#define DEBUG_TYPE "loop-unroll"
static cl::opt<unsigned>
-UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
- cl::desc("The cut-off point for automatic loop unrolling"));
+ UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
+ cl::desc("The baseline cost threshold for loop unrolling"));
+
+static cl::opt<unsigned> UnrollPercentDynamicCostSavedThreshold(
+ "unroll-percent-dynamic-cost-saved-threshold", cl::init(20), cl::Hidden,
+ cl::desc("The percentage of estimated dynamic cost which must be saved by "
+ "unrolling to allow unrolling up to the max threshold."));
+
+static cl::opt<unsigned> UnrollDynamicCostSavingsDiscount(
+ "unroll-dynamic-cost-savings-discount", cl::init(2000), cl::Hidden,
+ cl::desc("This is the amount discounted from the total unroll cost when "
+ "the unrolled form has a high dynamic cost savings (triggered by "
+ "the '-unroll-perecent-dynamic-cost-saved-threshold' flag)."));
static cl::opt<unsigned> UnrollMaxIterationsCountToAnalyze(
"unroll-max-iteration-count-to-analyze", cl::init(0), 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> UnrollMinPercentOfOptimized(
- "unroll-percent-of-optimized-for-complete-unroll", cl::init(20), cl::Hidden,
- cl::desc("If complete unrolling could trigger further optimizations, and, "
- "by that, remove the given percent of instructions, perform the "
- "complete unroll even if it's beyond the threshold"));
-
-static cl::opt<unsigned> UnrollAbsoluteThreshold(
- "unroll-absolute-threshold", cl::init(2000), cl::Hidden,
- cl::desc("Don't unroll if the unrolled size is bigger than this threshold,"
- " even if we can remove big portion of instructions later."));
-
static cl::opt<unsigned>
UnrollCount("unroll-count", cl::init(0), cl::Hidden,
cl::desc("Use this unroll count for all loops including those with "
static char ID; // Pass ID, replacement for typeid
LoopUnroll(int T = -1, int C = -1, int P = -1, int R = -1) : LoopPass(ID) {
CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T);
- CurrentAbsoluteThreshold = UnrollAbsoluteThreshold;
- CurrentMinPercentOfOptimized = UnrollMinPercentOfOptimized;
+ CurrentPercentDynamicCostSavedThreshold =
+ UnrollPercentDynamicCostSavedThreshold;
+ CurrentDynamicCostSavingsDiscount = UnrollDynamicCostSavingsDiscount;
CurrentCount = (C == -1) ? UnrollCount : unsigned(C);
CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P;
CurrentRuntime = (R == -1) ? UnrollRuntime : (bool)R;
UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0);
- UserAbsoluteThreshold = (UnrollAbsoluteThreshold.getNumOccurrences() > 0);
- UserPercentOfOptimized =
- (UnrollMinPercentOfOptimized.getNumOccurrences() > 0);
+ UserPercentDynamicCostSavedThreshold =
+ (UnrollPercentDynamicCostSavedThreshold.getNumOccurrences() > 0);
+ UserDynamicCostSavingsDiscount =
+ (UnrollDynamicCostSavingsDiscount.getNumOccurrences() > 0);
UserAllowPartial = (P != -1) ||
(UnrollAllowPartial.getNumOccurrences() > 0);
UserRuntime = (R != -1) || (UnrollRuntime.getNumOccurrences() > 0);
unsigned CurrentCount;
unsigned CurrentThreshold;
- unsigned CurrentAbsoluteThreshold;
- unsigned CurrentMinPercentOfOptimized;
- bool CurrentAllowPartial;
- bool CurrentRuntime;
- bool UserCount; // CurrentCount is user-specified.
- bool UserThreshold; // CurrentThreshold is user-specified.
- bool UserAbsoluteThreshold; // CurrentAbsoluteThreshold is
- // user-specified.
- bool UserPercentOfOptimized; // CurrentMinPercentOfOptimized is
- // user-specified.
- bool UserAllowPartial; // CurrentAllowPartial is user-specified.
- bool UserRuntime; // CurrentRuntime is user-specified.
+ unsigned CurrentPercentDynamicCostSavedThreshold;
+ unsigned CurrentDynamicCostSavingsDiscount;
+ bool CurrentAllowPartial;
+ bool CurrentRuntime;
+
+ // Flags for whether the 'current' settings are user-specified.
+ bool UserCount;
+ bool UserThreshold;
+ bool UserPercentDynamicCostSavedThreshold;
+ bool UserDynamicCostSavingsDiscount;
+ bool UserAllowPartial;
+ bool UserRuntime;
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
void getUnrollingPreferences(Loop *L, const TargetTransformInfo &TTI,
TargetTransformInfo::UnrollingPreferences &UP) {
UP.Threshold = CurrentThreshold;
- UP.AbsoluteThreshold = CurrentAbsoluteThreshold;
- UP.MinPercentOfOptimized = CurrentMinPercentOfOptimized;
+ UP.PercentDynamicCostSavedThreshold =
+ CurrentPercentDynamicCostSavedThreshold;
+ UP.DynamicCostSavingsDiscount = CurrentDynamicCostSavingsDiscount;
UP.OptSizeThreshold = OptSizeUnrollThreshold;
UP.PartialThreshold = CurrentThreshold;
UP.PartialOptSizeThreshold = OptSizeUnrollThreshold;
void selectThresholds(const Loop *L, bool HasPragma,
const TargetTransformInfo::UnrollingPreferences &UP,
unsigned &Threshold, unsigned &PartialThreshold,
- unsigned &AbsoluteThreshold,
- unsigned &PercentOfOptimizedForCompleteUnroll) {
+ unsigned &PercentDynamicCostSavedThreshold,
+ unsigned &DynamicCostSavingsDiscount) {
// 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
// specified.
Threshold = UserThreshold ? CurrentThreshold : UP.Threshold;
PartialThreshold = UserThreshold ? CurrentThreshold : UP.PartialThreshold;
- AbsoluteThreshold = UserAbsoluteThreshold ? CurrentAbsoluteThreshold
- : UP.AbsoluteThreshold;
- PercentOfOptimizedForCompleteUnroll = UserPercentOfOptimized
- ? CurrentMinPercentOfOptimized
- : UP.MinPercentOfOptimized;
+ PercentDynamicCostSavedThreshold =
+ UserPercentDynamicCostSavedThreshold
+ ? CurrentPercentDynamicCostSavedThreshold
+ : UP.PercentDynamicCostSavedThreshold;
+ DynamicCostSavingsDiscount = UserDynamicCostSavingsDiscount
+ ? CurrentDynamicCostSavingsDiscount
+ : UP.DynamicCostSavingsDiscount;
if (!UserThreshold &&
L->getHeader()->getParent()->hasFnAttribute(
}
}
bool canUnrollCompletely(Loop *L, unsigned Threshold,
- unsigned AbsoluteThreshold, uint64_t UnrolledSize,
- unsigned NumberOfOptimizedInstructions,
- unsigned PercentOfOptimizedForCompleteUnroll);
+ unsigned PercentDynamicCostSavedThreshold,
+ unsigned DynamicCostSavingsDiscount,
+ uint64_t UnrolledCost, uint64_t RolledDynamicCost);
};
}
}
namespace {
-/// \brief SCEV expressions visitor used for finding expressions that would
-/// become constants if the loop L is unrolled.
-struct FindConstantPointers {
- /// \brief Shows whether the expression is ConstAddress+Constant or not.
- bool IndexIsConstant;
-
- /// \brief Used for filtering out SCEV expressions with two or more AddRec
- /// subexpressions.
- ///
- /// Used to filter out complicated SCEV expressions, having several AddRec
- /// sub-expressions. We don't handle them, because unrolling one loop
- /// would help to replace only one of these inductions with a constant, and
- /// consequently, the expression would remain non-constant.
- bool HaveSeenAR;
-
- /// \brief If the SCEV expression becomes ConstAddress+Constant, this value
- /// holds ConstAddress. Otherwise, it's nullptr.
- Value *BaseAddress;
-
- /// \brief The loop, which we try to completely unroll.
- const Loop *L;
-
- ScalarEvolution &SE;
-
- FindConstantPointers(const Loop *L, ScalarEvolution &SE)
- : IndexIsConstant(true), HaveSeenAR(false), BaseAddress(nullptr),
- L(L), SE(SE) {}
-
- /// Examine the given expression S and figure out, if it can be a part of an
- /// expression, that could become a constant after the loop is unrolled.
- /// The routine sets IndexIsConstant and HaveSeenAR according to the analysis
- /// results.
- /// \returns true if we need to examine subexpressions, and false otherwise.
- 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.
- // If it's the only one SCEV-subexpression, then it might be a base
- // address of an index expression.
- // If we've already recorded base address, then just give up on this SCEV
- // - it's too complicated.
- if (BaseAddress) {
- IndexIsConstant = false;
- return false;
- }
- BaseAddress = SC->getValue();
- return false;
- }
- if (isa<SCEVConstant>(S))
- return false;
- 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) {
- IndexIsConstant = false;
- return false;
- }
- // We don't handle multiple AddRecs here, so give up in this case.
- if (HaveSeenAR) {
- IndexIsConstant = false;
- return false;
- }
- HaveSeenAR = true;
- }
-
- // Continue traversal.
- return true;
- }
- 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
// 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>;
-
- struct SCEVGEPDescriptor {
- Value *BaseAddr;
- unsigned Start;
- unsigned Step;
+class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
+ typedef InstVisitor<UnrolledInstAnalyzer, bool> Base;
+ friend class InstVisitor<UnrolledInstAnalyzer, bool>;
+ struct SimplifiedAddress {
+ Value *Base = nullptr;
+ ConstantInt *Offset = nullptr;
};
- /// \brief The loop we're going to analyze.
- const Loop *L;
-
- /// \brief TripCount of the given loop.
- unsigned TripCount;
-
- ScalarEvolution &SE;
-
- const TargetTransformInfo &TTI;
+public:
+ UnrolledInstAnalyzer(unsigned Iteration,
+ DenseMap<Value *, Constant *> &SimplifiedValues,
+ const Loop *L, ScalarEvolution &SE)
+ : Iteration(Iteration), SimplifiedValues(SimplifiedValues), L(L), SE(SE) {
+ IterationNumber = SE.getConstant(APInt(64, Iteration));
+ }
- // While we walk the loop instructions, we we build up and maintain a mapping
- // of simplified values specific to this iteration. The idea is to propagate
- // any special information we have about loads that can be replaced with
- // constants after complete unrolling, and account for likely simplifications
- // post-unrolling.
- DenseMap<Value *, Constant *> SimplifiedValues;
+ // Allow access to the initial visit method.
+ using Base::visit;
- // To avoid requesting SCEV info on every iteration, request it once, and
- // for each value that would become ConstAddress+Constant after loop
- // unrolling, save the corresponding data.
- SmallDenseMap<Value *, SCEVGEPDescriptor> SCEVCache;
+private:
+ /// \brief A cache of pointer bases and constant-folded offsets corresponding
+ /// to GEP (or derived from GEP) instructions.
+ ///
+ /// In order to find the base pointer one needs to perform non-trivial
+ /// traversal of the corresponding SCEV expression, so it's good to have the
+ /// results saved.
+ DenseMap<Value *, SimplifiedAddress> SimplifiedAddresses;
/// \brief Number of currently simulated iteration.
///
/// If an expression is ConstAddress+Constant, then the Constant is
/// Start + Iteration*Step, where Start and Step could be obtained from
- /// SCEVCache.
+ /// SCEVGEPCache.
unsigned Iteration;
- /// \brief Upper threshold for complete unrolling.
- unsigned MaxUnrolledLoopSize;
+ /// \brief SCEV expression corresponding to number of currently simulated
+ /// iteration.
+ const SCEV *IterationNumber;
+
+ /// \brief A Value->Constant map for keeping values that we managed to
+ /// constant-fold on the given iteration.
+ ///
+ /// While we walk the loop instructions, we build up and maintain a mapping
+ /// of simplified values specific to this iteration. The idea is to propagate
+ /// any special information we have about loads that can be replaced with
+ /// constants after complete unrolling, and account for likely simplifications
+ /// post-unrolling.
+ DenseMap<Value *, Constant *> &SimplifiedValues;
+
+ const Loop *L;
+ ScalarEvolution &SE;
+
+ /// \brief Try to simplify instruction \param I using its SCEV expression.
+ ///
+ /// The idea is that some AddRec expressions become constants, which then
+ /// could trigger folding of other instructions. However, that only happens
+ /// for expressions whose start value is also constant, which isn't always the
+ /// case. In another common and important case the start value is just some
+ /// address (i.e. SCEVUnknown) - in this case we compute the offset and save
+ /// it along with the base address instead.
+ bool simplifyInstWithSCEV(Instruction *I) {
+ if (!SE.isSCEVable(I->getType()))
+ return false;
+
+ const SCEV *S = SE.getSCEV(I);
+ if (auto *SC = dyn_cast<SCEVConstant>(S)) {
+ SimplifiedValues[I] = SC->getValue();
+ return true;
+ }
+
+ auto *AR = dyn_cast<SCEVAddRecExpr>(S);
+ if (!AR)
+ return false;
+
+ const SCEV *ValueAtIteration = AR->evaluateAtIteration(IterationNumber, SE);
+ // Check if the AddRec expression becomes a constant.
+ if (auto *SC = dyn_cast<SCEVConstant>(ValueAtIteration)) {
+ SimplifiedValues[I] = SC->getValue();
+ return true;
+ }
+
+ // Check if the offset from the base address becomes a constant.
+ auto *Base = dyn_cast<SCEVUnknown>(SE.getPointerBase(S));
+ if (!Base)
+ return false;
+ auto *Offset =
+ dyn_cast<SCEVConstant>(SE.getMinusSCEV(ValueAtIteration, Base));
+ if (!Offset)
+ return false;
+ SimplifiedAddress Address;
+ Address.Base = Base->getValue();
+ Address.Offset = Offset->getValue();
+ SimplifiedAddresses[I] = Address;
+ return true;
+ }
/// Base case for the instruction visitor.
- bool visitInstruction(Instruction &I) { return false; };
+ bool visitInstruction(Instruction &I) {
+ return simplifyInstWithSCEV(&I);
+ }
/// TODO: Add visitors for other instruction types, e.g. ZExt, SExt.
if (!isa<Constant>(RHS))
if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
RHS = SimpleRHS;
+
Value *SimpleV = nullptr;
const DataLayout &DL = I.getModule()->getDataLayout();
if (auto FI = dyn_cast<FPMathOperator>(&I))
else
SimpleV = SimplifyBinOp(I.getOpcode(), LHS, RHS, DL);
- if (SimpleV)
- NumberOfOptimizedInstructions += TTI.getUserCost(&I);
-
- if (Constant *C = dyn_cast_or_null<Constant>(SimpleV)) {
+ if (Constant *C = dyn_cast_or_null<Constant>(SimpleV))
SimplifiedValues[&I] = C;
+
+ if (SimpleV)
return true;
- }
- return false;
+ return Base::visitBinaryOperator(I);
}
/// Try to fold load I.
bool visitLoad(LoadInst &I) {
Value *AddrOp = I.getPointerOperand();
- if (!isa<Constant>(AddrOp))
- if (Constant *SimplifiedAddrOp = SimplifiedValues.lookup(AddrOp))
- AddrOp = SimplifiedAddrOp;
- auto It = SCEVCache.find(AddrOp);
- if (It == SCEVCache.end())
+ auto AddressIt = SimplifiedAddresses.find(AddrOp);
+ if (AddressIt == SimplifiedAddresses.end())
return false;
- SCEVGEPDescriptor GEPDesc = It->second;
+ ConstantInt *SimplifiedAddrOp = AddressIt->second.Offset;
- auto GV = dyn_cast<GlobalVariable>(GEPDesc.BaseAddr);
+ auto *GV = dyn_cast<GlobalVariable>(AddressIt->second.Base);
// We're only interested in loads that can be completely folded to a
// constant.
if (!GV || !GV->hasInitializer())
if (!CDS)
return false;
- // This calculation should never overflow because we bound Iteration quite
- // low and both the start and step are 32-bit integers. We use signed
- // integers so that UBSan will catch if a bug sneaks into the code.
int ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
- int64_t Index = ((int64_t)GEPDesc.Start +
- (int64_t)GEPDesc.Step * (int64_t)Iteration) /
- ElemSize;
+ assert(SimplifiedAddrOp->getValue().getActiveBits() < 64 &&
+ "Unexpectedly large index value.");
+ int64_t Index = SimplifiedAddrOp->getSExtValue() / ElemSize;
if (Index >= CDS->getNumElements()) {
// FIXME: For now we conservatively ignore out of bound accesses, but
// we're allowed to perform the optimization in this case.
assert(CV && "Constant expected.");
SimplifiedValues[&I] = CV;
- NumberOfOptimizedInstructions += TTI.getUserCost(&I);
return true;
}
+};
+} // namespace
- /// Visit all GEPs in the loop and find those which after complete loop
- /// unrolling would become a constant, or BaseAddress+Constant.
- ///
- /// Such GEPs could allow to evaluate a load to a constant later - for now we
- /// just store the corresponding BaseAddress and StartValue with StepValue in
- /// the SCEVCache.
- void cacheSCEVResults() {
- for (auto BB : L->getBlocks()) {
- for (Instruction &I : *BB) {
- if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(&I)) {
- Value *V = cast<Value>(GEP);
- if (!SE.isSCEVable(V->getType()))
- continue;
- const SCEV *S = SE.getSCEV(V);
- // FIXME: Hoist the initialization out of the loop.
- FindConstantPointers Visitor(L, SE);
- SCEVTraversal<FindConstantPointers> T(Visitor);
- // Try to find (BaseAddress+Step+Offset) tuple.
- // If succeeded, save it to the cache - it might help in folding
- // loads.
- T.visitAll(S);
- if (!Visitor.IndexIsConstant || !Visitor.BaseAddress)
- continue;
-
- const SCEV *BaseAddrSE = SE.getSCEV(Visitor.BaseAddress);
- if (BaseAddrSE->getType() != S->getType())
- continue;
- const SCEV *OffSE = SE.getMinusSCEV(S, BaseAddrSE);
- const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(OffSE);
-
- if (!AR)
- continue;
-
- const SCEVConstant *StepSE =
- dyn_cast<SCEVConstant>(AR->getStepRecurrence(SE));
- const SCEVConstant *StartSE = dyn_cast<SCEVConstant>(AR->getStart());
- if (!StepSE || !StartSE)
- continue;
-
- // Check and skip caching if doing so would require lots of bits to
- // avoid overflow.
- APInt Start = StartSE->getValue()->getValue();
- APInt Step = StepSE->getValue()->getValue();
- if (Start.getActiveBits() > 32 || Step.getActiveBits() > 32)
- continue;
-
- // We found a cacheable SCEV model for the GEP.
- SCEVCache[V] = {Visitor.BaseAddress,
- (unsigned)Start.getLimitedValue(),
- (unsigned)Step.getLimitedValue()};
- }
- }
- }
- }
-public:
- UnrollAnalyzer(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
- const TargetTransformInfo &TTI, unsigned MaxUnrolledLoopSize)
- : L(L), TripCount(TripCount), SE(SE), TTI(TTI),
- MaxUnrolledLoopSize(MaxUnrolledLoopSize),
- NumberOfOptimizedInstructions(0), UnrolledLoopSize(0) {}
+namespace {
+struct EstimatedUnrollCost {
+ /// \brief The estimated cost after unrolling.
+ unsigned UnrolledCost;
- /// \brief Count the number of optimized instructions.
- unsigned NumberOfOptimizedInstructions;
+ /// \brief The estimated dynamic cost of executing the instructions in the
+ /// rolled form.
+ unsigned RolledDynamicCost;
+};
+}
- /// \brief Count the total number of instructions.
- unsigned UnrolledLoopSize;
+/// \brief Figure out if the loop is worth full unrolling.
+///
+/// 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. It computes cost of unrolled loop
+/// (UnrolledCost) and dynamic cost of the original loop (RolledDynamicCost). By
+/// dynamic cost we mean that we won't count costs of blocks that are known not
+/// to be executed (i.e. if we have a branch in the loop and we know that at the
+/// given iteration its condition would be resolved to true, we won't add up the
+/// cost of the 'false'-block).
+/// \returns Optional value, holding the RolledDynamicCost and UnrolledCost. If
+/// the analysis failed (no benefits expected from the unrolling, or the loop is
+/// too big to analyze), the returned value is None.
+Optional<EstimatedUnrollCost>
+analyzeLoopUnrollCost(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
+ const TargetTransformInfo &TTI,
+ unsigned MaxUnrolledLoopSize) {
+ // We want to be able to scale offsets by the trip count and add more offsets
+ // to them without checking for overflows, and we already don't want to
+ // analyze *massive* trip counts, so we force the max to be reasonably small.
+ assert(UnrollMaxIterationsCountToAnalyze < (INT_MAX / 2) &&
+ "The unroll iterations max is too large!");
+
+ // Don't simulate loops with a big or unknown tripcount
+ if (!UnrollMaxIterationsCountToAnalyze || !TripCount ||
+ TripCount > UnrollMaxIterationsCountToAnalyze)
+ return None;
+
+ SmallSetVector<BasicBlock *, 16> BBWorklist;
+ DenseMap<Value *, Constant *> SimplifiedValues;
- /// \brief Figure out if the loop is worth full unrolling.
- ///
- /// 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. It assigns computed number of instructions,
- /// that potentially might be optimized away, to
- /// NumberOfOptimizedInstructions, and total number of instructions to
- /// UnrolledLoopSize (not counting blocks that won't be reached, if we were
- /// able to compute the condition).
- /// \returns false if we can't analyze the loop, or if we discovered that
- /// unrolling won't give anything. Otherwise, returns true.
- bool analyzeLoop() {
- SmallSetVector<BasicBlock *, 16> BBWorklist;
-
- // We want to be able to scale offsets by the trip count and add more
- // offsets to them without checking for overflows, and we already don't want
- // to analyze *massive* trip counts, so we force the max to be reasonably
- // small.
- assert(UnrollMaxIterationsCountToAnalyze < (INT_MAX / 2) &&
- "The unroll iterations max is too large!");
-
- // Don't simulate loops with a big or unknown tripcount
- if (!UnrollMaxIterationsCountToAnalyze || !TripCount ||
- TripCount > UnrollMaxIterationsCountToAnalyze)
- return false;
+ // The estimated cost of the unrolled form of the loop. We try to estimate
+ // this by simplifying as much as we can while computing the estimate.
+ unsigned UnrolledCost = 0;
+ // We also track the estimated dynamic (that is, actually executed) cost in
+ // the rolled form. This helps identify cases when the savings from unrolling
+ // aren't just exposing dead control flows, but actual reduced dynamic
+ // instructions due to the simplifications which we expect to occur after
+ // unrolling.
+ unsigned RolledDynamicCost = 0;
+
+ // Simulate execution of each iteration of the loop counting instructions,
+ // which would be simplified.
+ // Since the same load will take different values on different iterations,
+ // we literally have to go through all loop's iterations.
+ for (unsigned Iteration = 0; Iteration < TripCount; ++Iteration) {
+ SimplifiedValues.clear();
+ UnrolledInstAnalyzer Analyzer(Iteration, SimplifiedValues, L, SE);
+
+ BBWorklist.clear();
+ BBWorklist.insert(L->getHeader());
+ // Note that we *must not* cache the size, this loop grows the worklist.
+ for (unsigned Idx = 0; Idx != BBWorklist.size(); ++Idx) {
+ BasicBlock *BB = BBWorklist[Idx];
+
+ // Visit all instructions in the given basic block and try to simplify
+ // it. We don't change the actual IR, just count optimization
+ // opportunities.
+ for (Instruction &I : *BB) {
+ unsigned InstCost = TTI.getUserCost(&I);
- // To avoid compute SCEV-expressions on every iteration, compute them once
- // and store interesting to us in SCEVCache.
- cacheSCEVResults();
-
- // Simulate execution of each iteration of the loop counting instructions,
- // which would be simplified.
- // Since the same load will take different values on different iterations,
- // we literally have to go through all loop's iterations.
- for (Iteration = 0; Iteration < TripCount; ++Iteration) {
- SimplifiedValues.clear();
- BBWorklist.clear();
- BBWorklist.insert(L->getHeader());
- // Note that we *must not* cache the size, this loop grows the worklist.
- for (unsigned Idx = 0; Idx != BBWorklist.size(); ++Idx) {
- BasicBlock *BB = BBWorklist[Idx];
-
- // Visit all instructions in the given basic block and try to simplify
- // it. We don't change the actual IR, just count optimization
- // opportunities.
- for (Instruction &I : *BB) {
- UnrolledLoopSize += TTI.getUserCost(&I);
- Base::visit(I);
- // If unrolled body turns out to be too big, bail out.
- if (UnrolledLoopSize - NumberOfOptimizedInstructions >
- MaxUnrolledLoopSize)
- return false;
- }
+ // Visit the instruction to analyze its loop cost after unrolling,
+ // and if the visitor returns false, include this instruction in the
+ // unrolled cost.
+ if (!Analyzer.visit(I))
+ UnrolledCost += InstCost;
+
+ // Also track this instructions expected cost when executing the rolled
+ // loop form.
+ RolledDynamicCost += InstCost;
- // Add BB's successors to the worklist.
- for (BasicBlock *Succ : successors(BB))
- if (L->contains(Succ))
- BBWorklist.insert(Succ);
+ // If unrolled body turns out to be too big, bail out.
+ if (UnrolledCost > MaxUnrolledLoopSize)
+ return None;
}
- // If we found no optimization opportunities on the first iteration, we
- // won't find them on later ones too.
- if (!NumberOfOptimizedInstructions)
- return false;
+ // Add BB's successors to the worklist.
+ for (BasicBlock *Succ : successors(BB))
+ if (L->contains(Succ))
+ BBWorklist.insert(Succ);
}
- return true;
+
+ // If we found no optimization opportunities on the first iteration, we
+ // won't find them on later ones too.
+ if (UnrolledCost == RolledDynamicCost)
+ return None;
}
-};
-} // namespace
+ return {{UnrolledCost, RolledDynamicCost}};
+}
/// ApproximateLoopSize - Approximate the size of the loop.
static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
L->setLoopID(NewLoopID);
}
-bool LoopUnroll::canUnrollCompletely(
- Loop *L, unsigned Threshold, unsigned AbsoluteThreshold,
- uint64_t UnrolledSize, unsigned NumberOfOptimizedInstructions,
- unsigned PercentOfOptimizedForCompleteUnroll) {
+bool LoopUnroll::canUnrollCompletely(Loop *L, unsigned Threshold,
+ unsigned PercentDynamicCostSavedThreshold,
+ unsigned DynamicCostSavingsDiscount,
+ uint64_t UnrolledCost,
+ uint64_t RolledDynamicCost) {
if (Threshold == NoThreshold) {
DEBUG(dbgs() << " Can fully unroll, because no threshold is set.\n");
return true;
}
- if (UnrolledSize <= Threshold) {
- DEBUG(dbgs() << " Can fully unroll, because unrolled size: "
- << UnrolledSize << "<" << Threshold << "\n");
+ if (UnrolledCost <= Threshold) {
+ DEBUG(dbgs() << " Can fully unroll, because unrolled cost: "
+ << UnrolledCost << "<" << Threshold << "\n");
return true;
}
- assert(UnrolledSize && "UnrolledSize can't be 0 at this point.");
- unsigned PercentOfOptimizedInstructions =
- (uint64_t)NumberOfOptimizedInstructions * 100ull / UnrolledSize;
-
- if (UnrolledSize <= AbsoluteThreshold &&
- PercentOfOptimizedInstructions >= PercentOfOptimizedForCompleteUnroll) {
- DEBUG(dbgs() << " Can fully unroll, because unrolling will help removing "
- << PercentOfOptimizedInstructions
- << "% instructions (threshold: "
- << PercentOfOptimizedForCompleteUnroll << "%)\n");
- DEBUG(dbgs() << " Unrolled size (" << UnrolledSize
- << ") is less than the threshold (" << AbsoluteThreshold
- << ").\n");
+ assert(UnrolledCost && "UnrolledCost can't be 0 at this point.");
+ assert(RolledDynamicCost >= UnrolledCost &&
+ "Cannot have a higher unrolled cost than a rolled cost!");
+
+ // Compute the percentage of the dynamic cost in the rolled form that is
+ // saved when unrolled. If unrolling dramatically reduces the estimated
+ // dynamic cost of the loop, we use a higher threshold to allow more
+ // unrolling.
+ unsigned PercentDynamicCostSaved =
+ (uint64_t)(RolledDynamicCost - UnrolledCost) * 100ull / RolledDynamicCost;
+
+ if (PercentDynamicCostSaved >= PercentDynamicCostSavedThreshold &&
+ (int64_t)UnrolledCost - (int64_t)DynamicCostSavingsDiscount <=
+ (int64_t)Threshold) {
+ DEBUG(dbgs() << " Can fully unroll, because unrolling will reduce the "
+ "expected dynamic cost by " << PercentDynamicCostSaved
+ << "% (threshold: " << PercentDynamicCostSavedThreshold
+ << "%)\n"
+ << " and the unrolled cost (" << UnrolledCost
+ << ") is less than the max threshold ("
+ << DynamicCostSavingsDiscount << ").\n");
return true;
}
DEBUG(dbgs() << " Too large to fully unroll:\n");
- DEBUG(dbgs() << " Unrolled size: " << UnrolledSize << "\n");
- DEBUG(dbgs() << " Estimated number of optimized instructions: "
- << NumberOfOptimizedInstructions << "\n");
- DEBUG(dbgs() << " Absolute threshold: " << AbsoluteThreshold << "\n");
- DEBUG(dbgs() << " Minimum percent of removed instructions: "
- << PercentOfOptimizedForCompleteUnroll << "\n");
- DEBUG(dbgs() << " Threshold for small loops: " << Threshold << "\n");
+ DEBUG(dbgs() << " Threshold: " << Threshold << "\n");
+ DEBUG(dbgs() << " Max threshold: " << DynamicCostSavingsDiscount << "\n");
+ DEBUG(dbgs() << " Percent cost saved threshold: "
+ << PercentDynamicCostSavedThreshold << "%\n");
+ DEBUG(dbgs() << " Unrolled cost: " << UnrolledCost << "\n");
+ DEBUG(dbgs() << " Rolled dynamic cost: " << RolledDynamicCost << "\n");
+ DEBUG(dbgs() << " Percent cost saved: " << PercentDynamicCostSaved
+ << "\n");
return false;
}
}
unsigned Threshold, PartialThreshold;
- unsigned AbsoluteThreshold, PercentOfOptimizedForCompleteUnroll;
+ unsigned PercentDynamicCostSavedThreshold;
+ unsigned DynamicCostSavingsDiscount;
selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold,
- AbsoluteThreshold, PercentOfOptimizedForCompleteUnroll);
+ PercentDynamicCostSavedThreshold,
+ DynamicCostSavingsDiscount);
// Given Count, TripCount and thresholds determine the type of
// unrolling which is to be performed.
if (TripCount && Count == TripCount) {
Unrolling = Partial;
// If the loop is really small, we don't need to run an expensive analysis.
- if (canUnrollCompletely(
- L, Threshold, AbsoluteThreshold,
- UnrolledSize, 0, 100)) {
+ if (canUnrollCompletely(L, Threshold, 100, DynamicCostSavingsDiscount,
+ UnrolledSize, UnrolledSize)) {
Unrolling = Full;
} else {
// The loop isn't that small, but we still can fully unroll it if that
// helps to remove a significant number of instructions.
// To check that, run additional analysis on the loop.
- UnrollAnalyzer UA(L, TripCount, *SE, TTI, AbsoluteThreshold);
- if (UA.analyzeLoop() &&
- canUnrollCompletely(L, Threshold, AbsoluteThreshold,
- UA.UnrolledLoopSize,
- UA.NumberOfOptimizedInstructions,
- PercentOfOptimizedForCompleteUnroll)) {
- Unrolling = Full;
- }
+ if (Optional<EstimatedUnrollCost> Cost = analyzeLoopUnrollCost(
+ L, TripCount, *SE, TTI, Threshold + DynamicCostSavingsDiscount))
+ if (canUnrollCompletely(L, Threshold, PercentDynamicCostSavedThreshold,
+ DynamicCostSavingsDiscount, Cost->UnrolledCost,
+ Cost->RolledDynamicCost)) {
+ Unrolling = Full;
+ }
}
} else if (TripCount && Count < TripCount) {
Unrolling = Partial;