LanguageExtensions.rst

.. code-block:: c++

  #pragma clang loop unroll(enable)
  for(...) {
    ...
  }

If ``unroll(full)`` is specified the unroller will attempt to fully unroll the
loop if the trip count is known at compile time identically to
``unroll(enable)``. However, with ``unroll(full)`` the loop will not be unrolled
if the loop count is not known at compile time.

.. code-block:: c++

  #pragma clang loop unroll(full)
  for(...) {
    ...
  }

The unroll count can be specified explicitly with ``unroll_count(_value_)`` where
_value_ is a positive integer. If this value is greater than the trip count the
loop will be fully unrolled. Otherwise the loop is partially unrolled subject
to the same code size limit as with ``unroll(enable)``.

.. code-block:: c++

  #pragma clang loop unroll_count(8)
  for(...) {
    ...
  }

Unrolling of a loop can be prevented by specifying ``unroll(disable)``.

Additional Information
----------------------

For convenience multiple loop hints can be specified on a single line.

.. code-block:: c++

  #pragma clang loop vectorize_width(4) interleave_count(8)
  for(...) {
    ...
  }

If an optimization cannot be applied any hints that apply to it will be ignored.
For example, the hint ``vectorize_width(4)`` is ignored if the loop is not
proven safe to vectorize. To identify and diagnose optimization issues use
`-Rpass`, `-Rpass-missed`, and `-Rpass-analysis` command line options. See the
user guide for details.