[CodeMoverUtils] Added an API to check if an instruction can be safely

moved before another instruction.
Summary:Added an API to check if an instruction can be safely moved
before another instruction. In future PRs, we will like to add support
of moving instructions between blocks that are not control flow
equivalent, and add other APIs to enhance usability, e.g. moving basic
blocks, moving list of instructions...
Loop Fusion will be its first user. When there is intervening code in
between two loops, fusion is currently unable to fuse them. Loop Fusion
can use this utility to check if the intervening code can be safely
moved before or after the two loops, and move them, then it can
successfully fuse them.
Reviewer:kbarton,jdoerfert,Meinersbur,bmahjour,etiotto
Reviewed By:bmahjour
Subscribers:mgorny,hiraditya,llvm-commits
Tag:LLVM
Differential Revision:https://reviews.llvm.org/D70049

llvm/include/llvm/Analysis/PostDominators.h

@@ -34,6 +34,13 @@ public:
   /// Handle invalidation explicitly.
   bool invalidate(Function &F, const PreservedAnalyses &PA,
                   FunctionAnalysisManager::Invalidator &);
+
+  // Ensure base-class overloads are visible.
+  using Base::dominates;
+
+  /// Return true if \p I1 dominates \p I2. This checks if \p I2 comes before
+  /// \p I1 if they belongs to the same basic block.
+  bool dominates(const Instruction *I1, const Instruction *I2) const;
 };
 
 /// Analysis pass which computes a \c PostDominatorTree.

llvm/include/llvm/Transforms/Utils/CodeMoverUtils.h

@@ -0,0 +1,40 @@
+//===- Transform/Utils/CodeMoverUtils.h - CodeMover Utils -------*- 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This family of functions determine movements are safe on basic blocks, and
+// instructions contained within a function.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_UTILS_CODEMOVERUTILS_H
+#define LLVM_TRANSFORMS_UTILS_CODEMOVERUTILS_H
+
+namespace llvm {
+
+class DependenceInfo;
+class DominatorTree;
+class Instruction;
+class PostDominatorTree;
+
+/// Return true if \p I0 and \p I1 are control flow equivalent.
+/// Two instructions are control flow equivalent if when one executes,
+/// the other is guaranteed to execute. This is determined using dominators
+/// and post-dominators: if A dominates B and B post-dominates A then A and B
+/// are control-flow equivalent.
+bool isControlFlowEquivalent(const Instruction &I0, const Instruction &I1,
+                             const DominatorTree &DT,
+                             const PostDominatorTree &PDT);
+
+/// Return true if \p I can be safely moved before \p InsertPoint.
+bool isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
+                        const DominatorTree &DT, const PostDominatorTree &PDT,
+                        DependenceInfo &DI);
+
+} // end namespace llvm
+
+#endif // LLVM_TRANSFORMS_UTILS_CODEMOVERUTILS_H

llvm/lib/Analysis/PostDominators.cpp

@@ -12,6 +12,7 @@
 
 #include "llvm/Analysis/PostDominators.h"
 #include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Pass.h"
@@ -50,6 +51,28 @@ bool PostDominatorTree::invalidate(Function &F, const PreservedAnalyses &PA,
            PAC.preservedSet<CFGAnalyses>());
 }
 
+bool PostDominatorTree::dominates(const Instruction *I1,
+                                  const Instruction *I2) const {
+  assert(I1 && I2 && "Expecting valid I1 and I2");
+
+  const BasicBlock *BB1 = I1->getParent();
+  const BasicBlock *BB2 = I2->getParent();
+
+  if (BB1 != BB2)
+    return Base::dominates(BB1, BB2);
+
+  // PHINodes in a block are unordered.
+  if (isa<PHINode>(I1) && isa<PHINode>(I2))
+    return false;
+
+  // Loop through the basic block until we find I1 or I2.
+  BasicBlock::const_iterator I = BB1->begin();
+  for (; &*I != I1 && &*I != I2; ++I)
+    /*empty*/;
+
+  return &*I == I2;
+}
+
 bool PostDominatorTreeWrapperPass::runOnFunction(Function &F) {
   DT.recalculate(F);
   return false;

llvm/lib/Transforms/Utils/CMakeLists.txt

@@ -10,6 +10,7 @@ add_llvm_component_library(LLVMTransformUtils
   CloneFunction.cpp
   CloneModule.cpp
   CodeExtractor.cpp
+  CodeMoverUtils.cpp
   CtorUtils.cpp
   Debugify.cpp
   DemoteRegToStack.cpp

llvm/lib/Transforms/Utils/CodeMoverUtils.cpp

@@ -0,0 +1,168 @@
+//===- CodeMoverUtils.cpp - CodeMover Utilities ----------------------------==//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This family of functions perform movements on basic blocks, and instructions
+// contained within a function.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/CodeMoverUtils.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/DependenceAnalysis.h"
+#include "llvm/Analysis/PostDominators.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/Dominators.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "codemover-utils"
+
+STATISTIC(HasDependences,
+          "Cannot move across instructions that has memory dependences");
+STATISTIC(MayThrowException, "Cannot move across instructions that may throw");
+STATISTIC(NotControlFlowEquivalent,
+          "Instructions are not control flow equivalent");
+STATISTIC(NotMovedPHINode, "Movement of PHINodes are not supported");
+STATISTIC(NotMovedTerminator, "Movement of Terminator are not supported");
+
+bool llvm::isControlFlowEquivalent(const Instruction &I0, const Instruction &I1,
+                                   const DominatorTree &DT,
+                                   const PostDominatorTree &PDT) {
+  const BasicBlock *BB0 = I0.getParent();
+  const BasicBlock *BB1 = I1.getParent();
+  return ((DT.dominates(BB0, BB1) && PDT.dominates(BB1, BB0)) ||
+          (PDT.dominates(BB0, BB1) && DT.dominates(BB1, BB0)));
+}
+
+static bool reportInvalidCandidate(const Instruction &I,
+                                   llvm::Statistic &Stat) {
+  ++Stat;
+  LLVM_DEBUG(dbgs() << "Unable to move instruction: " << I << ". "
+                    << Stat.getDesc());
+  return false;
+}
+
+/// Collect all instructions in between \p StartInst and \p EndInst, and store
+/// them in \p InBetweenInsts.
+static void
+collectInstructionsInBetween(Instruction &StartInst, const Instruction &EndInst,
+                             SmallPtrSetImpl<Instruction *> &InBetweenInsts) {
+  assert(InBetweenInsts.empty() && "Expecting InBetweenInsts to be empty");
+
+  /// Get the next instructions of \p I, and push them to \p WorkList.
+  auto getNextInsts = [](Instruction &I,
+                         SmallPtrSetImpl<Instruction *> &WorkList) {
+    if (Instruction *NextInst = I.getNextNode())
+      WorkList.insert(NextInst);
+    else {
+      assert(I.isTerminator() && "Expecting a terminator instruction");
+      for (BasicBlock *Succ : successors(&I))
+        WorkList.insert(&Succ->front());
+    }
+  };
+
+  SmallPtrSet<Instruction *, 10> WorkList;
+  getNextInsts(StartInst, WorkList);
+  while (!WorkList.empty()) {
+    Instruction *CurInst = *WorkList.begin();
+    WorkList.erase(CurInst);
+
+    if (CurInst == &EndInst)
+      continue;
+
+    if (!InBetweenInsts.insert(CurInst).second)
+      continue;
+
+    getNextInsts(*CurInst, WorkList);
+  }
+}
+
+bool llvm::isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
+                              const DominatorTree &DT,
+                              const PostDominatorTree &PDT,
+                              DependenceInfo &DI) {
+  // Cannot move itself before itself.
+  if (&I == &InsertPoint)
+    return false;
+
+  // Not moved.
+  if (I.getNextNode() == &InsertPoint)
+    return true;
+
+  if (isa<PHINode>(I) || isa<PHINode>(InsertPoint))
+    return reportInvalidCandidate(I, NotMovedPHINode);
+
+  if (I.isTerminator())
+    return reportInvalidCandidate(I, NotMovedTerminator);
+
+  // TODO remove this limitation.
+  if (!isControlFlowEquivalent(I, InsertPoint, DT, PDT))
+    return reportInvalidCandidate(I, NotControlFlowEquivalent);
+
+  // As I and InsertPoint are control flow equivalent, if I dominates
+  // InsertPoint, then I comes before InsertPoint.
+  const bool MoveForward = DT.dominates(&I, &InsertPoint);
+  if (MoveForward) {
+    // When I is being moved forward, we need to make sure the InsertPoint
+    // dominates every users. Or else, a user may be using an undefined I.
+    for (const Value *User : I.users())
+      if (auto *UserInst = dyn_cast<Instruction>(User))
+        if (!DT.dominates(&InsertPoint, UserInst))
+          return false;
+  } else {
+    // When I is being moved backward, we need to make sure all its opernads
+    // dominates the InsertPoint. Or else, an operand may be undefined for I.
+    for (const Value *Op : I.operands())
+      if (auto *OpInst = dyn_cast<Instruction>(Op))
+        if (&InsertPoint == OpInst || !DT.dominates(OpInst, &InsertPoint))
+          return false;
+  }
+
+  Instruction &StartInst = (MoveForward ? I : InsertPoint);
+  Instruction &EndInst = (MoveForward ? InsertPoint : I);
+  SmallPtrSet<Instruction *, 10> InstsToCheck;
+  collectInstructionsInBetween(StartInst, EndInst, InstsToCheck);
+  if (!MoveForward)
+    InstsToCheck.insert(&InsertPoint);
+
+  // Check if there exists instructions which may throw, may synchonize, or may
+  // never return, from I to InsertPoint.
+  if (!isSafeToSpeculativelyExecute(&I))
+    if (std::any_of(InstsToCheck.begin(), InstsToCheck.end(),
+                    [](Instruction *I) {
+                      if (I->mayThrow())
+                        return true;
+
+                      const CallBase *CB = dyn_cast<CallBase>(I);
+                      if (!CB)
+                        return false;
+                      if (!CB->hasFnAttr(Attribute::WillReturn))
+                        return true;
+                      if (!CB->hasFnAttr(Attribute::NoSync))
+                        return true;
+
+                      return false;
+                    })) {
+      return reportInvalidCandidate(I, MayThrowException);
+    }
+
+  // Check if I has any output/flow/anti dependences with instructions from \p
+  // StartInst to \p EndInst.
+  if (std::any_of(InstsToCheck.begin(), InstsToCheck.end(),
+                  [&DI, &I](Instruction *CurInst) {
+                    auto DepResult = DI.depends(&I, CurInst, true);
+                    if (DepResult &&
+                        (DepResult->isOutput() || DepResult->isFlow() ||
+                         DepResult->isAnti()))
+                      return true;
+                    return false;
+                  }))
+    return reportInvalidCandidate(I, HasDependences);
+
+  return true;
+}

llvm/unittests/Transforms/Utils/CMakeLists.txt

@@ -11,6 +11,7 @@ add_llvm_unittest(UtilsTests
   BasicBlockUtilsTest.cpp
   CloningTest.cpp
   CodeExtractorTest.cpp
+  CodeMoverUtilsTest.cpp
   FunctionComparatorTest.cpp
   IntegerDivisionTest.cpp
   LocalTest.cpp

llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp

@@ -0,0 +1,172 @@
+//===- CodeMoverUtils.cpp - Unit tests for CodeMoverUtils ---------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/CodeMoverUtils.h"
+#include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/DependenceAnalysis.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/PostDominators.h"
+#include "llvm/Analysis/ScalarEvolutionExpander.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/Support/SourceMgr.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+static std::unique_ptr<Module> parseIR(LLVMContext &C, const char *IR) {
+  SMDiagnostic Err;
+  std::unique_ptr<Module> Mod = parseAssemblyString(IR, Err, C);
+  if (!Mod)
+    Err.print("CodeMoverUtilsTests", errs());
+  return Mod;
+}
+
+static void run(Module &M, StringRef FuncName,
+                function_ref<void(Function &F, DominatorTree &DT,
+                                  PostDominatorTree &PDT, DependenceInfo &DI)>
+                    Test) {
+  auto *F = M.getFunction(FuncName);
+  DominatorTree DT(*F);
+  PostDominatorTree PDT(*F);
+  TargetLibraryInfoImpl TLII;
+  TargetLibraryInfo TLI(TLII);
+  AssumptionCache AC(*F);
+  AliasAnalysis AA(TLI);
+  LoopInfo LI(DT);
+  ScalarEvolution SE(*F, TLI, AC, DT, LI);
+  DependenceInfo DI(F, &AA, &SE, &LI);
+  Test(*F, DT, PDT, DI);
+}
+
+TEST(CodeMoverUtils, BasicTest) {
+  LLVMContext C;
+
+  // void safecall() noexcept willreturn nosync;
+  // void unsafecall();
+  // void foo(int * __restrict__ A, int * __restrict__ B, int * __restrict__ C,
+  //          long N) {
+  //   X = N / 1;
+  //   safecall();
+  //   unsafecall1();
+  //   unsafecall2();
+  //   for (long i = 0; i < N; ++i) {
+  //     A[5] = 5;
+  //     A[i] = 0;
+  //     B[i] = A[i];
+  //     C[i] = A[i];
+  //     A[6] = 6;
+  //   }
+  // }
+  std::unique_ptr<Module> M = parseIR(
+      C,
+      "define void @foo(i32* noalias %A, i32* noalias %B, i32* noalias %C\n"
+      "                  , i64 %N) {\n"
+      "entry:\n"
+      "  %X = sdiv i64 1, %N\n"
+      "  call void @safecall()\n"
+      "  %cmp1 = icmp slt i64 0, %N\n"
+      "  call void @unsafecall1()\n"
+      "  call void @unsafecall2()\n"
+      "  br i1 %cmp1, label %for.body, label %for.end\n"
+      "for.body:\n"
+      "  %i = phi i64 [ 0, %entry ], [ %inc, %for.body ]\n"
+      "  %arrayidx_A5 = getelementptr inbounds i32, i32* %A, i64 5\n"
+      "  store i32 5, i32* %arrayidx_A5, align 4\n"
+      "  %arrayidx_A = getelementptr inbounds i32, i32* %A, i64 %i\n"
+      "  store i32 0, i32* %arrayidx_A, align 4\n"
+      "  %load1 = load i32, i32* %arrayidx_A, align 4\n"
+      "  %arrayidx_B = getelementptr inbounds i32, i32* %B, i64 %i\n"
+      "  store i32 %load1, i32* %arrayidx_B, align 4\n"
+      "  %load2 = load i32, i32* %arrayidx_A, align 4\n"
+      "  %arrayidx_C = getelementptr inbounds i32, i32* %C, i64 %i\n"
+      "  store i32 %load2, i32* %arrayidx_C, align 4\n"
+      "  %arrayidx_A6 = getelementptr inbounds i32, i32* %A, i64 6\n"
+      "  store i32 6, i32* %arrayidx_A6, align 4\n"
+      "  %inc = add nsw i64 %i, 1\n"
+      "  %cmp = icmp slt i64 %inc, %N\n"
+      "  br i1 %cmp, label %for.body, label %for.end\n"
+      "for.end:\n"
+      "  ret void\n"
+      "}\n"
+      "declare void @safecall() nounwind nosync willreturn\n"
+      "declare void @unsafecall1()\n"
+      "declare void @unsafecall2()\n");
+
+  run(*M, "foo",
+      [&](Function &F, DominatorTree &DT, PostDominatorTree &PDT,
+          DependenceInfo &DI) {
+        Function::iterator FI = F.begin();
+        BasicBlock *Entry = &*(FI++);
+        assert(Entry->getName() == "entry" && "Expecting BasicBlock entry");
+        Instruction *CI_safecall = Entry->front().getNextNode();
+        assert(isa<CallInst>(CI_safecall) && "Expecting CI_safecall to be a CallInst");
+        Instruction *CI_unsafecall = CI_safecall->getNextNode()->getNextNode();
+        assert(isa<CallInst>(CI_unsafecall) && "Expecting CI_unsafecall to be a CallInst");
+        BasicBlock *ForBody = &*(FI++);
+        assert(ForBody->getName() == "for.body" &&
+               "Expecting BasicBlock for.body");
+        Instruction &PN = ForBody->front();
+        assert(isa<PHINode>(PN) && "Expecting PN to be a PHINode");
+        Instruction *SI_A5 = PN.getNextNode()->getNextNode();
+        assert(isa<StoreInst>(SI_A5) &&
+               SI_A5->getOperand(1)->getName() == "arrayidx_A5" &&
+               "Expecting store to arrayidx_A5");
+        Instruction *SI = SI_A5->getNextNode()->getNextNode();
+        assert(isa<StoreInst>(SI) &&
+               SI->getOperand(1)->getName() == "arrayidx_A" &&
+               "Expecting store to arrayidx_A");
+        Instruction *LI1 = SI->getNextNode();
+        assert(LI1->getName() == "load1" && "Expecting LI1 to be load1");
+        Instruction *LI2 = LI1->getNextNode()->getNextNode()->getNextNode();
+        assert(LI2->getName() == "load2" && "Expecting LI2 to be load2");
+        Instruction *SI_A6 = LI2->getNextNode()->getNextNode()->getNextNode()->getNextNode();
+        assert(isa<StoreInst>(SI_A6) &&
+               SI_A6->getOperand(1)->getName() == "arrayidx_A6" &&
+               "Expecting store to arrayidx_A6");
+
+        // Can move after CI_safecall, as it does not throw, not synchronize, or must return.
+        EXPECT_TRUE(isSafeToMoveBefore(*CI_safecall->getPrevNode(), *CI_safecall->getNextNode(), DT, PDT, DI));
+
+        // Cannot move CI_unsafecall, as it may throw.
+        EXPECT_FALSE(isSafeToMoveBefore(*CI_unsafecall->getNextNode(), *CI_unsafecall, DT, PDT, DI));
+
+        // Moving instruction to non control flow equivalent places are not
+        // supported.
+        EXPECT_FALSE(isSafeToMoveBefore(*SI_A5, *Entry->getTerminator(), DT, PDT, DI));
+
+        // Moving PHINode is not supported.
+        EXPECT_FALSE(isSafeToMoveBefore(PN, *PN.getPrevNode(), DT, PDT, DI));
+
+        // Cannot move non-PHINode before PHINode.
+        EXPECT_FALSE(isSafeToMoveBefore(*PN.getNextNode(), PN, DT, PDT, DI));
+
+        // Moving Terminator is not supported.
+        EXPECT_FALSE(isSafeToMoveBefore(*Entry->getTerminator(), *PN.getNextNode(), DT,
+                                PDT, DI));
+
+        // Cannot move %arrayidx_A after SI, as SI is its user.
+        EXPECT_FALSE(isSafeToMoveBefore(*SI->getPrevNode(), *SI->getNextNode(), DT, PDT, DI));
+
+        // Cannot move SI before %arrayidx_A, as %arrayidx_A is its operand.
+        EXPECT_FALSE(isSafeToMoveBefore(*SI, *SI->getPrevNode(), DT, PDT, DI));
+
+        // Cannot move LI2 after SI_A6, as there is a flow dependence.
+        EXPECT_FALSE(isSafeToMoveBefore(*LI2, *SI_A6->getNextNode(), DT, PDT, DI));
+
+        // Cannot move SI after LI1, as there is a anti dependence.
+        EXPECT_FALSE(isSafeToMoveBefore(*SI, *LI1->getNextNode(), DT, PDT, DI));
+
+        // Cannot move SI_A5 after SI, as there is a output dependence.
+        EXPECT_FALSE(isSafeToMoveBefore(*SI_A5, *LI1, DT, PDT, DI));
+
+        // Can move LI2 before LI1, as there is only an input dependence.
+        EXPECT_TRUE(isSafeToMoveBefore(*LI2, *LI1, DT, PDT, DI));
+      });
+}