RE: [avr-gcc-list] Clobber list isn't working

[Eric Weddington]

> -----Original Message-----
> From:
> address@hidden
> [mailto:address@hidden
> org] On Behalf Of Gre7g Luterman
> Sent: Wednesday, July 25, 2007 6:35 AM
> To: address@hidden
> Subject: RE: [avr-gcc-list] Clobber list isn't working
>
> --- Eric Weddington <address@hidden> wrote:
>
> > Can you provide your real test case instead of the
> > degenerate case above?
>
> Huh?  But that's the whole point of making a simple
> test case.  That way I don't have to make you slog
> through hundreds of pages of code and I don't have to
> put my customer's code up on the 'net.

True, but you said yourself that it was an absurd test case. Obviously a nop
doesn't clobber any registers.


> Here, just change the nop to something that actually
> abuses r28 and you'll see a perfectly valid example:
>
> void test(void)
> {
>     struct
>     {
>         int a, b, c, d, e, f;
>     } x;
>
>     x.d = 5;
>     asm volatile("in r28, 0x2F" : : : "r28");
>     x.d = 6;
> }
>
> which compiles to:
> .
> :
>     x.d = 5;
>   14:   85 e0           ldi     r24, 0x05       ; 5
>   16:   90 e0           ldi     r25, 0x00       ; 0
>   18:   98 87           std     Y+8, r25        ; 0x08
>   1a:   8f 83           std     Y+7, r24        ; 0x07
>     asm volatile("in r28, 0x2F" : : : "r28");
>   1c:   cf b5           in      r28, 0x2f       ; 47
>     x.d = 6;
>   1e:   86 e0           ldi     r24, 0x06       ; 6
>   20:   90 e0           ldi     r25, 0x00       ; 0
>   22:   98 87           std     Y+8, r25        ; 0x08
>   24:   8f 83           std     Y+7, r24        ; 0x07
> .
> :
> As you can see, this code really does mess up r28, but
> gcc presumes that Y is untouched. Obviously, this code
> could use r0 instead of r28, but that's not the point.
>  The problem is that gcc should be respecting the
> clobber list or spitting out an error.
>
> Ideas?

Hmm. I see what you mean. AVR GCC 4.2.1 generates the same code too.

Right now, all I can recommend is that you fill out a GCC Bug Report:
<http://gcc.gnu.org/bugzilla/>
Put me in the CC list so I can track the bug.

Eric Weddington