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llvm-svn: 171449
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Nadav Rotem 2013-01-03 01:56:33 +00:00
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docs/Vectorizers.rst
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@ -208,15 +208,14 @@ Partial unrolling during vectorization
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Modern processors feature multiple execution units, and only programs that contain a
high degree of parallelism can fully utilize the entire width of the machine.
high degree of parallelism can fully utilize the entire width of the machine.
The Loop Vectorizer increases the instruction level parallelism (ILP) by
performing partial-unrolling of loops.
In the example below the entire array is accumulated into the variable 'sum'.
This is inefficient because only a single 'adder' can be used by the processor.
This is inefficient because only a single execution port can be used by the processor.
By unrolling the code the Loop Vectorizer allows two or more execution ports
to be used.
to be used simultaneously.
.. code-block:: c++