[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, 4 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 0dc44c660f1..16ce04b0e80 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -95,10 +95,6 @@ static cl::opt<bool> EnableLoopInterchange(
     "enable-loopinterchange", cl::init(false), cl::Hidden,
     cl::desc("Enable the new, experimental LoopInterchange Pass"));
 
-static cl::opt<bool> EnableLoopDistribute(
-    "enable-loop-distribute", cl::init(false), cl::Hidden,
-    cl::desc("Enable the new, experimental LoopDistribution Pass"));
-
 static cl::opt<bool> EnableNonLTOGlobalsModRef(
     "enable-non-lto-gmr", cl::init(true), cl::Hidden,
     cl::desc(
@@ -480,9 +476,10 @@ void PassManagerBuilder::populateModulePassManager(
   MPM.add(createLoopRotatePass(SizeLevel == 2 ? 0 : -1));
 
   // Distribute loops to allow partial vectorization.  I.e. isolate dependences
-  // into separate loop that would otherwise inhibit vectorization.
-  if (EnableLoopDistribute)
-    MPM.add(createLoopDistributePass());
+  // into separate loop that would otherwise inhibit vectorization.  This is
+  // currently only performed for loops marked with the metadata
+  // llvm.loop.distribute=true or when -enable-loop-distribute is specified.
+  MPM.add(createLoopDistributePass(/*ProcessAllLoopsByDefault=*/false));
 
   MPM.add(createLoopVectorizePass(DisableUnrollLoops, LoopVectorize));