2014-04-04 05:18:25 +08:00
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//===-- NVPTXFavorNonGenericAddrSpace.cpp - ---------------------*- C++ -*-===//
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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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// When a load/store accesses the generic address space, checks whether the
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// address is casted from a non-generic address space. If so, remove this
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// addrspacecast because accessing non-generic address spaces is typically
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// faster. Besides seeking addrspacecasts, this optimization also traces into
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// the base pointer of a GEP.
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//
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// For instance, the code below loads a float from an array allocated in
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// addrspace(3).
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//
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// %0 = addrspacecast [10 x float] addrspace(3)* @a to [10 x float]*
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// %1 = gep [10 x float]* %0, i64 0, i64 %i
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// %2 = load float* %1 ; emits ld.f32
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//
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// First, function hoistAddrSpaceCastFromGEP reorders the addrspacecast
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// and the GEP to expose more optimization opportunities to function
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// optimizeMemoryInst. The intermediate code looks like:
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//
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// %0 = gep [10 x float] addrspace(3)* @a, i64 0, i64 %i
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// %1 = addrspacecast float addrspace(3)* %0 to float*
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// %2 = load float* %1 ; still emits ld.f32, but will be optimized shortly
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//
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// Then, function optimizeMemoryInstruction detects a load from addrspacecast'ed
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// generic pointers, and folds the load and the addrspacecast into a load from
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// the original address space. The final code looks like:
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//
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// %0 = gep [10 x float] addrspace(3)* @a, i64 0, i64 %i
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// %2 = load float addrspace(3)* %0 ; emits ld.shared.f32
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//
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// This pass may remove an addrspacecast in a different BB. Therefore, we
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// implement it as a FunctionPass.
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//
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//===----------------------------------------------------------------------===//
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#include "NVPTX.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Operator.h"
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#include "llvm/Support/CommandLine.h"
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using namespace llvm;
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// An option to disable this optimization. Enable it by default.
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static cl::opt<bool> DisableFavorNonGeneric(
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"disable-nvptx-favor-non-generic",
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cl::init(false),
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cl::desc("Do not convert generic address space usage "
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"to non-generic address space usage"),
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cl::Hidden);
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namespace {
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/// \brief NVPTXFavorNonGenericAddrSpaces
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class NVPTXFavorNonGenericAddrSpaces : public FunctionPass {
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public:
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static char ID;
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NVPTXFavorNonGenericAddrSpaces() : FunctionPass(ID) {}
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2014-05-23 14:30:12 +08:00
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bool runOnFunction(Function &F) override;
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2014-04-04 05:18:25 +08:00
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/// Optimizes load/store instructions. Idx is the index of the pointer operand
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/// (0 for load, and 1 for store). Returns true if it changes anything.
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bool optimizeMemoryInstruction(Instruction *I, unsigned Idx);
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/// Transforms "gep (addrspacecast X), indices" into "addrspacecast (gep X,
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/// indices)". This reordering exposes to optimizeMemoryInstruction more
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/// optimization opportunities on loads and stores. Returns true if it changes
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/// the program.
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bool hoistAddrSpaceCastFromGEP(GEPOperator *GEP);
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};
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}
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char NVPTXFavorNonGenericAddrSpaces::ID = 0;
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namespace llvm {
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void initializeNVPTXFavorNonGenericAddrSpacesPass(PassRegistry &);
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}
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INITIALIZE_PASS(NVPTXFavorNonGenericAddrSpaces, "nvptx-favor-non-generic",
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"Remove unnecessary non-generic-to-generic addrspacecasts",
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false, false)
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// Decides whether removing Cast is valid and beneficial. Cast can be an
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// instruction or a constant expression.
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static bool IsEliminableAddrSpaceCast(Operator *Cast) {
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// Returns false if not even an addrspacecast.
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if (Cast->getOpcode() != Instruction::AddrSpaceCast)
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return false;
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Value *Src = Cast->getOperand(0);
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PointerType *SrcTy = cast<PointerType>(Src->getType());
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PointerType *DestTy = cast<PointerType>(Cast->getType());
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// TODO: For now, we only handle the case where the addrspacecast only changes
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// the address space but not the type. If the type also changes, we could
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// still get rid of the addrspacecast by adding an extra bitcast, but we
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// rarely see such scenarios.
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if (SrcTy->getElementType() != DestTy->getElementType())
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return false;
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// Checks whether the addrspacecast is from a non-generic address space to the
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// generic address space.
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return (SrcTy->getAddressSpace() != AddressSpace::ADDRESS_SPACE_GENERIC &&
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DestTy->getAddressSpace() == AddressSpace::ADDRESS_SPACE_GENERIC);
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}
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bool NVPTXFavorNonGenericAddrSpaces::hoistAddrSpaceCastFromGEP(
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GEPOperator *GEP) {
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Operator *Cast = dyn_cast<Operator>(GEP->getPointerOperand());
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if (!Cast)
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2014-04-04 05:18:25 +08:00
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return false;
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if (!IsEliminableAddrSpaceCast(Cast))
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return false;
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SmallVector<Value *, 8> Indices(GEP->idx_begin(), GEP->idx_end());
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if (Instruction *GEPI = dyn_cast<Instruction>(GEP)) {
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// %1 = gep (addrspacecast X), indices
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// =>
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// %0 = gep X, indices
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// %1 = addrspacecast %0
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GetElementPtrInst *NewGEPI = GetElementPtrInst::Create(Cast->getOperand(0),
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Indices,
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GEP->getName(),
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GEPI);
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NewGEPI->setIsInBounds(GEP->isInBounds());
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GEP->replaceAllUsesWith(
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new AddrSpaceCastInst(NewGEPI, GEP->getType(), "", GEPI));
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} else {
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// GEP is a constant expression.
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Constant *NewGEPCE = ConstantExpr::getGetElementPtr(
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cast<Constant>(Cast->getOperand(0)),
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Indices,
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GEP->isInBounds());
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GEP->replaceAllUsesWith(
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ConstantExpr::getAddrSpaceCast(NewGEPCE, GEP->getType()));
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}
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return true;
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}
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bool NVPTXFavorNonGenericAddrSpaces::optimizeMemoryInstruction(Instruction *MI,
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unsigned Idx) {
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// If the pointer operand is a GEP, hoist the addrspacecast if any from the
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// GEP to expose more optimization opportunites.
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if (GEPOperator *GEP = dyn_cast<GEPOperator>(MI->getOperand(Idx))) {
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hoistAddrSpaceCastFromGEP(GEP);
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}
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// load/store (addrspacecast X) => load/store X if shortcutting the
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// addrspacecast is valid and can improve performance.
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//
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// e.g.,
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// %1 = addrspacecast float addrspace(3)* %0 to float*
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// %2 = load float* %1
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// ->
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// %2 = load float addrspace(3)* %0
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//
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// Note: the addrspacecast can also be a constant expression.
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if (Operator *Cast = dyn_cast<Operator>(MI->getOperand(Idx))) {
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if (IsEliminableAddrSpaceCast(Cast)) {
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MI->setOperand(Idx, Cast->getOperand(0));
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return true;
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}
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}
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return false;
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}
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bool NVPTXFavorNonGenericAddrSpaces::runOnFunction(Function &F) {
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if (DisableFavorNonGeneric)
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return false;
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bool Changed = false;
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for (Function::iterator B = F.begin(), BE = F.end(); B != BE; ++B) {
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for (BasicBlock::iterator I = B->begin(), IE = B->end(); I != IE; ++I) {
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if (isa<LoadInst>(I)) {
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// V = load P
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Changed |= optimizeMemoryInstruction(I, 0);
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} else if (isa<StoreInst>(I)) {
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// store V, P
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Changed |= optimizeMemoryInstruction(I, 1);
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}
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}
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}
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return Changed;
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}
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FunctionPass *llvm::createNVPTXFavorNonGenericAddrSpacesPass() {
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return new NVPTXFavorNonGenericAddrSpaces();
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}
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