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[oota-llvm.git] / docs / Vectorizers.rst

diff --git a/docs/Vectorizers.rst b/docs/Vectorizers.rst
index d679ca495404683dc05b0211dde46e4b54511e1d..e2d3667bc116bcbdfa108a4a209b572ca0fc3167 100644 (file)
--- a/docs/Vectorizers.rst
+++ b/docs/Vectorizers.rst
@@ -35,6 +35,27 @@ will only vectorize loops that do not require a major increase in code size.
 
 We plan to enable the Loop Vectorizer by default as part of the LLVM 3.3 release.
 
+Command line flags
+^^^^^^^^^^^^^^^^^^
+
+The loop vectorizer uses a cost model to decide on the optimal vectorization factor
+and unroll factor. However, users of the vectorizer can force the vectorizer to use
+specific values. Both 'clang' and 'opt' support the flags below.
+
+Users can control the vectorization SIMD width using the command line flag "-force-vector-width".
+
+.. code-block:: console
+
+  $ clang  -mllvm -force-vector-width=8 ...
+  $ opt -loop-vectorize -force-vector-width=8 ...
+
+Users can control the unroll factor using the command line flag "-force-vector-unroll"
+
+.. code-block:: console
+
+  $ clang  -mllvm -force-vector-unroll=2 ...
+  $ opt -loop-vectorize -force-vector-unroll=2 ...
+
 Features
 --------
 
@@ -99,6 +120,8 @@ reduction operations, such as addition, multiplication, XOR, AND and OR.
     return sum;
   }
 
+We support floating point reduction operations when `-ffast-math` is used.
+
 Inductions
 ^^^^^^^^^^
 
@@ -183,6 +206,25 @@ vectorization is profitable.
       A[i] += 4 * B[i];
   }
 
+Global Structures Alias Analysis
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Access to global structures can also be vectorized, with alias analysis being
+used to make sure accesses don't alias. Run-time checks can also be added on
+pointer access to structure members.
+
+Many variations are supported, but some that rely on undefined behaviour being
+ignored (as other compilers do) are still being left un-vectorized.
+
+.. code-block:: c++
+
+  struct { int A[100], K, B[100]; } Foo;
+
+  int foo() {
+    for (int i = 0; i < 100; ++i)
+      Foo.A[i] = Foo.B[i] + 100;
+  }
+
 Vectorization of function calls
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -203,6 +245,17 @@ See the table below for a list of these functions.
 |     |     | fmuladd |
 +-----+-----+---------+
 
+The loop vectorizer knows about special instructions on the target and will
+vectorize a loop containing a function call that maps to the instructions. For
+example, the loop below will be vectorized on Intel x86 if the SSE4.1 roundps
+instruction is available.
+
+.. code-block:: c++
+
+  void foo(float *f) {
+    for (int i = 0; i != 1024; ++i)
+      f[i] = floorf(f[i]);
+  }
 
 Partial unrolling during vectorization
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -226,13 +279,8 @@ to be used simultaneously.
     return sum;
   }
 
-At the moment the unrolling feature is not enabled by default and needs to be enabled
-in opt or clang using the following flag:
-
-.. code-block:: console
-
-  -force-vector-unroll=2 
-
+The Loop Vectorizer uses a cost model to decide when it is profitable to unroll loops.
+The decision to unroll the loop depends on the register pressure and the generated code size. 
 
 Performance
 -----------
@@ -247,6 +295,10 @@ The Y-axis shows the time in msec. Lower is better. The last column shows the ge
 
 .. image:: gcc-loops.png
 
+And Linpack-pc with the same configuration. Result is Mflops, higher is better.
+
+.. image:: linpack-pc.png
+
 .. _bb-vectorizer:
 
 The Basic Block Vectorizer