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SubjectRe: Invalid compilation without -fno-strict-aliasing
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?


Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.


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