[Docs] add note to avoid 'errno' for better vectorization (PR40265)

This is a partial fix for the documentation improvements requested in:
https://bugs.llvm.org/show_bug.cgi?id=40265

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350845 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/docs/Vectorizers.rst b/docs/Vectorizers.rst
index 42e8d02f337eead4f993cfd34c27f532291631fd..4a0158007414cb46b87bc808f992f642eb9ca64d 100644 (file)
--- a/docs/Vectorizers.rst
+++ b/docs/Vectorizers.rst
@@ -340,6 +340,10 @@ instruction is available.
       f[i] = floorf(f[i]);
   }
 
+Note that the optimizer may not be able to vectorize math library functions 
+that access external state such as "errno". To allow better optimization of 
+C/C++ math library functions, use "-fno-math-errno".
+
 Partial unrolling during vectorization
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^