Optimize vector select from all 0s or all 1s

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

diff --git a/docs/Vectorizers.rst b/docs/Vectorizers.rst
index 0894b1eb3f10f3f3ac4bd26c9896937110945764..fa0f49c3ddfd675a31bc0ea12421c5fbc191711a 100644 (file)
--- a/docs/Vectorizers.rst
+++ b/docs/Vectorizers.rst
@@ -21,19 +21,12 @@ The Loop Vectorizer
 Usage
 -----
 
-LLVM's Loop Vectorizer is now available and will be useful for many people.
-It is not enabled by default, but can be enabled through clang using the
-command line flag:
+LLVM's Loop Vectorizer is now enabled by default for -O3.
+The vectorizer can be disabled using the command line:
 
 .. code-block:: console
 
-   $ clang -fvectorize -O3 file.c
-
-If the ``-fvectorize`` flag is used then the loop vectorizer will be enabled
-when running with ``-O3``, ``-O2``. When ``-Os`` is used, the loop vectorizer
-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.
+   $ clang ... -fno-vectorize  file.c
 
 Command line flags
 ^^^^^^^^^^^^^^^^^^
@@ -245,6 +238,17 @@ 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^