Introduce runtime unrolling disable matadata and use it to mark the scalar loop from vectorization.

Runtime unrolling is an expensive optimization which can bring benefit
only if the loop is hot and iteration number is relatively large enough.
For some loops, we know they are not worth to be runtime unrolled.
The scalar loop from vectorization is one of the cases.

llvm-svn: 231631

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 218be344b6b..ff9b4bf06f0 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -3509,6 +3509,16 @@ which is the string ``llvm.loop.unroll.disable``.  For example:
 
    !0 = !{!"llvm.loop.unroll.disable"}
 
+'``llvm.loop.unroll.runtime.disable``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This metadata either disables runtime loop unrolling. The metadata has a single
+operand which is the string ``llvm.loop.unroll.runtime.disable``.  For example:
+
+.. code-block:: llvm
+
+   !0 = !{!"llvm.loop.unroll.runtime.disable"}
+
 '``llvm.loop.unroll.full``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^