xiph / rav1e / 4464

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travis-ci

Add explicit bounds checks in txfm functions (#1644)

Provides about 2.5% overall speedup at -s1.

When manually accessing slice indices in series,
the compiler cannot determine the highest index you
are going to access, so it adds a bounds check around
every access. Many transform functions were suffering
from this. For example, our dct16 function would have
32 extra branches from this, 16 for the input array
and 16 for the output array. By adding explicit bounds
checks at the start of the function, or by using an iterator,
the compiler knows that it can elide the other bounds
checks, and you end up with 2 bounds checks per function.
This also allows the compiler to auto-vectorize the code
in these functions where it would have previously been
unable to do so.

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