Re: [9fans] how to avoid a memset() optimization

["Douglas A. Gwyn" <DAGwyn@null.net>]

Russ Cox wrote:
> This is a stupid argument anyway.  Volatile is a crock.
> Here's another problem.  Suppose I'm worried that
> leaking the processor flags at the end of my computation
> might reveal something about the secret key.  I want to
> clear them.  I could do something like:
> 	x = 0 & 0;
> which in this particular case will clear the appropriate
> flags.  But now the compiler comes along and optimizes
> away the whole expression.  Where do I put my volatile
> to make that stay?  Hmm?  The answer is not more crappy
> qualifiers.  The answer is clearly dumber compilers.

Do you really want the compiler to always generate code
	MOV	R1,#0
	MOV	R2,#0
	AND	R2,R1
	MOV	R2,X(SP)
?  Almost any modern compiler will peform the AND at
compiler time (constant folding), and most customers want
that.  Anyway, I don't see why you need to AND in this
context.  Are you talking about the C and V bits?  If some
other process can inspect those for your process I'd think
it could inspect a whole lot more..

It is certainly true that we could really use better access
to the C bit from higher-level language source code.  This
shows up especially in the MP library.  But exploiting too
specific knowledge about the workings of a specific platform
can be dangerous, as we've seen historically.  If anybody
can devise a *nice* way to get at the C bit in MP coding,
please let me know so that it can be proposed for C0x.