The ``#pragma clang loop`` directive is used to specify hints for optimizing the
subsequent for, while, do-while, or c++11 range-based for loop. The directive
-provides options for vectorization and interleaving. Loop hints can be specified
-before any loop and will be ignored if the optimization is not safe to apply.
+provides options for vectorization, interleaving, and unrolling. Loop hints can
+be specified before any loop and will be ignored if the optimization is not safe
+to apply.
+
+Vectorization and Interleaving
+------------------------------
A vectorized loop performs multiple iterations of the original loop
in parallel using vector instructions. The instruction set of the target
.. code-block:: c++
-
#pragma clang loop vectorize_width(2)
#pragma clang loop interleave_count(2)
for(...) {
Specifying a width/count of 1 disables the optimization, and is equivalent to
``vectorize(disable)`` or ``interleave(disable)``.
+Loop Unrolling
+--------------
+
+Unrolling a loop reduces the loop control overhead and exposes more
+opportunities for ILP. Loops can be fully or partially unrolled. Full unrolling
+eliminates the loop and replaces it with an enumerated sequence of loop
+iterations. Full unrolling is only possible if the loop trip count is known at
+compile time. Partial unrolling replicates the loop body within the loop and
+reduces the trip count.
+
+If ``unroll(enable)`` is specified the unroller will attempt to fully unroll the
+loop if the trip count is known at compile time. If the loop count is not known
+or the fully unrolled code size is greater than the limit specified by the
+`-pragma-unroll-threshold` command line option the loop will be partially
+unrolled subject to the same limit.
+
+.. code-block:: c++
+
+ #pragma clang loop unroll(enable)
+ 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 `-pragma-unroll-threshold` limit.
+
+.. 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++
-----------------------
Loop optimization hints can be specified using the new `#pragma clang loop`
-directive just prior to the desired loop. The directive allows vectorization
-and interleaving to be enabled or disabled, and the vector width and interleave
-count to be manually specified. See language extensions for details.
+directive just prior to the desired loop. The directive allows vectorization,
+interleaving, and unrolling to be enabled or disabled. Vector width as well
+as interleave and unrolling count can be manually specified. See language
+extensions for details.
C Language Changes in Clang
---------------------------
let Category = DocCatStmt;
let Content = [{
The ``#pragma clang loop'' directive allows loop optimization hints to be
-specified for the subsequent loop. The directive allows vectorization
-and interleaving to be enabled or disabled, and the vector width and interleave
-count to be manually specified. See `language extensions
+specified for the subsequent loop. The directive allows vectorization,
+interleaving, and unrolling to be enabled or disabled. Vector width as well
+as interleave and unrolling count can be manually specified. See
+`language extensions
<http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>'_
for details.
}];