New Loop Versioning LICM Pass

Summary:
When alias analysis is uncertain about the aliasing between any two accesses,
it will return MayAlias. This uncertainty from alias analysis restricts LICM
from proceeding further. In cases where alias analysis is uncertain we might
use loop versioning as an alternative.

Loop Versioning will create a version of the loop with aggressive aliasing
assumptions in addition to the original with conservative (default) aliasing
assumptions. The version of the loop making aggressive aliasing assumptions
will have all the memory accesses marked as no-alias. These two versions of
loop will be preceded by a memory runtime check. This runtime check consists
of bound checks for all unique memory accessed in loop, and it ensures the
lack of memory aliasing. The result of the runtime check determines which of
the loop versions is executed: If the runtime check detects any memory
aliasing, then the original loop is executed. Otherwise, the version with
aggressive aliasing assumptions is used.

The pass is off by default and can be enabled with command line option 
-enable-loop-versioning-licm.

Reviewers: hfinkel, anemet, chatur01, reames

Subscribers: MatzeB, grosser, joker.eph, sanjoy, javed.absar, sbaranga,
             llvm-commits

Differential Revision: http://reviews.llvm.org/D9151



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@259986 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 11 insertions, 0 deletions

diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 1932cb6d843..5f745d2b06f 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -4554,6 +4554,17 @@ For example:
 
    !0 = !{!"llvm.loop.unroll.full"}
 
+'``llvm.loop.licm_versioning.disable``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This metadata indicates that the loop should not be versioned for the purpose
+of enabling loop-invariant code motion (LICM). The metadata has a single operand
+which is the string ``llvm.loop.licm_versioning.disable``. For example:
+
+.. code-block:: llvm
+
+   !0 = !{!"llvm.loop.licm_versioning.disable"}
+
 '``llvm.mem``'
 ^^^^^^^^^^^^^^^