >
> >         break;
> >     }
> > }
>
> ...
>
> > We can see that one compare is missing and this causes functional change
> > to code.
> >
> > Could someone verify this bug and possibly fix it ;)
> >
> > I used following command line:
> > avr-gcc -c -mmcu=atmega128 -Os -Wall -Wstrict-prototypes
> > -Wa,-ahlms=test.lst test.c -o test.o
>
>
> You program read from `*ppv' BEFORE checking. This is a reason.
> Look 'gcc.info (3.3.5)':
>
> `-fdelete-null-pointer-checks'
>      Use global dataflow analysis to identify and eliminate useless
>      checks for null pointers.  The compiler assumes that dereferencing
>      a null pointer would have halted the program.  If a pointer is
>      checked after it has already been dereferenced, it cannot be null.
>
>      In some environments, this assumption is not true, and programs can
>      safely dereference null pointers.  Use
>      `-fno-delete-null-pointer-checks' to disable this optimization for
>      programs which depend on that behavior.
>
>      Enabled at levels `-O2', `-O3', `-Os'.
>
> Regards.

Thank you for this information. This indeed seems to cure the problem in 
this case. But shouldn't this be default for AVR ?

Is there a "right" way to write this code then ?

I have personally used this in numerous places (except that I compare it 
 to NULL). I usually has some elements that pass either NULL pointer or 
pointer to some structure. First one usually disables option and latter 
one enables it.

I would like to know what is wrong in this and how could it be invalid 
in some architectures ?

Thanks,
Vesa Jääskeläinen