Author: eliben Date: Thu Jun 19 13:12:44 2014 New Revision: 211286 URL: http://llvm.org/viewvc/llvm-project?rev=211286&view=rev Log: Document unroll and unroll_count directives.
Extend the documentation for "#pragma clang loop" hints to include the unroll and unroll_count directives. Patch by Mark Heffernan [http://reviews.llvm.org/D4198] Modified: cfe/trunk/docs/LanguageExtensions.rst cfe/trunk/docs/ReleaseNotes.rst cfe/trunk/include/clang/Basic/AttrDocs.td Modified: cfe/trunk/docs/LanguageExtensions.rst URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/LanguageExtensions.rst?rev=211286&r1=211285&r2=211286&view=diff ============================================================================== --- cfe/trunk/docs/LanguageExtensions.rst (original) +++ cfe/trunk/docs/LanguageExtensions.rst Thu Jun 19 13:12:44 2014 @@ -1770,8 +1770,12 @@ Extensions for loop hint optimizations 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 @@ -1805,7 +1809,6 @@ width/count of the set of target archite .. code-block:: c++ - #pragma clang loop vectorize_width(2) #pragma clang loop interleave_count(2) for(...) { @@ -1815,6 +1818,46 @@ width/count of the set of target archite 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++ Modified: cfe/trunk/docs/ReleaseNotes.rst URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/ReleaseNotes.rst?rev=211286&r1=211285&r2=211286&view=diff ============================================================================== --- cfe/trunk/docs/ReleaseNotes.rst (original) +++ cfe/trunk/docs/ReleaseNotes.rst Thu Jun 19 13:12:44 2014 @@ -101,9 +101,10 @@ New Pragmas in Clang ----------------------- 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 --------------------------- Modified: cfe/trunk/include/clang/Basic/AttrDocs.td URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/AttrDocs.td?rev=211286&r1=211285&r2=211286&view=diff ============================================================================== --- cfe/trunk/include/clang/Basic/AttrDocs.td (original) +++ cfe/trunk/include/clang/Basic/AttrDocs.td Thu Jun 19 13:12:44 2014 @@ -1016,9 +1016,10 @@ def LoopHintDocs : Documentation { 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. }]; _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
