[Bug rtl-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong

[bastian dot schick at sciopta dot com]

------- Comment #2 from bastian dot schick at sciopta dot com  2009-07-08 08:24 -------
(In reply to comment #1)
> 
> Sent from my iPhone

Oh, dude (which one :-)

> On Jul 8, 2009, at 12:32 AM, "bastian dot schick at sciopta dot com"
> <gcc-bugzilla@gcc.gnu.org 
>  > wrote:
> 
> > If the following code is compiled with -Os for ARM or ColdFire, the  
> > exit
> > condition for the loop is removed.
> > Replacing *tbl++ with tbl[i] or using unsigned long instead of  
> > volatile
> > unsigned long does not show the problem.
> > I suspect the post-increment optimization to be the problem, because  
> > the
> > PowerPC version does not show the problem.
> > Also: Using a different start-address for tbl, does not show the  
> > problem.
> 
> It looks more like a wrapping issue. 4*64 = 256. So we go from -256u  
> to 0 which causes wrapping of the pointer which is undefined and  

If so, it should not be used, but it seems to be a new optimization as 3.4.4
does not use it.

> therefor I think gcc is doing the correct thing (If got my numbers  
> correct).

Replacing *tbl++ by tbl[i] gives this ARM code:
.L2:
        mov     r3, #10
        str     r3, [r2], #4
        cmp     r2, #0
        bne     .L2
        bx      lr

See, gcc knows about the wrapping but still the *tbl++ version misses the
end-condition which is the bug.

Addenum: x86-gcc 4.3.3 (openSUSE 10.0) has the same problem.

Compiling with -O, gives correct code with *tbl++ :

.L2:
        mov     r3, #10
        str     r3, [r2], #4
        cmp     r2, #0
        bne     .L2
        bx      lr


-- 

bastian dot schick at sciopta dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bastian dot schick at
                   |                            |sciopta dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679