Re: [PATCH v1] compiler: prevent dead store elimination

[Bill Davidsen]

Andi Kleen wrote:
> > Every byte in the [p,p+n[ range must be used. If you only use the
> > first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
> > _will_ skip scrubbing bytes beyond the first. This works with
> > gcc-3.2.3 up to gcc-4.4.3.
>
> You forgot to credit Mikael who did all the hard work figuring
> this out?
>
> >  /*
> > + * Dead store elimination (DSE) is an optimization that may remove a write to
> > + * a buffer that is not used anymore. Use ARRAY_PREVENT_DSE after a write when
> > + * the scrub is required for security reasons.
> > + */
> > +#define ARRAY_PREVENT_DSE(p, n)				\
>
> Maybe it's just me, but the name is ugly.
>
> > +	do {						\
> > +		struct __scrub { char c[n]; };		\
>
>
> Better typeof(*p)[n]
>
> > +++ b/include/linux/compiler-intel.h
> > @@ -14,9 +14,11 @@
> >   * It uses intrinsics to do the equivalent things.
> >   */
> >  #undef barrier
> > +#undef ARRAY_PREVENT_DSE
> >  #undef RELOC_HIDE
> >  
> >  #define barrier() __memory_barrier()
> > +#define ARRAY_PREVENT_DSE(p, n)
>
> Who says the Intel compiler doesn't need this?
>
> I'm sure it does dead store elimination too and it understands
> gcc asm syntax.
According to the Intel forum, it not only doesn't, but a request for this as a 
feature was rejected, so it won't. Or am I misreading this?

http://software.intel.com/en-us/forums/showthread.php?t=46770

--
Bill Davidsen <davidsen@xxxxxxx>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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