[UnJ] Document unroll and jam pass and loop metadata

Add some quick words for unroll and jam to the list of passes and add
unroll_and_jam metadata to the language ref.

Differential Revision: https://reviews.llvm.org/D49349

llvm-svn: 337448

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/docs/Passes.rst b/llvm/docs/Passes.rst
index 39874e4c8a0..9ab214984d2 100644
--- a/llvm/docs/Passes.rst
+++ b/llvm/docs/Passes.rst
@@ -83,6 +83,8 @@ Yet to be written.
 A pass which can be used to count how many alias queries are being made and how
 the alias analysis implementation being used responds.
 
+.. _passes-da:
+
 ``-da``: Dependence Analysis
 ----------------------------
 
@@ -825,6 +827,27 @@ This pass implements a simple loop unroller.  It works best when loops have
 been canonicalized by the :ref:`indvars <passes-indvars>` pass, allowing it to
 determine the trip counts of loops easily.
 
+``-loop-unroll-and-jam``: Unroll and Jam loops
+----------------------------------------------
+
+This pass implements a simple unroll and jam classical loop optimisation pass.
+It transforms loop from:
+
+.. code-block:: c++
+
+  for i.. i+= 1              for i.. i+= 4
+    for j..                    for j..
+      code(i, j)                 code(i, j)
+                                 code(i+1, j)
+                                 code(i+2, j)
+                                 code(i+3, j)
+                             remainder loop
+
+Which can be seen as unrolling the outer loop and "jamming" (fusing) the inner
+loops into one. When variables or loads can be shared in the new inner loop, this
+can lead to significant performance improvements. It uses
+:ref:`Dependence Analysis <passes-da>` for proving the transformations are safe.
+
 ``-loop-unswitch``: Unswitch loops
 ----------------------------------