Re: [Xen-devel] [PATCH] slightly consolidate code in free_domheap_pages()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.06.14 at 12:04, <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> On Fri, 2014-06-20 at 13:40 +0100, Jan Beulich wrote:
> 
>> +        if ( likely(d) && likely(d != dom_cow) )
> 
> OOI is this more or less efficient than a single likely around the
> entire thing?

likely()/unlikely() around && or || expressions is never having the
intended effect unless these expressions can be guaranteed to
result in only a single branch instruction in the compiled code.
That's because branch likelihood needs to be determined for each
branch instruction individually (and e.g. likely(x && y) doesn't
necessarily mean likely(x) && likely(y), i.e. it may only be the
particular combination of the two that is likely).

>> +        else
>> +        {
>> +            ASSERT(!d || !order);
> 
> Is this effectively replacing the ASSERT(order == 0) In the previous d
> == dom_cow case?

Yes.

> If so, can d at this point ever be anything other than dom_cow or NULL?
> I don't think so. Given that I think ASSERT(!(d == dom_cow && order !=
> 0)) would more clearly capture the intent of the test (with the spelling
> out of the conditions being more important than the de morganing of the
> expression).

Indeed, d can only be NULL or dom_cow here (being in the else part
of the if() you quoted at the top). So an alternative might indeed be
ASSERT(d != dom_cow || !order), but that seems less desirable to
me as it opens up ways to pass the ASSERT() with d != NULL should
the if() condition ever get modified. I.e. I'd prefer the assertion to be
as restrictive as possible, getting relaxed only when in fact necessary.

Jan


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