[PM] Port LoopSink to the new pass manager.

Like several other loop passes (the vectorizer, etc) this pass doesn't
really fit the model of a loop pass. The critical distinction is that it
isn't intended to be pipelined together with other loop passes. I plan
to add some documentation to the loop pass manager to make this more
clear on that side.

LoopSink is also different because it doesn't really need a lot of the
infrastructure of our loop passes. For example, if there aren't loop
invariant instructions causing a preheader to exist, there is no need to
form a preheader. It also doesn't need LCSSA because this pass is
only involved in sinking invariant instructions from a preheader into
the loop, not reasoning about live-outs.

This allows some nice simplifications to the pass in the new PM where we
can directly walk the loops once without restructuring them.

Differential Revision: https://reviews.llvm.org/D28921

llvm-svn: 292589

llvm/include/llvm/Transforms/Scalar/LoopSink.h

@@ -0,0 +1,40 @@
+//===- LoopSink.h - Loop Sink Pass ------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides the interface for the Loop Sink pass.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_SCALAR_LOOPSINK_H
+#define LLVM_TRANSFORMS_SCALAR_LOOPSINK_H
+
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/IR/PassManager.h"
+#include "llvm/Transforms/Scalar/LoopPassManager.h"
+
+namespace llvm {
+
+/// A pass that does profile-guided sinking of instructions into loops.
+///
+/// This is a function pass as it shouldn't be composed into any kind of
+/// unified loop pass pipeline. The goal of it is to sink code into loops that
+/// is loop invariant but only required within the loop body when doing so
+/// reduces the global expected dynamic frequency with which it executes.
+/// A classic example is an extremely cold branch within a loop body.
+///
+/// We do this as a separate pass so that during normal optimization all
+/// invariant operations can be held outside the loop body to simplify
+/// fundamental analyses and transforms of the loop.
+class LoopSinkPass : public PassInfoMixin<LoopSinkPass> {
+public:
+  PreservedAnalyses run(Function &F, FunctionAnalysisManager &FAM);
+};
+}
+
+#endif // LLVM_TRANSFORMS_SCALAR_LOOPSINK_H

llvm/lib/Passes/PassBuilder.cpp

@@ -107,6 +107,7 @@
 #include "llvm/Transforms/Scalar/LoopPassManager.h"
 #include "llvm/Transforms/Scalar/LoopRotation.h"
 #include "llvm/Transforms/Scalar/LoopSimplifyCFG.h"
+#include "llvm/Transforms/Scalar/LoopSink.h"
 #include "llvm/Transforms/Scalar/LoopStrengthReduce.h"
 #include "llvm/Transforms/Scalar/LoopUnrollPass.h"
 #include "llvm/Transforms/Scalar/LowerAtomic.h"

llvm/lib/Passes/PassRegistry.def

@@ -159,6 +159,7 @@ FUNCTION_PASS("lower-guard-intrinsic", LowerGuardIntrinsicPass())
 FUNCTION_PASS("guard-widening", GuardWideningPass())
 FUNCTION_PASS("gvn", GVN())
 FUNCTION_PASS("loop-simplify", LoopSimplifyPass())
+FUNCTION_PASS("loop-sink", LoopSinkPass())
 FUNCTION_PASS("lowerinvoke", LowerInvokePass())
 FUNCTION_PASS("mem2reg", PromotePass())
 FUNCTION_PASS("memcpyopt", MemCpyOptPass())

llvm/lib/Transforms/Scalar/LoopSink.cpp

@@ -31,6 +31,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/Transforms/Scalar/LoopSink.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/AliasSetTracker.h"
@@ -298,6 +299,42 @@ static bool sinkLoopInvariantInstructions(Loop &L, AAResults &AA, LoopInfo &LI,
   return Changed;
 }
 
+PreservedAnalyses LoopSinkPass::run(Function &F, FunctionAnalysisManager &FAM) {
+  LoopInfo &LI = FAM.getResult<LoopAnalysis>(F);
+  // Nothing to do if there are no loops.
+  if (LI.empty())
+    return PreservedAnalyses::all();
+
+  AAResults &AA = FAM.getResult<AAManager>(F);
+  DominatorTree &DT = FAM.getResult<DominatorTreeAnalysis>(F);
+  BlockFrequencyInfo &BFI = FAM.getResult<BlockFrequencyAnalysis>(F);
+
+  // We want to do a postorder walk over the loops. Since loops are a tree this
+  // is equivalent to a reversed preorder walk and preorder is easy to compute
+  // without recursion. Since we reverse the preorder, we will visit siblings
+  // in reverse program order. This isn't expected to matter at all but is more
+  // consistent with sinking algorithms which generally work bottom-up.
+  SmallVector<Loop *, 4> PreorderLoops = LI.getLoopsInPreorder();
+
+  bool Changed = false;
+  do {
+    Loop &L = *PreorderLoops.pop_back_val();
+
+    // Note that we don't pass SCEV here because it is only used to invalidate
+    // loops in SCEV and we don't preserve (or request) SCEV at all making that
+    // unnecessary.
+    Changed |= sinkLoopInvariantInstructions(L, AA, LI, DT, BFI,
+                                             /*ScalarEvolution*/ nullptr);
+  } while (!PreorderLoops.empty());
+
+  if (!Changed)
+    return PreservedAnalyses::all();
+
+  PreservedAnalyses PA;
+  PA.preserveSet<CFGAnalyses>();
+  return PA;
+}
+
 namespace {
 struct LegacyLoopSinkPass : public LoopPass {
   static char ID;

llvm/test/Transforms/LICM/loopsink.ll

@@ -1,4 +1,5 @@
 ; RUN: opt -S -loop-sink < %s | FileCheck %s
+; RUN: opt -S -passes=loop-sink < %s | FileCheck %s
 
 @g = global i32 0, align 4
 

llvm/test/Transforms/LICM/sink.ll

@@ -1,5 +1,7 @@
 ; RUN: opt -S -licm < %s | FileCheck %s --check-prefix=CHECK-LICM
 ; RUN: opt -S -licm < %s | opt -S -loop-sink | FileCheck %s --check-prefix=CHECK-SINK
+; RUN: opt -S < %s -passes='require<opt-remark-emit>,loop(licm),loop-sink' \
+; RUN:     | FileCheck %s --check-prefix=CHECK-SINK
 
 ; Original source code:
 ; int g;