Imported Upstream version 6.0.0.172

Former-commit-id: f3cc9b82f3e5bd8f0fd3ebc098f789556b44e9cd

external/llvm/lib/Transforms/Scalar/Sink.cpp

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+//===-- Sink.cpp - Code Sinking -------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass moves instructions into successor blocks, when possible, so that
+// they aren't executed on paths where their results aren't needed.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar/Sink.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Scalar.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "sink"
+
+STATISTIC(NumSunk, "Number of instructions sunk");
+STATISTIC(NumSinkIter, "Number of sinking iterations");
+
+/// AllUsesDominatedByBlock - Return true if all uses of the specified value
+/// occur in blocks dominated by the specified block.
+static bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB,
+                                    DominatorTree &DT) {
+  // Ignoring debug uses is necessary so debug info doesn't affect the code.
+  // This may leave a referencing dbg_value in the original block, before
+  // the definition of the vreg.  Dwarf generator handles this although the
+  // user might not get the right info at runtime.
+  for (Use &U : Inst->uses()) {
+    // Determine the block of the use.
+    Instruction *UseInst = cast<Instruction>(U.getUser());
+    BasicBlock *UseBlock = UseInst->getParent();
+    if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
+      // PHI nodes use the operand in the predecessor block, not the block with
+      // the PHI.
+      unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo());
+      UseBlock = PN->getIncomingBlock(Num);
+    }
+    // Check that it dominates.
+    if (!DT.dominates(BB, UseBlock))
+      return false;
+  }
+  return true;
+}
+
+static bool isSafeToMove(Instruction *Inst, AliasAnalysis &AA,
+                         SmallPtrSetImpl<Instruction *> &Stores) {
+
+  if (Inst->mayWriteToMemory()) {
+    Stores.insert(Inst);
+    return false;
+  }
+
+  if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
+    MemoryLocation Loc = MemoryLocation::get(L);
+    for (Instruction *S : Stores)
+      if (isModSet(AA.getModRefInfo(S, Loc)))
+        return false;
+  }
+
+  if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst) || Inst->isEHPad() ||
+      Inst->mayThrow())
+    return false;
+
+  if (auto CS = CallSite(Inst)) {
+    // Convergent operations cannot be made control-dependent on additional
+    // values.
+    if (CS.hasFnAttr(Attribute::Convergent))
+      return false;
+
+    for (Instruction *S : Stores)
+      if (isModSet(AA.getModRefInfo(S, CS)))
+        return false;
+  }
+
+  return true;
+}
+
+/// IsAcceptableTarget - Return true if it is possible to sink the instruction
+/// in the specified basic block.
+static bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo,
+                               DominatorTree &DT, LoopInfo &LI) {
+  assert(Inst && "Instruction to be sunk is null");
+  assert(SuccToSinkTo && "Candidate sink target is null");
+
+  // It is not possible to sink an instruction into its own block.  This can
+  // happen with loops.
+  if (Inst->getParent() == SuccToSinkTo)
+    return false;
+
+  // It's never legal to sink an instruction into a block which terminates in an
+  // EH-pad.
+  if (SuccToSinkTo->getTerminator()->isExceptional())
+    return false;
+
+  // If the block has multiple predecessors, this would introduce computation
+  // on different code paths.  We could split the critical edge, but for now we
+  // just punt.
+  // FIXME: Split critical edges if not backedges.
+  if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
+    // We cannot sink a load across a critical edge - there may be stores in
+    // other code paths.
+    if (isa<LoadInst>(Inst))
+      return false;
+
+    // We don't want to sink across a critical edge if we don't dominate the
+    // successor. We could be introducing calculations to new code paths.
+    if (!DT.dominates(Inst->getParent(), SuccToSinkTo))
+      return false;
+
+    // Don't sink instructions into a loop.
+    Loop *succ = LI.getLoopFor(SuccToSinkTo);
+    Loop *cur = LI.getLoopFor(Inst->getParent());
+    if (succ != nullptr && succ != cur)
+      return false;
+  }
+
+  // Finally, check that all the uses of the instruction are actually
+  // dominated by the candidate
+  return AllUsesDominatedByBlock(Inst, SuccToSinkTo, DT);
+}
+
+/// SinkInstruction - Determine whether it is safe to sink the specified machine
+/// instruction out of its current block into a successor.
+static bool SinkInstruction(Instruction *Inst,
+                            SmallPtrSetImpl<Instruction *> &Stores,
+                            DominatorTree &DT, LoopInfo &LI, AAResults &AA) {
+
+  // Don't sink static alloca instructions.  CodeGen assumes allocas outside the
+  // entry block are dynamically sized stack objects.
+  if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst))
+    if (AI->isStaticAlloca())
+      return false;
+
+  // Check if it's safe to move the instruction.
+  if (!isSafeToMove(Inst, AA, Stores))
+    return false;
+
+  // FIXME: This should include support for sinking instructions within the
+  // block they are currently in to shorten the live ranges.  We often get
+  // instructions sunk into the top of a large block, but it would be better to
+  // also sink them down before their first use in the block.  This xform has to
+  // be careful not to *increase* register pressure though, e.g. sinking
+  // "x = y + z" down if it kills y and z would increase the live ranges of y
+  // and z and only shrink the live range of x.
+
+  // SuccToSinkTo - This is the successor to sink this instruction to, once we
+  // decide.
+  BasicBlock *SuccToSinkTo = nullptr;
+
+  // Instructions can only be sunk if all their uses are in blocks
+  // dominated by one of the successors.
+  // Look at all the dominated blocks and see if we can sink it in one.
+  DomTreeNode *DTN = DT.getNode(Inst->getParent());
+  for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end();
+      I != E && SuccToSinkTo == nullptr; ++I) {
+    BasicBlock *Candidate = (*I)->getBlock();
+    // A node always immediate-dominates its children on the dominator
+    // tree.
+    if (IsAcceptableTarget(Inst, Candidate, DT, LI))
+      SuccToSinkTo = Candidate;
+  }
+
+  // If no suitable postdominator was found, look at all the successors and
+  // decide which one we should sink to, if any.
+  for (succ_iterator I = succ_begin(Inst->getParent()),
+      E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) {
+    if (IsAcceptableTarget(Inst, *I, DT, LI))
+      SuccToSinkTo = *I;
+  }
+
+  // If we couldn't find a block to sink to, ignore this instruction.
+  if (!SuccToSinkTo)
+    return false;
+
+  DEBUG(dbgs() << "Sink" << *Inst << " (";
+        Inst->getParent()->printAsOperand(dbgs(), false);
+        dbgs() << " -> ";
+        SuccToSinkTo->printAsOperand(dbgs(), false);
+        dbgs() << ")\n");
+
+  // Move the instruction.
+  Inst->moveBefore(&*SuccToSinkTo->getFirstInsertionPt());
+  return true;
+}
+
+static bool ProcessBlock(BasicBlock &BB, DominatorTree &DT, LoopInfo &LI,
+                         AAResults &AA) {
+  // Can't sink anything out of a block that has less than two successors.
+  if (BB.getTerminator()->getNumSuccessors() <= 1) return false;
+
+  // Don't bother sinking code out of unreachable blocks. In addition to being
+  // unprofitable, it can also lead to infinite looping, because in an
+  // unreachable loop there may be nowhere to stop.
+  if (!DT.isReachableFromEntry(&BB)) return false;
+
+  bool MadeChange = false;
+
+  // Walk the basic block bottom-up.  Remember if we saw a store.
+  BasicBlock::iterator I = BB.end();
+  --I;
+  bool ProcessedBegin = false;
+  SmallPtrSet<Instruction *, 8> Stores;
+  do {
+    Instruction *Inst = &*I; // The instruction to sink.
+
+    // Predecrement I (if it's not begin) so that it isn't invalidated by
+    // sinking.
+    ProcessedBegin = I == BB.begin();
+    if (!ProcessedBegin)
+      --I;
+
+    if (isa<DbgInfoIntrinsic>(Inst))
+      continue;
+
+    if (SinkInstruction(Inst, Stores, DT, LI, AA)) {
+      ++NumSunk;
+      MadeChange = true;
+    }
+
+    // If we just processed the first instruction in the block, we're done.
+  } while (!ProcessedBegin);
+
+  return MadeChange;
+}
+
+static bool iterativelySinkInstructions(Function &F, DominatorTree &DT,
+                                        LoopInfo &LI, AAResults &AA) {
+  bool MadeChange, EverMadeChange = false;
+
+  do {
+    MadeChange = false;
+    DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
+    // Process all basic blocks.
+    for (BasicBlock &I : F)
+      MadeChange |= ProcessBlock(I, DT, LI, AA);
+    EverMadeChange |= MadeChange;
+    NumSinkIter++;
+  } while (MadeChange);
+
+  return EverMadeChange;
+}
+
+PreservedAnalyses SinkingPass::run(Function &F, FunctionAnalysisManager &AM) {
+  auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
+  auto &LI = AM.getResult<LoopAnalysis>(F);
+  auto &AA = AM.getResult<AAManager>(F);
+
+  if (!iterativelySinkInstructions(F, DT, LI, AA))
+    return PreservedAnalyses::all();
+
+  PreservedAnalyses PA;
+  PA.preserveSet<CFGAnalyses>();
+  return PA;
+}
+
+namespace {
+  class SinkingLegacyPass : public FunctionPass {
+  public:
+    static char ID; // Pass identification
+    SinkingLegacyPass() : FunctionPass(ID) {
+      initializeSinkingLegacyPassPass(*PassRegistry::getPassRegistry());
+    }
+
+    bool runOnFunction(Function &F) override {
+      auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+      auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+      auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
+
+      return iterativelySinkInstructions(F, DT, LI, AA);
+    }
+
+    void getAnalysisUsage(AnalysisUsage &AU) const override {
+      AU.setPreservesCFG();
+      FunctionPass::getAnalysisUsage(AU);
+      AU.addRequired<AAResultsWrapperPass>();
+      AU.addRequired<DominatorTreeWrapperPass>();
+      AU.addRequired<LoopInfoWrapperPass>();
+      AU.addPreserved<DominatorTreeWrapperPass>();
+      AU.addPreserved<LoopInfoWrapperPass>();
+    }
+  };
+} // end anonymous namespace
+
+char SinkingLegacyPass::ID = 0;
+INITIALIZE_PASS_BEGIN(SinkingLegacyPass, "sink", "Code sinking", false, false)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
+INITIALIZE_PASS_END(SinkingLegacyPass, "sink", "Code sinking", false, false)
+
+FunctionPass *llvm::createSinkingPass() { return new SinkingLegacyPass(); }