[clang] Patch for 6037

[Nathan Sidwell]

On 01/14/15 23:16, Richard Smith wrote:
> On Wed, Dec 24, 2014 at 8:46 AM, Nathan Sidwell <nathan at acm.org> wrote:
>> [*] As an aside, I wonder if completing the std:set with the direct bases
>> and keeping it with the RecordDecl would speed up base conversion checks --
>> the base conversion machinery could use it for a quick 'is this even worth
>> figuring out' check.
>
> It shouldn't be a std::set; node-based containers are particularly bad
> for malloc traffic. Also, for a deep hierarchy, we'd end up storing
> O(N^2) nodes, which seems unfortunate (we already get that as a time
> cost with this patch; it'd be nice to avoid that too).

I'm not sure what your N is here.  For a hierarchy I'd expect O(N^M) size, where 
N is the mean number of direct bases and M is the mean depth.  Anyway, it'd be a 
time-space tradeoff -- we're already doing an O(whatever) walk on every 
class-related overload argument check.  I just don't know how much that costs in 
performance.

> I wonder if
> there's some relatively compact way of representing this information
> that supports fast query and fast updates...

A pointer hash table would suffice,  I'd've thought.  We don't care about 
ordering, just uniqueness.  That'd have O(1) lookup and insertion costs.

nathan