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Re: [RFC] [PATCH] Support explicit_bzero, memset_s, memzero_explicit, or similar.


On Tue, 16 Dec 2014, Rich Felker wrote:

> On Tue, Dec 16, 2014 at 10:24:17AM -0600, Richard Henderson wrote:
> > On 12/15/2014 11:26 PM, Rich Felker wrote:
> > > I don't think the implementation as written is valid -- at least, not
> > > if you allow LTO. The compiler barrier does not prevent the memset
> > > from being optimized out unless the address of the buffer being memset
> > > has been leaked to code the compiler cannot see. As long as it sees
> > > that the asm has no way of observing the output of the memset, it can
> > > optimize out the memset. Simply making the memset buffer visible to
> > > the asm by passing its address (or better yet, it as a memory object)
> > > in an asm constraint would probably fix this, but I'd like to have
> > > someone from the GCC side confirm this.
> > 
> > I believe a simple memory clobber (without even passing the buffer address)
> > should be sufficient.  The memory clobber is a very large hammer, indicating
> > that *all* memory is both read and written.  Thus the memset cannot be dead,
> > because its results may be read by the asm.
> 
> But in the case of a function like:
> 
> 	int foo = 42;
> 	memset(&foo, 0, sizeof foo);
> 	__asm__ __volatile__ ( "" : : : "memory" );

Note that the asm in the proposed patch is written in a different way: it
accepts the address of the memset 'dest' pointer.  I don't understand why you
said "as long the asm has no way of observing the output of memset" while it
clearly can do so via the passed pointer.

Alexander

> after analysis based on compiler knowledge of the memset function, the
> object foo is not "memory" because its address has never leaked. Since
> the asm cannot see it, it can be optimized out to never exist at all
> independently of handling the asm. If this weren't the case, presence
> of asm with memory clobbers anywhere in the whole program would
> prevent the compiler from optimizing out objects like this anywhere in
> the whole program, which is obviously not correct.


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