#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
+#include "llvm/IR/InstVisitor.h"
+#include "llvm/Analysis/InstructionSimplify.h"
#include <climits>
using namespace llvm;
UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
cl::desc("The cut-off point for automatic loop unrolling"));
+static cl::opt<unsigned> UnrollMaxIterationsCountToAnalyze(
+ "unroll-max-iteration-count-to-analyze", cl::init(1000), cl::Hidden,
+ cl::desc("Don't allow loop unrolling to simulate more than this number of"
+ "iterations when checking full unroll profitability"));
+
static cl::opt<unsigned>
UnrollCount("unroll-count", cl::init(0), cl::Hidden,
cl::desc("Use this unroll count for all loops including those with "
// unrolled loops respectively.
void selectThresholds(const Loop *L, bool HasPragma,
const TargetTransformInfo::UnrollingPreferences &UP,
- unsigned &Threshold, unsigned &PartialThreshold) {
+ unsigned &Threshold, unsigned &PartialThreshold,
+ unsigned NumberOfSimplifiedInstructions) {
// Determine the current unrolling threshold. While this is
// normally set from UnrollThreshold, it is overridden to a
// smaller value if the current function is marked as
PartialThreshold =
std::max<unsigned>(PartialThreshold, PragmaUnrollThreshold);
}
+ Threshold += NumberOfSimplifiedInstructions;
}
};
}
return llvm::createLoopUnrollPass(-1, -1, 0, 0);
}
+static bool IsLoadFromConstantInitializer(Value *V) {
+ if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
+ if (GV->isConstant() && GV->hasDefinitiveInitializer())
+ return GV->getInitializer();
+ return false;
+}
+
+struct FindConstantPointers {
+ bool LoadCanBeConstantFolded;
+ bool IndexIsConstant;
+ APInt Step;
+ APInt StartValue;
+ Value *BaseAddress;
+ const Loop *L;
+ ScalarEvolution &SE;
+ FindConstantPointers(const Loop *loop, ScalarEvolution &SE)
+ : LoadCanBeConstantFolded(true), IndexIsConstant(true), L(loop), SE(SE) {}
+
+ bool follow(const SCEV *S) {
+ if (const SCEVUnknown *SC = dyn_cast<SCEVUnknown>(S)) {
+ // We've reached the leaf node of SCEV, it's most probably just a
+ // variable. Now it's time to see if it corresponds to a global constant
+ // global (in which case we can eliminate the load), or not.
+ BaseAddress = SC->getValue();
+ LoadCanBeConstantFolded =
+ IndexIsConstant && IsLoadFromConstantInitializer(BaseAddress);
+ return false;
+ }
+ if (isa<SCEVConstant>(S))
+ return true;
+ if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
+ // If the current SCEV expression is AddRec, and its loop isn't the loop
+ // we are about to unroll, then we won't get a constant address after
+ // unrolling, and thus, won't be able to eliminate the load.
+ if (AR->getLoop() != L)
+ return IndexIsConstant = false;
+ // If the step isn't constant, we won't get constant addresses in unrolled
+ // version. Bail out.
+ if (const SCEVConstant *StepSE =
+ dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE)))
+ Step = StepSE->getValue()->getValue();
+ else
+ return IndexIsConstant = false;
+
+ return IndexIsConstant;
+ }
+ // If Result is true, continue traversal.
+ // Otherwise, we have found something that prevents us from (possible) load
+ // elimination.
+ return IndexIsConstant;
+ }
+ bool isDone() const { return !IndexIsConstant; }
+};
+
+// This class is used to get an estimate of the optimization effects that we
+// could get from complete loop unrolling. It comes from the fact that some
+// loads might be replaced with concrete constant values and that could trigger
+// a chain of instruction simplifications.
+//
+// E.g. we might have:
+// int a[] = {0, 1, 0};
+// v = 0;
+// for (i = 0; i < 3; i ++)
+// v += b[i]*a[i];
+// If we completely unroll the loop, we would get:
+// v = b[0]*a[0] + b[1]*a[1] + b[2]*a[2]
+// Which then will be simplified to:
+// v = b[0]* 0 + b[1]* 1 + b[2]* 0
+// And finally:
+// v = b[1]
+class UnrollAnalyzer : public InstVisitor<UnrollAnalyzer, bool> {
+ typedef InstVisitor<UnrollAnalyzer, bool> Base;
+ friend class InstVisitor<UnrollAnalyzer, bool>;
+
+ const Loop *L;
+ unsigned TripCount;
+ ScalarEvolution &SE;
+ const TargetTransformInfo &TTI;
+ unsigned NumberOfOptimizedInstructions;
+
+ DenseMap<Value *, Constant *> SimplifiedValues;
+ DenseMap<LoadInst *, Value *> LoadBaseAddresses;
+ SmallPtrSet<Instruction *, 32> CountedInsns;
+
+ // Provide base case for our instruction visit.
+ bool visitInstruction(Instruction &I) { return false; };
+ // TODO: We should also visit ICmp, FCmp, GetElementPtr, Trunc, ZExt, SExt,
+ // FPTrunc, FPExt, FPToUI, FPToSI, UIToFP, SIToFP, BitCast, Select,
+ // ExtractElement, InsertElement, ShuffleVector, ExtractValue, InsertValue.
+ //
+ // Probaly it's worth to hoist the code for estimating the simplifications
+ // effects to a separate class, since we have a very similar code in
+ // InlineCost already.
+ bool visitBinaryOperator(BinaryOperator &I) {
+ Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
+ if (!isa<Constant>(LHS))
+ if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
+ LHS = SimpleLHS;
+ if (!isa<Constant>(RHS))
+ if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
+ RHS = SimpleRHS;
+ Value *SimpleV = SimplifyBinOp(I.getOpcode(), LHS, RHS);
+
+ if (SimpleV && CountedInsns.insert(&I).second)
+ NumberOfOptimizedInstructions += TTI.getUserCost(&I);
+
+ if (Constant *C = dyn_cast_or_null<Constant>(SimpleV)) {
+ SimplifiedValues[&I] = C;
+ return true;
+ }
+ return false;
+ }
+
+ Constant *computeLoadValue(LoadInst *LI, unsigned Iteration) {
+ if (!LI)
+ return nullptr;
+ Value *BaseAddr = LoadBaseAddresses[LI];
+ if (!BaseAddr)
+ return nullptr;
+
+ auto GV = dyn_cast<GlobalVariable>(BaseAddr);
+ if (!GV)
+ return nullptr;
+
+ ConstantDataSequential *CDS =
+ dyn_cast<ConstantDataSequential>(GV->getInitializer());
+ if (!CDS)
+ return nullptr;
+
+ const SCEV *BaseAddrSE = SE.getSCEV(BaseAddr);
+ const SCEV *S = SE.getSCEV(LI->getPointerOperand());
+ const SCEV *OffSE = SE.getMinusSCEV(S, BaseAddrSE);
+
+ APInt StepC, StartC;
+ const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(OffSE);
+ if (!AR)
+ return nullptr;
+
+ if (const SCEVConstant *StepSE =
+ dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE)))
+ StepC = StepSE->getValue()->getValue();
+ else
+ return nullptr;
+
+ if (const SCEVConstant *StartSE = dyn_cast<SCEVConstant>(AR->getStart()))
+ StartC = StartSE->getValue()->getValue();
+ else
+ return nullptr;
+
+ unsigned ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
+ unsigned Start = StartC.getLimitedValue();
+ unsigned Step = StepC.getLimitedValue();
+
+ unsigned Index = (Start + Step * Iteration) / ElemSize;
+ if (Index >= CDS->getNumElements())
+ return nullptr;
+
+ Constant *CV = CDS->getElementAsConstant(Index);
+
+ return CV;
+ }
+
+public:
+ UnrollAnalyzer(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
+ const TargetTransformInfo &TTI)
+ : L(L), TripCount(TripCount), SE(SE), TTI(TTI),
+ NumberOfOptimizedInstructions(0) {}
+
+ // Visit all loads the loop L, and for those that, after complete loop
+ // unrolling, would have a constant address and it will point to a known
+ // constant initializer, record its base address for future use. It is used
+ // when we estimate number of potentially simplified instructions.
+ void FindConstFoldableLoads() {
+ for (auto BB : L->getBlocks()) {
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
+ if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
+ if (!LI->isSimple())
+ continue;
+ Value *AddrOp = LI->getPointerOperand();
+ const SCEV *S = SE.getSCEV(AddrOp);
+ FindConstantPointers Visitor(L, SE);
+ SCEVTraversal<FindConstantPointers> T(Visitor);
+ T.visitAll(S);
+ if (Visitor.IndexIsConstant && Visitor.LoadCanBeConstantFolded) {
+ LoadBaseAddresses[LI] = Visitor.BaseAddress;
+ }
+ }
+ }
+ }
+ }
+
+ // Given a list of loads that could be constant-folded (LoadBaseAddresses),
+ // estimate number of optimized instructions after substituting the concrete
+ // values for the given Iteration.
+ // Fill in SimplifiedInsns map for future use in DCE-estimation.
+ unsigned EstimateNumberOfSimplifiedInsns(unsigned Iteration) {
+ SmallVector<Instruction *, 8> Worklist;
+ SimplifiedValues.clear();
+ CountedInsns.clear();
+
+ NumberOfOptimizedInstructions = 0;
+ // We start by adding all loads to the worklist.
+ for (auto LoadDescr : LoadBaseAddresses) {
+ LoadInst *LI = LoadDescr.first;
+ SimplifiedValues[LI] = computeLoadValue(LI, Iteration);
+ if (CountedInsns.insert(LI).second)
+ NumberOfOptimizedInstructions += TTI.getUserCost(LI);
+
+ for (auto U : LI->users()) {
+ Instruction *UI = dyn_cast<Instruction>(U);
+ if (!UI)
+ continue;
+ if (!L->contains(UI))
+ continue;
+ Worklist.push_back(UI);
+ }
+ }
+
+ // And then we try to simplify every user of every instruction from the
+ // worklist. If we do simplify a user, add it to the worklist to process
+ // its users as well.
+ while (!Worklist.empty()) {
+ Instruction *I = Worklist.pop_back_val();
+ if (!visit(I))
+ continue;
+ for (auto U : I->users()) {
+ Instruction *UI = dyn_cast<Instruction>(U);
+ if (!UI)
+ continue;
+ if (!L->contains(UI))
+ continue;
+ Worklist.push_back(UI);
+ }
+ }
+ return NumberOfOptimizedInstructions;
+ }
+
+ // Given a list of potentially simplifed instructions, estimate number of
+ // instructions that would become dead if we do perform the simplification.
+ unsigned EstimateNumberOfDeadInsns() {
+ NumberOfOptimizedInstructions = 0;
+ SmallVector<Instruction *, 8> Worklist;
+ DenseMap<Instruction *, bool> DeadInstructions;
+ // Start by initializing worklist with simplified instructions.
+ for (auto Folded : SimplifiedValues) {
+ if (auto FoldedInsn = dyn_cast<Instruction>(Folded.first)) {
+ Worklist.push_back(FoldedInsn);
+ DeadInstructions[FoldedInsn] = true;
+ }
+ }
+ // If a definition of an insn is only used by simplified or dead
+ // instructions, it's also dead. Check defs of all instructions from the
+ // worklist.
+ while (!Worklist.empty()) {
+ Instruction *FoldedInsn = Worklist.pop_back_val();
+ for (Value *Op : FoldedInsn->operands()) {
+ if (auto I = dyn_cast<Instruction>(Op)) {
+ if (!L->contains(I))
+ continue;
+ if (SimplifiedValues[I])
+ continue; // This insn has been counted already.
+ if (I->getNumUses() == 0)
+ continue;
+ bool AllUsersFolded = true;
+ for (auto U : I->users()) {
+ Instruction *UI = dyn_cast<Instruction>(U);
+ if (!SimplifiedValues[UI] && !DeadInstructions[UI]) {
+ AllUsersFolded = false;
+ break;
+ }
+ }
+ if (AllUsersFolded) {
+ NumberOfOptimizedInstructions += TTI.getUserCost(I);
+ Worklist.push_back(I);
+ DeadInstructions[I] = true;
+ }
+ }
+ }
+ }
+ return NumberOfOptimizedInstructions;
+ }
+};
+
+// Complete loop unrolling can make some loads constant, and we need to know if
+// that would expose any further optimization opportunities.
+// This routine estimates this optimization effect and returns the number of
+// instructions, that potentially might be optimized away.
+static unsigned
+ApproximateNumberOfOptimizedInstructions(const Loop *L, ScalarEvolution &SE,
+ unsigned TripCount,
+ const TargetTransformInfo &TTI) {
+ if (!TripCount)
+ return 0;
+
+ UnrollAnalyzer UA(L, TripCount, SE, TTI);
+ UA.FindConstFoldableLoads();
+
+ // Estimate number of instructions, that could be simplified if we replace a
+ // load with the corresponding constant. Since the same load will take
+ // different values on different iterations, we have to go through all loop's
+ // iterations here. To limit ourselves here, we check only first N
+ // iterations, and then scale the found number, if necessary.
+ unsigned IterationsNumberForEstimate =
+ std::min<unsigned>(UnrollMaxIterationsCountToAnalyze, TripCount);
+ unsigned NumberOfOptimizedInstructions = 0;
+ for (unsigned i = 0; i < IterationsNumberForEstimate; ++i) {
+ NumberOfOptimizedInstructions += UA.EstimateNumberOfSimplifiedInsns(i);
+ NumberOfOptimizedInstructions += UA.EstimateNumberOfDeadInsns();
+ }
+ NumberOfOptimizedInstructions *= TripCount / IterationsNumberForEstimate;
+
+ return NumberOfOptimizedInstructions;
+}
+
/// ApproximateLoopSize - Approximate the size of the loop.
static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
bool &NotDuplicatable,
return false;
}
+ unsigned NumberOfOptimizedInstructions =
+ ApproximateNumberOfOptimizedInstructions(L, *SE, TripCount, TTI);
+ DEBUG(dbgs() << " Complete unrolling could save: "
+ << NumberOfOptimizedInstructions << "\n");
+
unsigned Threshold, PartialThreshold;
- selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold);
+ selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold,
+ NumberOfOptimizedInstructions);
// Given Count, TripCount and thresholds determine the type of
// unrolling which is to be performed.
projects / llvm / commitdiff