Re: Invalid compilation without -fno-strict-aliasing

[Daniel Jacobowitz <dan@debian.org>]

On Wed, Feb 26, 2003 at 02:40:27PM -0500, Richard B. Johnson wrote:
> On Wed, 26 Feb 2003, Daniel Jacobowitz wrote:
> 
> > On Wed, Feb 26, 2003 at 01:23:19PM -0500, Richard B. Johnson wrote:
> > > On Wed, 26 Feb 2003, Jean Tourrilhes wrote:
> > > 
> > > > On Tue, Feb 25, 2003 at 11:33:09PM -0500, Albert Cahalan wrote:
> > > > > Jean Tourrilhes writes:
> > > > > 
> > > > > > It looks like a compiler bug to me...
> > > > > > Some users have complained that when the following
> > > > > > code is compiled without the -fno-strict-aliasing,
> > > > > > the order of the write and memcpy is inverted (which
> > > > > > mean a bogus len is mem-copied into the stream).
> > > > > > Code (from linux/include/net/iw_handler.h) :
> > > > > > --------------------------------------------
> > > > > > static inline char *
> > > > > > iwe_stream_add_event(char * stream,  /* Stream of events */
> > > > > >        char * ends,  /* End of stream */
> > > > > >        struct iw_event *iwe, /* Payload */
> > > > > >        int event_len) /* Real size of payload */
> > > > > > {
> > > > > >  /* Check if it's possible */
> > > > > >  if((stream + event_len) < ends) {
> > > > > >   iwe->len = event_len;
> > > > > >   memcpy(stream, (char *) iwe, event_len);
> > > > > >   stream += event_len;
> > > > > >  } return stream;
> > > > > > }
> > > > > > --------------------------------------------
> > > > > > IMHO, the compiler should have enough context to
> > > > > > know that the reordering is dangerous. Any suggestion
> > > > > > to make this simple code more bullet proof is welcomed.
> > > > > >
> > > > > > Have fun...
> > > > > 
> > > > > Since (char*) is special, I agree that it's a bug.
> > > > > In any case, a warning sure would be nice!
> > > > > 
> > > > > Now for the fun. Pass iwe->len into this
> > > > > macro before the memcpy, and all should be well.
> > > > > 
> > > > > #define FORCE_TO_MEM(x) asm volatile(""::"r"(&(x)))
> > > > > 
> > > > > Like this:
> > > > > 
> > > > >    iwe->len = event_len;
> > > > >    FORCE_TO_MEM(iwe->len);
> > > > >    memcpy(stream, (char *) iwe, event_len);
> > > > 
> > > > 	I'll try that, that sounds absolutely clever (but I only
> > > > understand half of it).
> > > > 	Thanks a lot !
> > > > 
> > > > 	Jean
> > > > -
> > > 
> > > This does absoultely nothing with egcs-2.91.66. I also modified
> > > 
> > >  #define FORCE_TO_MEM(x) asm volatile(""::"r"(&(x)))
> > >                                            |________ this to "memory"
> > > 
> > > and it still does nothing. The result of gcc -O2 -S -o xxx xxx.c just
> > > shows:
> > > 
> > > #AP
> > > #NOAP
> > > 
> > > With no code in-between.
> > > 
> > > I also changed it to:
> > >  #define FORCE_TO_MEM(x) __asm__ __volatile__(""::"r"(&(x)))
> > > to no avail.
> > > 
> > > What's up?
> > 
> > That's "working".  Does it prevent the reordering?
> 
> It was supposed to force x, which may be cached in a register,
> to be written to memory __now__. It doesn't seem to do anything.
> I think FORCE_TO_MEM() needs to claim that it uses most all the
> registers. That will make sure that any register values get
> written to their final memory locations.

If so it wouldn't be inside the #APP/#NOAPP markers.  You didn't answer
my other question: was X in memory at the time?

You should be using something like __asm__ __volatile__ (""::"m"(x))
anyway.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer