[LoopDist] Add llvm.loop.distribute.enable loop metadata

Summary:
D19403 adds a new pragma for loop distribution.  This change adds
support for the corresponding metadata that the pragma is translated to
by the FE.

As part of this I had to rethink the flag -enable-loop-distribute.  My
goal was to be backward compatible with the existing behavior:

  A1. pass is off by default from the optimization pipeline
  unless -enable-loop-distribute is specified

  A2. pass is on when invoked directly from opt (e.g. for unit-testing)

The new pragma/metadata overrides these defaults so the new behavior is:

  B1. A1 + enable distribution for individual loop with the pragma/metadata

  B2. A2 + disable distribution for individual loop with the pragma/metadata

The default value whether the pass is on or off comes from the initiator
of the pass.  From the PassManagerBuilder the default is off, from opt
it's on.

I moved -enable-loop-distribute under the pass.  If the flag is
specified it overrides the default from above.

Then the pragma/metadata can further modifies this per loop.

As a side-effect, we can now also use -enable-loop-distribute=0 from opt
to emulate the default from the optimization pipeline.  So to be precise
this is the new behavior:

  C1. pass is off by default from the optimization pipeline
  unless -enable-loop-distribute or the pragma/metadata enables it

  C2. pass is on when invoked directly from opt
  unless -enable-loop-distribute=0 or the pragma/metadata disables it

Reviewers: hfinkel

Subscribers: joker.eph, mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D19431

llvm-svn: 267672

1 files changed, 21 insertions, 0 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 197439f5750..cd57c24e9ae 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -4711,6 +4711,27 @@ which is the string ``llvm.loop.licm_versioning.disable``. For example:
 
    !0 = !{!"llvm.loop.licm_versioning.disable"}
 
+'``llvm.loop.distribute.enable``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Loop distribution allows splitting a loop into multiple loops.  Currently,
+this is only performed if the entire loop cannot be vectorized due to unsafe
+memory dependencies.  The transformation will atempt to isolate the unsafe
+dependencies into their own loop.
+
+This metadata can be used to selectively enable or disable distribution of the
+loop.  The first operand is the string ``llvm.loop.distribute.enable`` and the
+second operand is a bit. If the bit operand value is 1 distribution is
+enabled. A value of 0 disables distribution:
+
+.. code-block:: llvm
+
+   !0 = !{!"llvm.loop.distribute.enable", i1 0}
+   !1 = !{!"llvm.loop.distribute.enable", i1 1}
+
+This metadata should be used in conjunction with ``llvm.loop`` loop
+identification metadata.
+
 '``llvm.mem``'
 ^^^^^^^^^^^^^^^