Add basic loop fusion pass.

This patch adds a basic loop fusion pass. It will fuse loops that conform to the
following 4 conditions:
  1. Adjacent (no code between them)
  2. Control flow equivalent (if one loop executes, the other loop executes)
  3. Identical bounds (both loops iterate the same number of iterations)
  4. No negative distance dependencies between the loop bodies.

The pass does not make any changes to the IR to create opportunities for fusion.
Instead, it checks if the necessary conditions are met and if so it fuses two
loops together.

The pass has not been added to the pass pipeline yet, and thus is not enabled by
default. It can be run stand alone using the -loop-fusion option.

Phabricator: https://reviews.llvm.org/D55851
llvm-svn: 358543

Author: kitbarton

llvm/include/llvm/InitializePasses.h

@@ -219,6 +219,7 @@ void initializeLoopDeletionLegacyPassPass(PassRegistry&);
 void initializeLoopDistributeLegacyPass(PassRegistry&);
 void initializeLoopExtractorPass(PassRegistry&);
 void initializeLoopGuardWideningLegacyPassPass(PassRegistry&);
+void initializeLoopFuseLegacyPass(PassRegistry&);
 void initializeLoopIdiomRecognizeLegacyPassPass(PassRegistry&);
 void initializeLoopInfoWrapperPassPass(PassRegistry&);
 void initializeLoopInstSimplifyLegacyPassPass(PassRegistry&);

llvm/include/llvm/Transforms/Scalar.h

@@ -458,6 +458,12 @@ FunctionPass *createNaryReassociatePass();
 //
 FunctionPass *createLoopDistributePass();
 
+//===----------------------------------------------------------------------===//
+//
+// LoopFuse - Fuse loops.
+//
+FunctionPass *createLoopFusePass();
+
 //===----------------------------------------------------------------------===//
 //
 // LoopLoadElimination - Perform loop-aware load elimination.

llvm/include/llvm/Transforms/Scalar/LoopFuse.h

@@ -0,0 +1,30 @@
+//===- LoopFuse.h - Loop Fusion Pass ----------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file implements the Loop Fusion pass.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_SCALAR_LOOPFUSE_H
+#define LLVM_TRANSFORMS_SCALAR_LOOPFUSE_H
+
+#include "llvm/IR/PassManager.h"
+
+namespace llvm {
+
+class Function;
+
+class LoopFusePass : public PassInfoMixin<LoopFusePass> {
+public:
+  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+};
+
+} // end namespace llvm
+
+#endif // LLVM_TRANSFORMS_SCALAR_LOOPFUSE_H

llvm/lib/Passes/PassBuilder.cpp

@@ -122,6 +122,7 @@
 #include "llvm/Transforms/Scalar/LoopDataPrefetch.h"
 #include "llvm/Transforms/Scalar/LoopDeletion.h"
 #include "llvm/Transforms/Scalar/LoopDistribute.h"
+#include "llvm/Transforms/Scalar/LoopFuse.h"
 #include "llvm/Transforms/Scalar/LoopIdiomRecognize.h"
 #include "llvm/Transforms/Scalar/LoopInstSimplify.h"
 #include "llvm/Transforms/Scalar/LoopLoadElimination.h"

llvm/lib/Passes/PassRegistry.def

@@ -197,6 +197,7 @@ FUNCTION_PASS("partially-inline-libcalls", PartiallyInlineLibCallsPass())
 FUNCTION_PASS("lcssa", LCSSAPass())
 FUNCTION_PASS("loop-data-prefetch", LoopDataPrefetchPass())
 FUNCTION_PASS("loop-load-elim", LoopLoadEliminationPass())
+FUNCTION_PASS("loop-fuse", LoopFusePass())
 FUNCTION_PASS("loop-distribute", LoopDistributePass())
 FUNCTION_PASS("loop-vectorize", LoopVectorizePass())
 FUNCTION_PASS("pgo-memop-opt", PGOMemOPSizeOpt())

llvm/lib/Transforms/Scalar/CMakeLists.txt

@@ -28,6 +28,7 @@ add_llvm_library(LLVMScalarOpts
   LoopDeletion.cpp
   LoopDataPrefetch.cpp
   LoopDistribute.cpp
+  LoopFuse.cpp
   LoopIdiomRecognize.cpp
   LoopInstSimplify.cpp
   LoopInterchange.cpp