[lit] Delete the now-unused SyntaxCheckTest format.

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

diff --git a/docs/Vectorizers.rst b/docs/Vectorizers.rst
index 5ec3da43ac7a660950f710810fca7cfb877ece8c..07486347e374ece62f1c2e612ba51f8d50bf4b3c 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.
 
 
 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
 --------
 
 Features
 --------
 


@@ -99,6 +120,8 @@ reduction operations, such as addition, multiplication, XOR, AND and OR.
     return sum;
   }
 
     return sum;
   }
 

+We support floating point reduction operations when `-ffast-math` is used.
+

 Inductions
 ^^^^^^^^^^
 
 Inductions
 ^^^^^^^^^^
 


@@ -159,8 +182,8 @@ The Loop Vectorizer can vectorize loops that count backwards.
 Scatter / Gather
 ^^^^^^^^^^^^^^^^
 
 Scatter / Gather
 ^^^^^^^^^^^^^^^^
 

-The Loop Vectorizer can vectorize code that becomes scatter/gather
-memory accesses.
+The Loop Vectorizer can vectorize code that becomes a sequence of scalar instructions 
+that scatter/gathers memory.

 
 .. code-block:: c++
 
 
 .. code-block:: c++
 


@@ -200,6 +223,34 @@ See the table below for a list of these functions.
 +-----+-----+---------+
 |fma  |trunc|nearbyint|
 +-----+-----+---------+
 +-----+-----+---------+
 |fma  |trunc|nearbyint|
 +-----+-----+---------+

+|     |     | fmuladd |
++-----+-----+---------+
+
+
+Partial unrolling during vectorization
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Modern processors feature multiple execution units, and only programs that contain a
+high degree of parallelism can fully utilize the entire width of the machine. 
+The Loop Vectorizer increases the instruction level parallelism (ILP) by 
+performing partial-unrolling of loops.
+
+In the example below the entire array is accumulated into the variable 'sum'.
+This is inefficient because only a single execution port can be used by the processor.
+By unrolling the code the Loop Vectorizer allows two or more execution ports
+to be used simultaneously.
+
+.. code-block:: c++
+
+  int foo(int *A, int *B, int n) {
+    unsigned sum = 0;
+    for (int i = 0; i < n; ++i)
+        sum += A[i];
+    return sum;
+  }
+
+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
 -----------
 
 Performance
 -----------


@@ -213,7 +264,10 @@ The chart below compares GCC-4.7, ICC-13, and Clang-SVN with and without loop ve
 The Y-axis shows the time in msec. Lower is better. The last column shows the geomean of all the kernels.
 
 .. image:: gcc-loops.png
 The Y-axis shows the time in msec. Lower is better. The last column shows the geomean of all the kernels.
 
 .. image:: gcc-loops.png

-   :width: 100%
+
+And Linpack-pc with the same configuration. Result is Mflops, higher is better.
+
+.. image:: linpack-pc.png

 
 .. _bb-vectorizer:
 
 
 .. _bb-vectorizer: