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

llvm-svn: 259986

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 9d4fa68c67c..4798d4e88d8 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -116,6 +116,10 @@ static cl::opt<std::string> RunPGOInstrUse(
     cl::desc("Enable use phase of PGO instrumentation and specify the path "
              "of profile data file"));
 
+static cl::opt<bool> UseLoopVersioningLICM(
+    "enable-loop-versioning-licm", cl::init(false), cl::Hidden,
+    cl::desc("Enable the experimental Loop Versioning LICM pass"));
+
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
@@ -375,6 +379,16 @@ void PassManagerBuilder::populateModulePassManager(
   // we must insert a no-op module pass to reset the pass manager.
   MPM.add(createBarrierNoopPass());
 
+  // Scheduling LoopVersioningLICM when inining is over, because after that
+  // we may see more accurate aliasing. Reason to run this late is that too
+  // early versioning may prevent further inlining due to increase of code
+  // size. By placing it just after inlining other optimizations which runs 
+  // later might get benefit of no-alias assumption in clone loop. 
+  if (UseLoopVersioningLICM) {
+    MPM.add(createLoopVersioningLICMPass());    // Do LoopVersioningLICM
+    MPM.add(createLICMPass());                  // Hoist loop invariants
+  }
+
   if (!DisableUnitAtATime)
     MPM.add(createReversePostOrderFunctionAttrsPass());