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From: Andy Glew (glew_at_[hidden])
Date: 1999-08-01 19:59:11

Although it seems impossible to create a non-intrusive weak
> pointer class that can track the liveness of an arbitrary class of
> object, it is possible to create one that tracks the liveness of
> objects controlled by shared_ptr.
> The changes to out current shared_ptr would be:
> Switch to the "indirect" implementation of shared_ptr.

Good solution.
> The net costs are whatever the indirection costs (nothing much,
> according to my measurements, it might even be a net gain)

However, I am somewhat astonished by this statement.

My background is in computer architecture - I was one of the principal
architects of the P6 microarchitecture and MMX - and I can assure you
folks that extra levels of indirection are one of the most costly things
for next generation computer architectures. When we are, more and
more, limited by pointer chasing cache misses, that extra level of
indirection doubles the number of pointer chasing cache misses.

On the other hand... this is a problem that I know how to fix in hardware,
using data traces and jump pointers. In fact, you may even want to consider
jump pointers in software --- no, non-faulting loads are not portable yet.
I just don't expect these ideas to get into hardware for 5-10 years.

So: I like the indirect solution, but I would not be surprised if in next generation
machines - the machines coming out next year - on really big programs it hurts
performance significantly. (On small programs, where the pointed to data structures
fit into the cache, it should not hurt much - worst case halving the cache size.)

Software functionality first; performance later.

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