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

[Dmitry K.]

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.