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+//===-- Sink.cpp - Code Sinking -------------------------------------------===//
+//
+// 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 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 (Inst->isTerminator() || isa<PHINode>(Inst) || Inst->isEHPad() ||
+ Inst->mayThrow())
+ return false;
+
+ if (auto *Call = dyn_cast<CallBase>(Inst)) {
+ // Convergent operations cannot be made control-dependent on additional
+ // values.
+ if (Call->hasFnAttr(Attribute::Convergent))
+ return false;
+
+ for (Instruction *S : Stores)
+ if (isModSet(AA.getModRefInfo(S, Call)))
+ 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()->isExceptionalTerminator())
+ 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 (Inst->mayReadFromMemory())
+ 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;
+
+ LLVM_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;
+ LLVM_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(); }
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