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llvm-6502/lib/Transforms/Scalar/LoopUnrollPass.cpp
Chandler Carruth 36ae8103b1 [Unrolling] Refactor the start and step offsets to simplify overflow 
checking and make the cache faster and smaller.

I had thought that using an APInt here would be useful, but I think
I was just wrong. Notably, we don't have to do any fancy overflow
checking, we can just bound the values as quite small and do the math in
a higher precision integer. I've switched to a signed integer so that
UBSan will even point out if we ever have integer overflow. I've added
various asserts to try to catch things as well and hoisted the overflow
checks so that we just leave the too-large offsets out of the SCEV-GEP
cache. This makes the value in the cache quite a bit smaller which is
probably worthwhile.

No functionality changed here (for trip counts under 1 billion).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237209 91177308-0d34-0410-b5e6-96231b3b80d8
2015-05-12 23:32:56 +00:00

964 lines
38 KiB
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//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a simple loop unroller. It works best when loops have
// been canonicalized by the -indvars pass, allowing it to determine the trip
// counts of loops easily.
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/InstructionSimplify.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/IR/Dominators.h"
#include "llvm/IR/InstVisitor.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Metadata.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
#include <climits>
using namespace llvm;
#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"));
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 "
"unroll_count pragma values, for testing purposes"));
static cl::opt<bool>
UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
cl::desc("Allows loops to be partially unrolled until "
"-unroll-threshold loop size is reached."));
static cl::opt<bool>
UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
cl::desc("Unroll loops with run-time trip counts"));
static cl::opt<unsigned>
PragmaUnrollThreshold("pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden,
cl::desc("Unrolled size limit for loops with an unroll(full) or "
"unroll_count pragma."));
namespace {
class LoopUnroll : public LoopPass {
public:
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;
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);
UserAllowPartial = (P != -1) ||
(UnrollAllowPartial.getNumOccurrences() > 0);
UserRuntime = (R != -1) || (UnrollRuntime.getNumOccurrences() > 0);
UserCount = (C != -1) || (UnrollCount.getNumOccurrences() > 0);
initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
}
/// A magic value for use with the Threshold parameter to indicate
/// that the loop unroll should be performed regardless of how much
/// code expansion would result.
static const unsigned NoThreshold = UINT_MAX;
// Threshold to use when optsize is specified (and there is no
// explicit -unroll-threshold).
static const unsigned OptSizeUnrollThreshold = 50;
// Default unroll count for loops with run-time trip count if
// -unroll-count is not set
static const unsigned UnrollRuntimeCount = 8;
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.
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG...
///
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addRequiredID(LoopSimplifyID);
AU.addPreservedID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addPreservedID(LCSSAID);
AU.addRequired<ScalarEvolution>();
AU.addPreserved<ScalarEvolution>();
AU.addRequired<TargetTransformInfoWrapperPass>();
// FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
// If loop unroll does not preserve dom info then LCSSA pass on next
// loop will receive invalid dom info.
// For now, recreate dom info, if loop is unrolled.
AU.addPreserved<DominatorTreeWrapperPass>();
}
// Fill in the UnrollingPreferences parameter with values from the
// TargetTransformationInfo.
void getUnrollingPreferences(Loop *L, const TargetTransformInfo &TTI,
TargetTransformInfo::UnrollingPreferences &UP) {
UP.Threshold = CurrentThreshold;
UP.AbsoluteThreshold = CurrentAbsoluteThreshold;
UP.MinPercentOfOptimized = CurrentMinPercentOfOptimized;
UP.OptSizeThreshold = OptSizeUnrollThreshold;
UP.PartialThreshold = CurrentThreshold;
UP.PartialOptSizeThreshold = OptSizeUnrollThreshold;
UP.Count = CurrentCount;
UP.MaxCount = UINT_MAX;
UP.Partial = CurrentAllowPartial;
UP.Runtime = CurrentRuntime;
UP.AllowExpensiveTripCount = false;
TTI.getUnrollingPreferences(L, UP);
}
// Select and return an unroll count based on parameters from
// user, unroll preferences, unroll pragmas, or a heuristic.
// SetExplicitly is set to true if the unroll count is is set by
// the user or a pragma rather than selected heuristically.
unsigned
selectUnrollCount(const Loop *L, unsigned TripCount, bool PragmaFullUnroll,
unsigned PragmaCount,
const TargetTransformInfo::UnrollingPreferences &UP,
bool &SetExplicitly);
// Select threshold values used to limit unrolling based on a
// total unrolled size. Parameters Threshold and PartialThreshold
// are set to the maximum unrolled size for fully and partially
// unrolled loops respectively.
void selectThresholds(const Loop *L, bool HasPragma,
const TargetTransformInfo::UnrollingPreferences &UP,
unsigned &Threshold, unsigned &PartialThreshold,
unsigned &AbsoluteThreshold,
unsigned &PercentOfOptimizedForCompleteUnroll) {
// 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
// optimize-for-size, and the unroll threshold was not user
// specified.
Threshold = UserThreshold ? CurrentThreshold : UP.Threshold;
PartialThreshold = UserThreshold ? CurrentThreshold : UP.PartialThreshold;
AbsoluteThreshold = UserAbsoluteThreshold ? CurrentAbsoluteThreshold
: UP.AbsoluteThreshold;
PercentOfOptimizedForCompleteUnroll = UserPercentOfOptimized
? CurrentMinPercentOfOptimized
: UP.MinPercentOfOptimized;
if (!UserThreshold &&
L->getHeader()->getParent()->hasFnAttribute(
Attribute::OptimizeForSize)) {
Threshold = UP.OptSizeThreshold;
PartialThreshold = UP.PartialOptSizeThreshold;
}
if (HasPragma) {
// If the loop has an unrolling pragma, we want to be more
// aggressive with unrolling limits. Set thresholds to at
// least the PragmaTheshold value which is larger than the
// default limits.
if (Threshold != NoThreshold)
Threshold = std::max<unsigned>(Threshold, PragmaUnrollThreshold);
if (PartialThreshold != NoThreshold)
PartialThreshold =
std::max<unsigned>(PartialThreshold, PragmaUnrollThreshold);
}
}
bool canUnrollCompletely(Loop *L, unsigned Threshold,
unsigned AbsoluteThreshold, uint64_t UnrolledSize,
unsigned NumberOfOptimizedInstructions,
unsigned PercentOfOptimizedForCompleteUnroll);
};
}
char LoopUnroll::ID = 0;
INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSA)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial,
int Runtime) {
return new LoopUnroll(Threshold, Count, AllowPartial, Runtime);
}
Pass *llvm::createSimpleLoopUnrollPass() {
return llvm::createLoopUnrollPass(-1, -1, 0, 0);
}
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
// 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>;
struct SCEVGEPDescriptor {
Value *BaseAddr;
unsigned Start;
unsigned Step;
};
/// \brief The loop we're going to analyze.
const Loop *L;
/// \brief TripCount of the given loop.
unsigned TripCount;
ScalarEvolution &SE;
const TargetTransformInfo &TTI;
// 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;
// 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;
/// \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.
unsigned Iteration;
/// \brief Upper threshold for complete unrolling.
unsigned MaxUnrolledLoopSize;
/// Base case for the instruction visitor.
bool visitInstruction(Instruction &I) { return false; };
/// TODO: Add visitors for other instruction types, e.g. ZExt, SExt.
/// Try to simplify binary operator I.
///
/// TODO: 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 = nullptr;
const DataLayout &DL = I.getModule()->getDataLayout();
if (auto FI = dyn_cast<FPMathOperator>(&I))
SimpleV =
SimplifyFPBinOp(I.getOpcode(), LHS, RHS, FI->getFastMathFlags(), DL);
else
SimpleV = SimplifyBinOp(I.getOpcode(), LHS, RHS, DL);
if (SimpleV)
NumberOfOptimizedInstructions += TTI.getUserCost(&I);
if (Constant *C = dyn_cast_or_null<Constant>(SimpleV)) {
SimplifiedValues[&I] = C;
return true;
}
return false;
}
/// 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())
return false;
SCEVGEPDescriptor GEPDesc = It->second;
auto GV = dyn_cast<GlobalVariable>(GEPDesc.BaseAddr);
// We're only interested in loads that can be completely folded to a
// constant.
if (!GV || !GV->hasInitializer())
return false;
ConstantDataSequential *CDS =
dyn_cast<ConstantDataSequential>(GV->getInitializer());
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;
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.
return false;
}
Constant *CV = CDS->getElementAsConstant(Index);
assert(CV && "Constant expected.");
SimplifiedValues[&I] = CV;
NumberOfOptimizedInstructions += TTI.getUserCost(&I);
return true;
}
/// 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) {}
/// \brief Count the number of optimized instructions.
unsigned NumberOfOptimizedInstructions;
/// \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 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;
// 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;
}
// Add BB's successors to the worklist.
for (BasicBlock *Succ : successors(BB))
if (L->contains(Succ))
BBWorklist.insert(Succ);
}
// If we found no optimization opportunities on the first iteration, we
// won't find them on later ones too.
if (!NumberOfOptimizedInstructions)
return false;
}
return true;
}
};
} // namespace
/// ApproximateLoopSize - Approximate the size of the loop.
static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
bool &NotDuplicatable,
const TargetTransformInfo &TTI,
AssumptionCache *AC) {
SmallPtrSet<const Value *, 32> EphValues;
CodeMetrics::collectEphemeralValues(L, AC, EphValues);
CodeMetrics Metrics;
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
Metrics.analyzeBasicBlock(*I, TTI, EphValues);
NumCalls = Metrics.NumInlineCandidates;
NotDuplicatable = Metrics.notDuplicatable;
unsigned LoopSize = Metrics.NumInsts;
// Don't allow an estimate of size zero. This would allows unrolling of loops
// with huge iteration counts, which is a compile time problem even if it's
// not a problem for code quality. Also, the code using this size may assume
// that each loop has at least three instructions (likely a conditional
// branch, a comparison feeding that branch, and some kind of loop increment
// feeding that comparison instruction).
LoopSize = std::max(LoopSize, 3u);
return LoopSize;
}
// Returns the loop hint metadata node with the given name (for example,
// "llvm.loop.unroll.count"). If no such metadata node exists, then nullptr is
// returned.
static MDNode *GetUnrollMetadataForLoop(const Loop *L, StringRef Name) {
if (MDNode *LoopID = L->getLoopID())
return GetUnrollMetadata(LoopID, Name);
return nullptr;
}
// Returns true if the loop has an unroll(full) pragma.
static bool HasUnrollFullPragma(const Loop *L) {
return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.full");
}
// Returns true if the loop has an unroll(disable) pragma.
static bool HasUnrollDisablePragma(const Loop *L) {
return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.disable");
}
// Returns true if the loop has an runtime unroll(disable) pragma.
static bool HasRuntimeUnrollDisablePragma(const Loop *L) {
return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.runtime.disable");
}
// If loop has an unroll_count pragma return the (necessarily
// positive) value from the pragma. Otherwise return 0.
static unsigned UnrollCountPragmaValue(const Loop *L) {
MDNode *MD = GetUnrollMetadataForLoop(L, "llvm.loop.unroll.count");
if (MD) {
assert(MD->getNumOperands() == 2 &&
"Unroll count hint metadata should have two operands.");
unsigned Count =
mdconst::extract<ConstantInt>(MD->getOperand(1))->getZExtValue();
assert(Count >= 1 && "Unroll count must be positive.");
return Count;
}
return 0;
}
// Remove existing unroll metadata and add unroll disable metadata to
// indicate the loop has already been unrolled. This prevents a loop
// from being unrolled more than is directed by a pragma if the loop
// unrolling pass is run more than once (which it generally is).
static void SetLoopAlreadyUnrolled(Loop *L) {
MDNode *LoopID = L->getLoopID();
if (!LoopID) return;
// First remove any existing loop unrolling metadata.
SmallVector<Metadata *, 4> MDs;
// Reserve first location for self reference to the LoopID metadata node.
MDs.push_back(nullptr);
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
bool IsUnrollMetadata = false;
MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
if (MD) {
const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll.");
}
if (!IsUnrollMetadata)
MDs.push_back(LoopID->getOperand(i));
}
// Add unroll(disable) metadata to disable future unrolling.
LLVMContext &Context = L->getHeader()->getContext();
SmallVector<Metadata *, 1> DisableOperands;
DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable"));
MDNode *DisableNode = MDNode::get(Context, DisableOperands);
MDs.push_back(DisableNode);
MDNode *NewLoopID = MDNode::get(Context, MDs);
// Set operand 0 to refer to the loop id itself.
NewLoopID->replaceOperandWith(0, NewLoopID);
L->setLoopID(NewLoopID);
}
bool LoopUnroll::canUnrollCompletely(
Loop *L, unsigned Threshold, unsigned AbsoluteThreshold,
uint64_t UnrolledSize, unsigned NumberOfOptimizedInstructions,
unsigned PercentOfOptimizedForCompleteUnroll) {
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");
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");
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");
return false;
}
unsigned LoopUnroll::selectUnrollCount(
const Loop *L, unsigned TripCount, bool PragmaFullUnroll,
unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP,
bool &SetExplicitly) {
SetExplicitly = true;
// User-specified count (either as a command-line option or
// constructor parameter) has highest precedence.
unsigned Count = UserCount ? CurrentCount : 0;
// If there is no user-specified count, unroll pragmas have the next
// highest precendence.
if (Count == 0) {
if (PragmaCount) {
Count = PragmaCount;
} else if (PragmaFullUnroll) {
Count = TripCount;
}
}
if (Count == 0)
Count = UP.Count;
if (Count == 0) {
SetExplicitly = false;
if (TripCount == 0)
// Runtime trip count.
Count = UnrollRuntimeCount;
else
// Conservative heuristic: if we know the trip count, see if we can
// completely unroll (subject to the threshold, checked below); otherwise
// try to find greatest modulo of the trip count which is still under
// threshold value.
Count = TripCount;
}
if (TripCount && Count > TripCount)
return TripCount;
return Count;
}
bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipOptnoneFunction(L))
return false;
Function &F = *L->getHeader()->getParent();
LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
const TargetTransformInfo &TTI =
getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
BasicBlock *Header = L->getHeader();
DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
<< "] Loop %" << Header->getName() << "\n");
if (HasUnrollDisablePragma(L)) {
return false;
}
bool PragmaFullUnroll = HasUnrollFullPragma(L);
unsigned PragmaCount = UnrollCountPragmaValue(L);
bool HasPragma = PragmaFullUnroll || PragmaCount > 0;
TargetTransformInfo::UnrollingPreferences UP;
getUnrollingPreferences(L, TTI, UP);
// Find trip count and trip multiple if count is not available
unsigned TripCount = 0;
unsigned TripMultiple = 1;
// If there are multiple exiting blocks but one of them is the latch, use the
// latch for the trip count estimation. Otherwise insist on a single exiting
// block for the trip count estimation.
BasicBlock *ExitingBlock = L->getLoopLatch();
if (!ExitingBlock || !L->isLoopExiting(ExitingBlock))
ExitingBlock = L->getExitingBlock();
if (ExitingBlock) {
TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);
}
// Select an initial unroll count. This may be reduced later based
// on size thresholds.
bool CountSetExplicitly;
unsigned Count = selectUnrollCount(L, TripCount, PragmaFullUnroll,
PragmaCount, UP, CountSetExplicitly);
unsigned NumInlineCandidates;
bool notDuplicatable;
unsigned LoopSize =
ApproximateLoopSize(L, NumInlineCandidates, notDuplicatable, TTI, &AC);
DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
// When computing the unrolled size, note that the conditional branch on the
// backedge and the comparison feeding it are not replicated like the rest of
// the loop body (which is why 2 is subtracted).
uint64_t UnrolledSize = (uint64_t)(LoopSize-2) * Count + 2;
if (notDuplicatable) {
DEBUG(dbgs() << " Not unrolling loop which contains non-duplicatable"
<< " instructions.\n");
return false;
}
if (NumInlineCandidates != 0) {
DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n");
return false;
}
unsigned Threshold, PartialThreshold;
unsigned AbsoluteThreshold, PercentOfOptimizedForCompleteUnroll;
selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold,
AbsoluteThreshold, PercentOfOptimizedForCompleteUnroll);
// Given Count, TripCount and thresholds determine the type of
// unrolling which is to be performed.
enum { Full = 0, Partial = 1, Runtime = 2 };
int Unrolling;
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)) {
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;
}
}
} else if (TripCount && Count < TripCount) {
Unrolling = Partial;
} else {
Unrolling = Runtime;
}
// Reduce count based on the type of unrolling and the threshold values.
unsigned OriginalCount = Count;
bool AllowRuntime = UserRuntime ? CurrentRuntime : UP.Runtime;
if (HasRuntimeUnrollDisablePragma(L)) {
AllowRuntime = false;
}
if (Unrolling == Partial) {
bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial;
if (!AllowPartial && !CountSetExplicitly) {
DEBUG(dbgs() << " will not try to unroll partially because "
<< "-unroll-allow-partial not given\n");
return false;
}
if (PartialThreshold != NoThreshold && UnrolledSize > PartialThreshold) {
// Reduce unroll count to be modulo of TripCount for partial unrolling.
Count = (std::max(PartialThreshold, 3u)-2) / (LoopSize-2);
while (Count != 0 && TripCount % Count != 0)
Count--;
}
} else if (Unrolling == Runtime) {
if (!AllowRuntime && !CountSetExplicitly) {
DEBUG(dbgs() << " will not try to unroll loop with runtime trip count "
<< "-unroll-runtime not given\n");
return false;
}
// Reduce unroll count to be the largest power-of-two factor of
// the original count which satisfies the threshold limit.
while (Count != 0 && UnrolledSize > PartialThreshold) {
Count >>= 1;
UnrolledSize = (LoopSize-2) * Count + 2;
}
if (Count > UP.MaxCount)
Count = UP.MaxCount;
DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n");
}
if (HasPragma) {
if (PragmaCount != 0)
// If loop has an unroll count pragma mark loop as unrolled to prevent
// unrolling beyond that requested by the pragma.
SetLoopAlreadyUnrolled(L);
// Emit optimization remarks if we are unable to unroll the loop
// as directed by a pragma.
DebugLoc LoopLoc = L->getStartLoc();
Function *F = Header->getParent();
LLVMContext &Ctx = F->getContext();
if (PragmaFullUnroll && PragmaCount == 0) {
if (TripCount && Count != TripCount) {
emitOptimizationRemarkMissed(
Ctx, DEBUG_TYPE, *F, LoopLoc,
"Unable to fully unroll loop as directed by unroll(full) pragma "
"because unrolled size is too large.");
} else if (!TripCount) {
emitOptimizationRemarkMissed(
Ctx, DEBUG_TYPE, *F, LoopLoc,
"Unable to fully unroll loop as directed by unroll(full) pragma "
"because loop has a runtime trip count.");
}
} else if (PragmaCount > 0 && Count != OriginalCount) {
emitOptimizationRemarkMissed(
Ctx, DEBUG_TYPE, *F, LoopLoc,
"Unable to unroll loop the number of times directed by "
"unroll_count pragma because unrolled size is too large.");
}
}
if (Unrolling != Full && Count < 2) {
// Partial unrolling by 1 is a nop. For full unrolling, a factor
// of 1 makes sense because loop control can be eliminated.
return false;
}
// Unroll the loop.
if (!UnrollLoop(L, Count, TripCount, AllowRuntime, UP.AllowExpensiveTripCount,
TripMultiple, LI, this, &LPM, &AC))
return false;
return true;
}
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