[PATCH] [Core] Update references in parallel

[Michael Spencer]

On Thu, Mar 19, 2015 at 6:33 PM, Sean Silva <chisophugis at gmail.com> wrote:
> I really don't like this approach. It has large memory contention in the inner loop even though this computation doesn't inherently have any. Also it is using an extremely cache-unfriendly design, along with putting a `new` into the inner loop, causing contention inside the memory allocator besides the usual allocation slowness.
>
> We could just store a bit inside the Atom that marks it as dead. That avoids a bunch of hash table lookups in the _deadAtoms map anyway. We have a ton of space in the Atom base class in the _definition field, which is only using 2 bits of a pointer-aligned field (aligned to the vptr). We already have the _definition field loaded (and in cache) due to the dyn_cast above (which checks _definition). So it's just an OR and a store to mark it: no extra loads, no extra cache misses, no synchronization.

Lots of stuff is sill read directly from the memory mapped file, so
making Atoms non-const by default is a bit weird conceptually, but I
do see how the benefits here could be significant. It would be nice to
see if just storing everything packed in memory up front is actually
better anyway. An alternative is to have a member function that
returns a non-const struct for modifiable properties.

- Michael Spencer