From: Eli Bendersky Date: Thu, 19 Jun 2014 18:12:44 +0000 (+0000) Subject: Document unroll and unroll_count directives. X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=47d52751cfb3a2c0491789ccdf097879d92ad0d6;p=clang 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] git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211286 91177308-0d34-0410-b5e6-96231b3b80d8 --- diff --git a/docs/LanguageExtensions.rst b/docs/LanguageExtensions.rst index a9ba9079c0..de9d513d5b 100644 --- a/docs/LanguageExtensions.rst +++ b/docs/LanguageExtensions.rst @@ -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 architectures supported by your application. .. 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 architectures supported by your application. 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++ diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst index a7bbbb5fdb..c99262eb35 100644 --- a/docs/ReleaseNotes.rst +++ b/docs/ReleaseNotes.rst @@ -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 --------------------------- diff --git a/include/clang/Basic/AttrDocs.td b/include/clang/Basic/AttrDocs.td index 6c8c9a37ae..5ba14462b2 100644 --- a/include/clang/Basic/AttrDocs.td +++ b/include/clang/Basic/AttrDocs.td @@ -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 '_ for details. }];