From: Arnold Schwaighofer <aschwaighofer@apple.com>
Date: Wed, 12 Mar 2014 23:23:44 +0000 (+0000)
Subject: Fix vectorizer docs.
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=6a7d263e8b86d13efa04a7fc64ec37103d03d73e;p=oota-llvm.git

Fix vectorizer docs.

This example is not vectorized because LLVM does not prove no-wrapping of
"a[i*7] += ...".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203734 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/docs/Vectorizers.rst b/docs/Vectorizers.rst
index 61ebca2bb52..823fd9e6702 100644
--- a/docs/Vectorizers.rst
+++ b/docs/Vectorizers.rst
@@ -182,11 +182,14 @@ that scatter/gathers memory.
 
 .. code-block:: c++
 
-  int foo(int *A, int *B, int n, int k) {
-    for (int i = 0; i < n; ++i)
-      A[i*7] += B[i*k];
+  int foo(int * A, int * B, int n) {
+    for (intptr_t i = 0; i < n; ++i)
+        A[i] += B[i*4];
   }
 
+In many situations the cost model will inform LLVM that this is not beneficial
+and LLVM will only vectorize such code if forced with "-mllvm -force-vector-width=#".
+
 Vectorization of Mixed Types
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^