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Subject: Re: [boost] Boost SIMD beta release
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2012-12-24 14:51:18

On December 24, 2012 8:16:13 PM "Domagoj Saric" <dsaritz_at_[hidden]> wrote:
> If we realize that protected memory
> systems have page-size-granularity which is much larger than any conceivable
> SIMD vector size it immediately follows that "overread" (reading outside of
> the specified range) of a maximum size of "SIMD-cardinal - 1" is always
> safe. Further more in a single threaded case or when you allocate your
> arrays with both their location and size aligned, overwrite is also always
> safe when you wrap the overwrite in a guard class that automatically aligns
> the array pointers/iterators and then simply restores any possibly
> overwritten elements in its destructor.


Call me old-fashioned but this would be an abomination. Every memory
checking tool will righteously raise a red flag on such code. In some
cases the overread garbage can also affect the processing result, so it
cannot be used as a general solution. Even if you provide such
behavior, please, don't make it a default and make it very explicit.

I'm prepared to have a somewhat slower tail processing, it won't be a
deal-breaking slowdown anyway. And if it will, I will probably
overallocate memory and fill the tail with neutral data and avoid the
undefined behavior.

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