Re: [Xen-devel] code question?

[Tim Newsham <newsham@xxxxxxxx>]

On Thu, 11 Aug 2005, Keir Fraser wrote:
> Forming compound predicates for bitwise operators can be faster than using 
> the logical operators because they are non 'short circuiting'. This means you 
> end up with fewer branches in the generated code and end up with nice 
> straight-line code that should execute fast.
Is it worth it to perform these optimizations?  For example the code to 
get time information in xen/386 linux:
       do {
                shadow_tv_version = s->wc_version;
                rmb();
                shadow_tv.tv_sec  = s->wc_sec;
                shadow_tv.tv_nsec = s->wc_nsec;
                rmb();
        }
        while ((s->wc_version & 1) | (shadow_tv_version ^ s->wc_version));

Granted most experienced programmers will recognize that the xor is used 
to test for inequality and the bitwise-or is used as a logical-or.  But 
how often does this piece of code even run?  Seems like a case of trading 
readability for speed when optimization isn't even important.
> -- Keir
Tim Newsham
http://www.lava.net/~newsham/

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