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Re: [PATCH] Hoist adjacent pointer loads


On 05/03/2012 10:47 AM, William J. Schmidt wrote:

Yes and no.  What's important is that you don't want to introduce page
faults (or less urgently, cache misses) by speculating the load.  So the
patch is currently extremely constrained, and likely will always stay
that way.  Only fields that are pointers and that are strictly adjacent
are hoisted, and only if they're in the same 16-byte block.  (The number
16 is a parameter that can be adjusted.)

Hopefully I didn't miss your point -- let me know if I did and I'll try
again. :)
You missed the point :-)

Under the C++11 memory model you can't introduce new data races on objects which might be visible to multiple threads. This requirement can restrict speculation in many cases. Furthermore, it sounds like C11 will have similar constraints.

I believe there's a wiki page which touches on these kinds of issues.

That doesn't mean we can't ever do the optimization, just that we have to be more careful than we have in the past when mucking around with memory optimizations.

jeff


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