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ReStructuredText
427 lines
13 KiB
ReStructuredText
==========================
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Auto-Vectorization in LLVM
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==========================
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.. contents::
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:local:
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LLVM has two vectorizers: The :ref:`Loop Vectorizer <loop-vectorizer>`,
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which operates on Loops, and the :ref:`SLP Vectorizer
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<slp-vectorizer>`. These vectorizers
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focus on different optimization opportunities and use different techniques.
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The SLP vectorizer merges multiple scalars that are found in the code into
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vectors while the Loop Vectorizer widens instructions in loops
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to operate on multiple consecutive iterations.
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Both the Loop Vectorizer and the SLP Vectorizer are enabled by default.
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.. _loop-vectorizer:
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The Loop Vectorizer
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===================
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Usage
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-----
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The Loop Vectorizer is enabled by default, but it can be disabled
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through clang using the command line flag:
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.. code-block:: console
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$ clang ... -fno-vectorize file.c
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Command line flags
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^^^^^^^^^^^^^^^^^^
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The loop vectorizer uses a cost model to decide on the optimal vectorization factor
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and unroll factor. However, users of the vectorizer can force the vectorizer to use
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specific values. Both 'clang' and 'opt' support the flags below.
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Users can control the vectorization SIMD width using the command line flag "-force-vector-width".
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.. code-block:: console
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$ clang -mllvm -force-vector-width=8 ...
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$ opt -loop-vectorize -force-vector-width=8 ...
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Users can control the unroll factor using the command line flag "-force-vector-unroll"
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.. code-block:: console
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$ clang -mllvm -force-vector-unroll=2 ...
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$ opt -loop-vectorize -force-vector-unroll=2 ...
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Pragma loop hint directives
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The ``#pragma clang loop`` directive allows loop vectorization hints to be
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specified for the subsequent for, while, do-while, or c++11 range-based for
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loop. The directive allows vectorization and interleaving to be enabled or
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disabled. Vector width as well as interleave count can also be manually
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specified. The following example explicitly enables vectorization and
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interleaving:
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.. code-block:: c++
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#pragma clang loop vectorize(enable) interleave(enable)
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while(...) {
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...
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}
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The following example implicitly enables vectorization and interleaving by
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specifying a vector width and interleaving count:
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.. code-block:: c++
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#pragma clang loop vectorize_width(2) interleave_count(2)
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for(...) {
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...
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}
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See the Clang
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`language extensions
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<http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>`_
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for details.
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Diagnostics
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-----------
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Many loops cannot be vectorized including loops with complicated control flow,
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unvectorizable types, and unvectorizable calls. The loop vectorizer generates
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optimization remarks which can be queried using command line options to identify
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and diagnose loops that are skipped by the loop-vectorizer.
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Optimization remarks are enabled using:
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``-Rpass=loop-vectorize`` identifies loops that were successfully vectorized.
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``-Rpass-missed=loop-vectorize`` identifies loops that failed vectorization and
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indicates if vectorization was specified.
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``-Rpass-analysis=loop-vectorize`` identifies the statements that caused
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vectorization to fail.
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Consider the following loop:
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.. code-block:: c++
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#pragma clang loop vectorize(enable)
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for (int i = 0; i < Length; i++) {
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switch(A[i]) {
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case 0: A[i] = i*2; break;
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case 1: A[i] = i; break;
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default: A[i] = 0;
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}
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}
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The command line ``-Rpass-missed=loop-vectorized`` prints the remark:
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.. code-block:: console
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no_switch.cpp:4:5: remark: loop not vectorized: vectorization is explicitly enabled [-Rpass-missed=loop-vectorize]
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And the command line ``-Rpass-analysis=loop-vectorize`` indicates that the
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switch statement cannot be vectorized.
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.. code-block:: console
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no_switch.cpp:4:5: remark: loop not vectorized: loop contains a switch statement [-Rpass-analysis=loop-vectorize]
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switch(A[i]) {
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^
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To ensure line and column numbers are produced include the command line options
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``-gline-tables-only`` and ``-gcolumn-info``. See the Clang `user manual
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<http://clang.llvm.org/docs/UsersManual.html#options-to-emit-optimization-reports>`_
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for details
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Features
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--------
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The LLVM Loop Vectorizer has a number of features that allow it to vectorize
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complex loops.
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Loops with unknown trip count
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The Loop Vectorizer supports loops with an unknown trip count.
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In the loop below, the iteration ``start`` and ``finish`` points are unknown,
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and the Loop Vectorizer has a mechanism to vectorize loops that do not start
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at zero. In this example, 'n' may not be a multiple of the vector width, and
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the vectorizer has to execute the last few iterations as scalar code. Keeping
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a scalar copy of the loop increases the code size.
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.. code-block:: c++
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void bar(float *A, float* B, float K, int start, int end) {
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for (int i = start; i < end; ++i)
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A[i] *= B[i] + K;
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}
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Runtime Checks of Pointers
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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In the example below, if the pointers A and B point to consecutive addresses,
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then it is illegal to vectorize the code because some elements of A will be
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written before they are read from array B.
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Some programmers use the 'restrict' keyword to notify the compiler that the
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pointers are disjointed, but in our example, the Loop Vectorizer has no way of
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knowing that the pointers A and B are unique. The Loop Vectorizer handles this
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loop by placing code that checks, at runtime, if the arrays A and B point to
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disjointed memory locations. If arrays A and B overlap, then the scalar version
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of the loop is executed.
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.. code-block:: c++
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void bar(float *A, float* B, float K, int n) {
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for (int i = 0; i < n; ++i)
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A[i] *= B[i] + K;
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}
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Reductions
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^^^^^^^^^^
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In this example the ``sum`` variable is used by consecutive iterations of
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the loop. Normally, this would prevent vectorization, but the vectorizer can
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detect that 'sum' is a reduction variable. The variable 'sum' becomes a vector
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of integers, and at the end of the loop the elements of the array are added
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together to create the correct result. We support a number of different
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reduction operations, such as addition, multiplication, XOR, AND and OR.
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.. code-block:: c++
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int foo(int *A, int *B, int n) {
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unsigned sum = 0;
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for (int i = 0; i < n; ++i)
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sum += A[i] + 5;
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return sum;
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}
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We support floating point reduction operations when `-ffast-math` is used.
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Inductions
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^^^^^^^^^^
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In this example the value of the induction variable ``i`` is saved into an
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array. The Loop Vectorizer knows to vectorize induction variables.
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.. code-block:: c++
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void bar(float *A, float* B, float K, int n) {
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for (int i = 0; i < n; ++i)
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A[i] = i;
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}
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If Conversion
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^^^^^^^^^^^^^
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The Loop Vectorizer is able to "flatten" the IF statement in the code and
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generate a single stream of instructions. The Loop Vectorizer supports any
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control flow in the innermost loop. The innermost loop may contain complex
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nesting of IFs, ELSEs and even GOTOs.
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.. code-block:: c++
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int foo(int *A, int *B, int n) {
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unsigned sum = 0;
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for (int i = 0; i < n; ++i)
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if (A[i] > B[i])
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sum += A[i] + 5;
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return sum;
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}
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Pointer Induction Variables
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This example uses the "accumulate" function of the standard c++ library. This
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loop uses C++ iterators, which are pointers, and not integer indices.
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The Loop Vectorizer detects pointer induction variables and can vectorize
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this loop. This feature is important because many C++ programs use iterators.
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.. code-block:: c++
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int baz(int *A, int n) {
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return std::accumulate(A, A + n, 0);
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}
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Reverse Iterators
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^^^^^^^^^^^^^^^^^
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The Loop Vectorizer can vectorize loops that count backwards.
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.. code-block:: c++
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int foo(int *A, int *B, int n) {
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for (int i = n; i > 0; --i)
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A[i] +=1;
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}
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Scatter / Gather
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^^^^^^^^^^^^^^^^
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The Loop Vectorizer can vectorize code that becomes a sequence of scalar instructions
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that scatter/gathers memory.
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.. code-block:: c++
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int foo(int * A, int * B, int n) {
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for (intptr_t i = 0; i < n; ++i)
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A[i] += B[i * 4];
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}
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In many situations the cost model will inform LLVM that this is not beneficial
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and LLVM will only vectorize such code if forced with "-mllvm -force-vector-width=#".
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Vectorization of Mixed Types
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The Loop Vectorizer can vectorize programs with mixed types. The Vectorizer
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cost model can estimate the cost of the type conversion and decide if
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vectorization is profitable.
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.. code-block:: c++
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int foo(int *A, char *B, int n, int k) {
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for (int i = 0; i < n; ++i)
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A[i] += 4 * B[i];
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}
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Global Structures Alias Analysis
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Access to global structures can also be vectorized, with alias analysis being
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used to make sure accesses don't alias. Run-time checks can also be added on
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pointer access to structure members.
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Many variations are supported, but some that rely on undefined behaviour being
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ignored (as other compilers do) are still being left un-vectorized.
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.. code-block:: c++
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struct { int A[100], K, B[100]; } Foo;
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int foo() {
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for (int i = 0; i < 100; ++i)
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Foo.A[i] = Foo.B[i] + 100;
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}
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Vectorization of function calls
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The Loop Vectorize can vectorize intrinsic math functions.
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See the table below for a list of these functions.
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+-----+-----+---------+
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| pow | exp | exp2 |
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+-----+-----+---------+
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| sin | cos | sqrt |
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+-----+-----+---------+
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| log |log2 | log10 |
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+-----+-----+---------+
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|fabs |floor| ceil |
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+-----+-----+---------+
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|fma |trunc|nearbyint|
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+-----+-----+---------+
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| | | fmuladd |
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+-----+-----+---------+
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The loop vectorizer knows about special instructions on the target and will
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vectorize a loop containing a function call that maps to the instructions. For
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example, the loop below will be vectorized on Intel x86 if the SSE4.1 roundps
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instruction is available.
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.. code-block:: c++
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void foo(float *f) {
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for (int i = 0; i != 1024; ++i)
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f[i] = floorf(f[i]);
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}
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Partial unrolling during vectorization
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Modern processors feature multiple execution units, and only programs that contain a
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high degree of parallelism can fully utilize the entire width of the machine.
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The Loop Vectorizer increases the instruction level parallelism (ILP) by
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performing partial-unrolling of loops.
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In the example below the entire array is accumulated into the variable 'sum'.
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This is inefficient because only a single execution port can be used by the processor.
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By unrolling the code the Loop Vectorizer allows two or more execution ports
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to be used simultaneously.
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.. code-block:: c++
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int foo(int *A, int *B, int n) {
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unsigned sum = 0;
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for (int i = 0; i < n; ++i)
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sum += A[i];
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return sum;
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}
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The Loop Vectorizer uses a cost model to decide when it is profitable to unroll loops.
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The decision to unroll the loop depends on the register pressure and the generated code size.
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Performance
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-----------
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This section shows the the execution time of Clang on a simple benchmark:
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`gcc-loops <http://llvm.org/viewvc/llvm-project/test-suite/trunk/SingleSource/UnitTests/Vectorizer/>`_.
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This benchmarks is a collection of loops from the GCC autovectorization
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`page <http://gcc.gnu.org/projects/tree-ssa/vectorization.html>`_ by Dorit Nuzman.
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The chart below compares GCC-4.7, ICC-13, and Clang-SVN with and without loop vectorization at -O3, tuned for "corei7-avx", running on a Sandybridge iMac.
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The Y-axis shows the time in msec. Lower is better. The last column shows the geomean of all the kernels.
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.. image:: gcc-loops.png
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And Linpack-pc with the same configuration. Result is Mflops, higher is better.
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.. image:: linpack-pc.png
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.. _slp-vectorizer:
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The SLP Vectorizer
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==================
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Details
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-------
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The goal of SLP vectorization (a.k.a. superword-level parallelism) is
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to combine similar independent instructions
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into vector instructions. Memory accesses, arithmetic operations, comparison
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operations, PHI-nodes, can all be vectorized using this technique.
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For example, the following function performs very similar operations on its
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inputs (a1, b1) and (a2, b2). The basic-block vectorizer may combine these
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into vector operations.
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.. code-block:: c++
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void foo(int a1, int a2, int b1, int b2, int *A) {
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A[0] = a1*(a1 + b1)/b1 + 50*b1/a1;
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A[1] = a2*(a2 + b2)/b2 + 50*b2/a2;
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}
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The SLP-vectorizer processes the code bottom-up, across basic blocks, in search of scalars to combine.
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Usage
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------
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The SLP Vectorizer is enabled by default, but it can be disabled
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through clang using the command line flag:
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.. code-block:: console
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$ clang -fno-slp-vectorize file.c
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LLVM has a second basic block vectorization phase
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which is more compile-time intensive (The BB vectorizer). This optimization
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can be enabled through clang using the command line flag:
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.. code-block:: console
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$ clang -fslp-vectorize-aggressive file.c
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