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606 lines
21 KiB
606 lines
21 KiB
//===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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//! \file
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//! \brief This pass performs merges of loads and stores on both sides of a
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// diamond (hammock). It hoists the loads and sinks the stores.
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//
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// The algorithm iteratively hoists two loads to the same address out of a
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// diamond (hammock) and merges them into a single load in the header. Similar
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// it sinks and merges two stores to the tail block (footer). The algorithm
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// iterates over the instructions of one side of the diamond and attempts to
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// find a matching load/store on the other side. It hoists / sinks when it
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// thinks it safe to do so. This optimization helps with eg. hiding load
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// latencies, triggering if-conversion, and reducing static code size.
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//
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//===----------------------------------------------------------------------===//
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//
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//
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// Example:
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// Diamond shaped code before merge:
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//
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// header:
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// br %cond, label %if.then, label %if.else
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// + +
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// + +
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// + +
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// if.then: if.else:
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// %lt = load %addr_l %le = load %addr_l
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// <use %lt> <use %le>
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// <...> <...>
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// store %st, %addr_s store %se, %addr_s
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// br label %if.end br label %if.end
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// + +
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// + +
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// + +
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// if.end ("footer"):
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// <...>
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//
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// Diamond shaped code after merge:
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//
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// header:
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// %l = load %addr_l
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// br %cond, label %if.then, label %if.else
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// + +
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// + +
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// + +
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// if.then: if.else:
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// <use %l> <use %l>
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// <...> <...>
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// br label %if.end br label %if.end
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// + +
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// + +
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// + +
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// if.end ("footer"):
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// %s.sink = phi [%st, if.then], [%se, if.else]
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// <...>
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// store %s.sink, %addr_s
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// <...>
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//
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//
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//===----------------------- TODO -----------------------------------------===//
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//
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// 1) Generalize to regions other than diamonds
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// 2) Be more aggressive merging memory operations
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// Note that both changes require register pressure control
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/MergedLoadStoreMotion.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/CFG.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/Loads.h"
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#include "llvm/Analysis/MemoryBuiltins.h"
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#include "llvm/Analysis/MemoryDependenceAnalysis.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/Metadata.h"
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#include "llvm/IR/PatternMatch.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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using namespace llvm;
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#define DEBUG_TYPE "mldst-motion"
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namespace {
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//===----------------------------------------------------------------------===//
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// MergedLoadStoreMotion Pass
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//===----------------------------------------------------------------------===//
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class MergedLoadStoreMotion {
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MemoryDependenceResults *MD = nullptr;
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AliasAnalysis *AA = nullptr;
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// The mergeLoad/Store algorithms could have Size0 * Size1 complexity,
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// where Size0 and Size1 are the #instructions on the two sides of
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// the diamond. The constant chosen here is arbitrary. Compiler Time
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// Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.
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const int MagicCompileTimeControl = 250;
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public:
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bool run(Function &F, MemoryDependenceResults *MD, AliasAnalysis &AA);
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private:
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///
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/// \brief Remove instruction from parent and update memory dependence
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/// analysis.
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///
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void removeInstruction(Instruction *Inst);
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BasicBlock *getDiamondTail(BasicBlock *BB);
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bool isDiamondHead(BasicBlock *BB);
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// Routines for hoisting loads
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bool isLoadHoistBarrierInRange(const Instruction &Start,
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const Instruction &End, LoadInst *LI,
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bool SafeToLoadUnconditionally);
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LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI);
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void hoistInstruction(BasicBlock *BB, Instruction *HoistCand,
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Instruction *ElseInst);
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bool isSafeToHoist(Instruction *I) const;
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bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst);
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bool mergeLoads(BasicBlock *BB);
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// Routines for sinking stores
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StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);
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PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);
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bool isStoreSinkBarrierInRange(const Instruction &Start,
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const Instruction &End, MemoryLocation Loc);
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bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst);
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bool mergeStores(BasicBlock *BB);
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};
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} // end anonymous namespace
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///
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/// \brief Remove instruction from parent and update memory dependence analysis.
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///
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void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) {
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// Notify the memory dependence analysis.
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if (MD) {
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MD->removeInstruction(Inst);
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if (auto *LI = dyn_cast<LoadInst>(Inst))
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MD->invalidateCachedPointerInfo(LI->getPointerOperand());
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if (Inst->getType()->isPtrOrPtrVectorTy()) {
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MD->invalidateCachedPointerInfo(Inst);
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}
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}
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Inst->eraseFromParent();
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}
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///
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/// \brief Return tail block of a diamond.
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///
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BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
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assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
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return BB->getTerminator()->getSuccessor(0)->getSingleSuccessor();
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}
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///
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/// \brief True when BB is the head of a diamond (hammock)
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///
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bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
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if (!BB)
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return false;
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auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (!BI || !BI->isConditional())
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return false;
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BasicBlock *Succ0 = BI->getSuccessor(0);
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BasicBlock *Succ1 = BI->getSuccessor(1);
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if (!Succ0->getSinglePredecessor())
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return false;
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if (!Succ1->getSinglePredecessor())
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return false;
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BasicBlock *Succ0Succ = Succ0->getSingleSuccessor();
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BasicBlock *Succ1Succ = Succ1->getSingleSuccessor();
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// Ignore triangles.
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if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ)
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return false;
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return true;
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}
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///
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/// \brief True when instruction is a hoist barrier for a load
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///
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/// Whenever an instruction could possibly modify the value
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/// being loaded or protect against the load from happening
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/// it is considered a hoist barrier.
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///
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bool MergedLoadStoreMotion::isLoadHoistBarrierInRange(
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const Instruction &Start, const Instruction &End, LoadInst *LI,
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bool SafeToLoadUnconditionally) {
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if (!SafeToLoadUnconditionally)
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for (const Instruction &Inst :
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make_range(Start.getIterator(), End.getIterator()))
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if (!isGuaranteedToTransferExecutionToSuccessor(&Inst))
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return true;
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MemoryLocation Loc = MemoryLocation::get(LI);
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return AA->canInstructionRangeModRef(Start, End, Loc, MRI_Mod);
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}
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///
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/// \brief Decide if a load can be hoisted
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///
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/// When there is a load in \p BB to the same address as \p LI
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/// and it can be hoisted from \p BB, return that load.
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/// Otherwise return Null.
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///
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LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB1,
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LoadInst *Load0) {
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BasicBlock *BB0 = Load0->getParent();
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BasicBlock *Head = BB0->getSinglePredecessor();
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bool SafeToLoadUnconditionally = isSafeToLoadUnconditionally(
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Load0->getPointerOperand(), Load0->getAlignment(),
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Load0->getModule()->getDataLayout(),
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/*ScanFrom=*/Head->getTerminator());
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for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE;
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++BBI) {
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Instruction *Inst = &*BBI;
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// Only merge and hoist loads when their result in used only in BB
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auto *Load1 = dyn_cast<LoadInst>(Inst);
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if (!Load1 || Inst->isUsedOutsideOfBlock(BB1))
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continue;
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MemoryLocation Loc0 = MemoryLocation::get(Load0);
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MemoryLocation Loc1 = MemoryLocation::get(Load1);
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if (Load0->isSameOperationAs(Load1) && AA->isMustAlias(Loc0, Loc1) &&
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!isLoadHoistBarrierInRange(BB1->front(), *Load1, Load1,
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SafeToLoadUnconditionally) &&
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!isLoadHoistBarrierInRange(BB0->front(), *Load0, Load0,
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SafeToLoadUnconditionally)) {
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return Load1;
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}
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}
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return nullptr;
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}
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///
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/// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into
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/// \p BB
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///
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/// BB is the head of a diamond
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///
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void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB,
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Instruction *HoistCand,
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Instruction *ElseInst) {
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DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump();
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dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n";
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dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n");
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// Hoist the instruction.
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assert(HoistCand->getParent() != BB);
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// Intersect optional metadata.
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HoistCand->intersectOptionalDataWith(ElseInst);
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HoistCand->dropUnknownNonDebugMetadata();
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// Prepend point for instruction insert
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Instruction *HoistPt = BB->getTerminator();
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// Merged instruction
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Instruction *HoistedInst = HoistCand->clone();
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// Hoist instruction.
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HoistedInst->insertBefore(HoistPt);
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HoistCand->replaceAllUsesWith(HoistedInst);
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removeInstruction(HoistCand);
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// Replace the else block instruction.
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ElseInst->replaceAllUsesWith(HoistedInst);
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removeInstruction(ElseInst);
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}
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///
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/// \brief Return true if no operand of \p I is defined in I's parent block
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///
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bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const {
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BasicBlock *Parent = I->getParent();
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for (Use &U : I->operands())
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if (auto *Instr = dyn_cast<Instruction>(&U))
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if (Instr->getParent() == Parent)
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return false;
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return true;
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}
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///
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/// \brief Merge two equivalent loads and GEPs and hoist into diamond head
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///
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bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0,
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LoadInst *L1) {
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// Only one definition?
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auto *A0 = dyn_cast<Instruction>(L0->getPointerOperand());
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auto *A1 = dyn_cast<Instruction>(L1->getPointerOperand());
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if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) &&
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A0->hasOneUse() && (A0->getParent() == L0->getParent()) &&
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A1->hasOneUse() && (A1->getParent() == L1->getParent()) &&
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isa<GetElementPtrInst>(A0)) {
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DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump();
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dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n";
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dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n");
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hoistInstruction(BB, A0, A1);
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hoistInstruction(BB, L0, L1);
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return true;
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}
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return false;
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}
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///
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/// \brief Try to hoist two loads to same address into diamond header
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///
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/// Starting from a diamond head block, iterate over the instructions in one
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/// successor block and try to match a load in the second successor.
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///
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bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) {
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bool MergedLoads = false;
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assert(isDiamondHead(BB));
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BranchInst *BI = cast<BranchInst>(BB->getTerminator());
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BasicBlock *Succ0 = BI->getSuccessor(0);
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BasicBlock *Succ1 = BI->getSuccessor(1);
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// #Instructions in Succ1 for Compile Time Control
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int Size1 = Succ1->size();
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int NLoads = 0;
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for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end();
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BBI != BBE;) {
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Instruction *I = &*BBI;
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++BBI;
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// Don't move non-simple (atomic, volatile) loads.
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auto *L0 = dyn_cast<LoadInst>(I);
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if (!L0 || !L0->isSimple() || L0->isUsedOutsideOfBlock(Succ0))
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continue;
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++NLoads;
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if (NLoads * Size1 >= MagicCompileTimeControl)
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break;
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if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) {
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bool Res = hoistLoad(BB, L0, L1);
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MergedLoads |= Res;
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// Don't attempt to hoist above loads that had not been hoisted.
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if (!Res)
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break;
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}
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}
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return MergedLoads;
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}
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///
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/// \brief True when instruction is a sink barrier for a store
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/// located in Loc
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///
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/// Whenever an instruction could possibly read or modify the
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/// value being stored or protect against the store from
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/// happening it is considered a sink barrier.
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///
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bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,
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const Instruction &End,
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MemoryLocation Loc) {
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for (const Instruction &Inst :
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make_range(Start.getIterator(), End.getIterator()))
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if (Inst.mayThrow())
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return true;
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return AA->canInstructionRangeModRef(Start, End, Loc, MRI_ModRef);
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}
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///
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/// \brief Check if \p BB contains a store to the same address as \p SI
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///
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/// \return The store in \p when it is safe to sink. Otherwise return Null.
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///
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StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1,
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StoreInst *Store0) {
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DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n");
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BasicBlock *BB0 = Store0->getParent();
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for (Instruction &Inst : reverse(*BB1)) {
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auto *Store1 = dyn_cast<StoreInst>(&Inst);
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if (!Store1)
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continue;
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MemoryLocation Loc0 = MemoryLocation::get(Store0);
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MemoryLocation Loc1 = MemoryLocation::get(Store1);
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if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) &&
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!isStoreSinkBarrierInRange(*Store1->getNextNode(), BB1->back(), Loc1) &&
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!isStoreSinkBarrierInRange(*Store0->getNextNode(), BB0->back(), Loc0)) {
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return Store1;
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}
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}
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return nullptr;
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}
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///
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/// \brief Create a PHI node in BB for the operands of S0 and S1
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///
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PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
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StoreInst *S1) {
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// Create a phi if the values mismatch.
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Value *Opd1 = S0->getValueOperand();
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Value *Opd2 = S1->getValueOperand();
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if (Opd1 == Opd2)
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return nullptr;
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auto *NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink",
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&BB->front());
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NewPN->addIncoming(Opd1, S0->getParent());
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NewPN->addIncoming(Opd2, S1->getParent());
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if (MD && NewPN->getType()->getScalarType()->isPointerTy())
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MD->invalidateCachedPointerInfo(NewPN);
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return NewPN;
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}
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///
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/// \brief Merge two stores to same address and sink into \p BB
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///
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/// Also sinks GEP instruction computing the store address
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///
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bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0,
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StoreInst *S1) {
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// Only one definition?
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auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
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auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
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if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() &&
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(A0->getParent() == S0->getParent()) && A1->hasOneUse() &&
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(A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) {
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DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();
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dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";
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dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");
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// Hoist the instruction.
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BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();
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// Intersect optional metadata.
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S0->intersectOptionalDataWith(S1);
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S0->dropUnknownNonDebugMetadata();
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// Create the new store to be inserted at the join point.
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StoreInst *SNew = cast<StoreInst>(S0->clone());
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Instruction *ANew = A0->clone();
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SNew->insertBefore(&*InsertPt);
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ANew->insertBefore(SNew);
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assert(S0->getParent() == A0->getParent());
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assert(S1->getParent() == A1->getParent());
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// New PHI operand? Use it.
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if (PHINode *NewPN = getPHIOperand(BB, S0, S1))
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SNew->setOperand(0, NewPN);
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removeInstruction(S0);
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removeInstruction(S1);
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A0->replaceAllUsesWith(ANew);
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removeInstruction(A0);
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A1->replaceAllUsesWith(ANew);
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removeInstruction(A1);
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return true;
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}
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return false;
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}
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///
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/// \brief True when two stores are equivalent and can sink into the footer
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///
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/// Starting from a diamond tail block, iterate over the instructions in one
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/// predecessor block and try to match a store in the second predecessor.
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///
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bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) {
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bool MergedStores = false;
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assert(T && "Footer of a diamond cannot be empty");
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pred_iterator PI = pred_begin(T), E = pred_end(T);
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assert(PI != E);
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BasicBlock *Pred0 = *PI;
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++PI;
|
|
BasicBlock *Pred1 = *PI;
|
|
++PI;
|
|
// tail block of a diamond/hammock?
|
|
if (Pred0 == Pred1)
|
|
return false; // No.
|
|
if (PI != E)
|
|
return false; // No. More than 2 predecessors.
|
|
|
|
// #Instructions in Succ1 for Compile Time Control
|
|
int Size1 = Pred1->size();
|
|
int NStores = 0;
|
|
|
|
for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
|
|
RBI != RBE;) {
|
|
|
|
Instruction *I = &*RBI;
|
|
++RBI;
|
|
|
|
// Don't sink non-simple (atomic, volatile) stores.
|
|
auto *S0 = dyn_cast<StoreInst>(I);
|
|
if (!S0 || !S0->isSimple())
|
|
continue;
|
|
|
|
++NStores;
|
|
if (NStores * Size1 >= MagicCompileTimeControl)
|
|
break;
|
|
if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {
|
|
bool Res = sinkStore(T, S0, S1);
|
|
MergedStores |= Res;
|
|
// Don't attempt to sink below stores that had to stick around
|
|
// But after removal of a store and some of its feeding
|
|
// instruction search again from the beginning since the iterator
|
|
// is likely stale at this point.
|
|
if (!Res)
|
|
break;
|
|
RBI = Pred0->rbegin();
|
|
RBE = Pred0->rend();
|
|
DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
|
|
}
|
|
}
|
|
return MergedStores;
|
|
}
|
|
|
|
bool MergedLoadStoreMotion::run(Function &F, MemoryDependenceResults *MD,
|
|
AliasAnalysis &AA) {
|
|
this->MD = MD;
|
|
this->AA = &AA;
|
|
|
|
bool Changed = false;
|
|
DEBUG(dbgs() << "Instruction Merger\n");
|
|
|
|
// Merge unconditional branches, allowing PRE to catch more
|
|
// optimization opportunities.
|
|
for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) {
|
|
BasicBlock *BB = &*FI++;
|
|
|
|
// Hoist equivalent loads and sink stores
|
|
// outside diamonds when possible
|
|
if (isDiamondHead(BB)) {
|
|
Changed |= mergeLoads(BB);
|
|
Changed |= mergeStores(getDiamondTail(BB));
|
|
}
|
|
}
|
|
return Changed;
|
|
}
|
|
|
|
namespace {
|
|
class MergedLoadStoreMotionLegacyPass : public FunctionPass {
|
|
public:
|
|
static char ID; // Pass identification, replacement for typeid
|
|
MergedLoadStoreMotionLegacyPass() : FunctionPass(ID) {
|
|
initializeMergedLoadStoreMotionLegacyPassPass(
|
|
*PassRegistry::getPassRegistry());
|
|
}
|
|
|
|
///
|
|
/// \brief Run the transformation for each function
|
|
///
|
|
bool runOnFunction(Function &F) override {
|
|
if (skipFunction(F))
|
|
return false;
|
|
MergedLoadStoreMotion Impl;
|
|
auto *MDWP = getAnalysisIfAvailable<MemoryDependenceWrapperPass>();
|
|
return Impl.run(F, MDWP ? &MDWP->getMemDep() : nullptr,
|
|
getAnalysis<AAResultsWrapperPass>().getAAResults());
|
|
}
|
|
|
|
private:
|
|
// This transformation requires dominator postdominator info
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override {
|
|
AU.setPreservesCFG();
|
|
AU.addRequired<AAResultsWrapperPass>();
|
|
AU.addPreserved<GlobalsAAWrapperPass>();
|
|
AU.addPreserved<MemoryDependenceWrapperPass>();
|
|
}
|
|
};
|
|
|
|
char MergedLoadStoreMotionLegacyPass::ID = 0;
|
|
} // anonymous namespace
|
|
|
|
///
|
|
/// \brief createMergedLoadStoreMotionPass - The public interface to this file.
|
|
///
|
|
FunctionPass *llvm::createMergedLoadStoreMotionPass() {
|
|
return new MergedLoadStoreMotionLegacyPass();
|
|
}
|
|
|
|
INITIALIZE_PASS_BEGIN(MergedLoadStoreMotionLegacyPass, "mldst-motion",
|
|
"MergedLoadStoreMotion", false, false)
|
|
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
|
|
INITIALIZE_PASS_END(MergedLoadStoreMotionLegacyPass, "mldst-motion",
|
|
"MergedLoadStoreMotion", false, false)
|
|
|
|
PreservedAnalyses
|
|
MergedLoadStoreMotionPass::run(Function &F, AnalysisManager<Function> &AM) {
|
|
MergedLoadStoreMotion Impl;
|
|
auto *MD = AM.getCachedResult<MemoryDependenceAnalysis>(F);
|
|
auto &AA = AM.getResult<AAManager>(F);
|
|
if (!Impl.run(F, MD, AA))
|
|
return PreservedAnalyses::all();
|
|
|
|
// FIXME: This should also 'preserve the CFG'.
|
|
PreservedAnalyses PA;
|
|
PA.preserve<GlobalsAA>();
|
|
PA.preserve<MemoryDependenceAnalysis>();
|
|
return PA;
|
|
}
|