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Vectorization documentation for loop hint pragmas and Rpass diagnostics.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211924 91177308-0d34-0410-b5e6-96231b3b80d8
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Tyler Nowicki 2014-06-27 18:30:08 +00:00
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docs/Vectorizers.rst
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@ -51,6 +51,89 @@ Users can control the unroll factor using the command line flag "-force-vector-u
$ clang -mllvm -force-vector-unroll=2 ...
$ opt -loop-vectorize -force-vector-unroll=2 ...
Pragma loop hint directives
^^^^^^^^^^^^^^^^^^^^^^^^^^^
The ``#pragma clang loop`` directive allows loop vectorization hints to be
specified for the subsequent for, while, do-while, or c++11 range-based for
loop. The directive allows vectorization and interleaving to be enabled or
disabled. Vector width as well as interleave count can also be manually
specified. The following example explicitly enables vectorization and
interleaving:
.. code-block:: c++
#pragma clang loop vectorize(enable) interleave(enable)
while(...) {
...
}
The following example implicitly enables vectorization and interleaving by
specifying a vector width and interleaving count:
.. code-block:: c++
#pragma clang loop vectorize_width(2) interleave_count(2)
for(...) {
...
}
See the Clang
`language extensions
<http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>`_
for details.
Diagnostics
-----------
Many loops cannot be vectorized including loops with complicated control flow,
unvectorizable types, and unvectorizable calls. The loop vectorizer generates
optimization remarks which can be queried using command line options to identify
and diagnose loops that are skipped by the loop-vectorizer.
Optimization remarks are enabled using:
``-Rpass=loop-vectorize`` identifies loops that were successfully vectorized.
``-Rpass-missed=loop-vectorize`` identifies loops that failed vectorization and
indicates if vectorization was specified.
``-Rpass-analysis=loop-vectorize`` identifies the statements that caused
vectorization to fail.
Consider the following loop:
.. code-block:: c++
#pragma clang loop vectorize(enable)
for (int i = 0; i < Length; i++) {
switch(A[i]) {
case 0: A[i] = i*2; break;
case 1: A[i] = i; break;
default: A[i] = 0;
}
}
The command line ``-Rpass-missed=loop-vectorized`` prints the remark:
.. code-block:: console
no_switch.cpp:4:5: remark: loop not vectorized: vectorization is explicitly enabled [-Rpass-missed=loop-vectorize]
And the command line ``-Rpass-analysis=loop-vectorize`` indicates that the
switch statement cannot be vectorized.
.. code-block:: console
no_switch.cpp:4:5: remark: loop not vectorized: loop contains a switch statement [-Rpass-analysis=loop-vectorize]
switch(A[i]) {
^
To ensure line and column numbers are produced include the command line options
``-gline-tables-only`` and ``-gcolumn-info``. See the Clang `user manual
<http://clang.llvm.org/docs/UsersManual.html#options-to-emit-optimization-reports>`_
for details
Features
--------