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//===- InductiveRangeCheckElimination.cpp - -------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// The InductiveRangeCheckElimination pass splits a loop's iteration space into
// three disjoint ranges. It does that in a way such that the loop running in
// the middle loop provably does not need range checks. As an example, it will
// convert
//
// len = < known positive >
// for (i = 0; i < n; i++) {
// if (0 <= i && i < len) {
// do_something();
// } else {
// throw_out_of_bounds();
// }
// }
//
// to
//
// len = < known positive >
// limit = smin(n, len)
// // no first segment
// for (i = 0; i < limit; i++) {
// if (0 <= i && i < len) { // this check is fully redundant
// do_something();
// } else {
// throw_out_of_bounds();
// }
// }
// for (i = limit; i < n; i++) {
// if (0 <= i && i < len) {
// do_something();
// } else {
// throw_out_of_bounds();
// }
// }
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/InductiveRangeCheckElimination.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/PriorityWorklist.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <limits>
#include <utility>
#include <vector>
using namespace llvm;
using namespace llvm::PatternMatch;
static cl::opt<unsigned> LoopSizeCutoff("irce-loop-size-cutoff", cl::Hidden,
cl::init(64));
static cl::opt<bool> PrintChangedLoops("irce-print-changed-loops", cl::Hidden,
cl::init(false));
static cl::opt<bool> PrintRangeChecks("irce-print-range-checks", cl::Hidden,
cl::init(false));
static cl::opt<bool> SkipProfitabilityChecks("irce-skip-profitability-checks",
cl::Hidden, cl::init(false));
static cl::opt<unsigned> MinRuntimeIterations("irce-min-runtime-iterations",
cl::Hidden, cl::init(10));
static cl::opt<bool> AllowUnsignedLatchCondition("irce-allow-unsigned-latch",
cl::Hidden, cl::init(true));
static cl::opt<bool> AllowNarrowLatchCondition(
"irce-allow-narrow-latch", cl::Hidden, cl::init(true),
cl::desc("If set to true, IRCE may eliminate wide range checks in loops "
"with narrow latch condition."));
static const char *ClonedLoopTag = "irce.loop.clone";
#define DEBUG_TYPE "irce"
namespace {
/// An inductive range check is conditional branch in a loop with
///
/// 1. a very cold successor (i.e. the branch jumps to that successor very
/// rarely)
///
/// and
///
/// 2. a condition that is provably true for some contiguous range of values
/// taken by the containing loop's induction variable.
///
class InductiveRangeCheck {
const SCEV *Begin = nullptr;
const SCEV *Step = nullptr;
const SCEV *End = nullptr;
Use *CheckUse = nullptr;
static bool parseRangeCheckICmp(Loop *L, ICmpInst *ICI, ScalarEvolution &SE,
Value *&Index, Value *&Length,
bool &IsSigned);
static void
extractRangeChecksFromCond(Loop *L, ScalarEvolution &SE, Use &ConditionUse,
SmallVectorImpl<InductiveRangeCheck> &Checks,
SmallPtrSetImpl<Value *> &Visited);
public:
const SCEV *getBegin() const { return Begin; }
const SCEV *getStep() const { return Step; }
const SCEV *getEnd() const { return End; }
void print(raw_ostream &OS) const {
OS << "InductiveRangeCheck:\n";
OS << " Begin: ";
Begin->print(OS);
OS << " Step: ";
Step->print(OS);
OS << " End: ";
End->print(OS);
OS << "\n CheckUse: ";
getCheckUse()->getUser()->print(OS);
OS << " Operand: " << getCheckUse()->getOperandNo() << "\n";
}
LLVM_DUMP_METHOD
void dump() {
print(dbgs());
}
Use *getCheckUse() const { return CheckUse; }
/// Represents an signed integer range [Range.getBegin(), Range.getEnd()). If
/// R.getEnd() le R.getBegin(), then R denotes the empty range.
class Range {
const SCEV *Begin;
const SCEV *End;
public:
Range(const SCEV *Begin, const SCEV *End) : Begin(Begin), End(End) {
assert(Begin->getType() == End->getType() && "ill-typed range!");
}
Type *getType() const { return Begin->getType(); }
const SCEV *getBegin() const { return Begin; }
const SCEV *getEnd() const { return End; }
bool isEmpty(ScalarEvolution &SE, bool IsSigned) const {
if (Begin == End)
return true;
if (IsSigned)
return SE.isKnownPredicate(ICmpInst::ICMP_SGE, Begin, End);
else
return SE.isKnownPredicate(ICmpInst::ICMP_UGE, Begin, End);
}
};
/// This is the value the condition of the branch needs to evaluate to for the
/// branch to take the hot successor (see (1) above).
bool getPassingDirection() { return true; }
/// Computes a range for the induction variable (IndVar) in which the range
/// check is redundant and can be constant-folded away. The induction
/// variable is not required to be the canonical {0,+,1} induction variable.
Optional<Range> computeSafeIterationSpace(ScalarEvolution &SE,
const SCEVAddRecExpr *IndVar,
bool IsLatchSigned) const;
/// Parse out a set of inductive range checks from \p BI and append them to \p
/// Checks.
///
/// NB! There may be conditions feeding into \p BI that aren't inductive range
/// checks, and hence don't end up in \p Checks.
static void
extractRangeChecksFromBranch(BranchInst *BI, Loop *L, ScalarEvolution &SE,
BranchProbabilityInfo *BPI,
SmallVectorImpl<InductiveRangeCheck> &Checks);
};
struct LoopStructure;
class InductiveRangeCheckElimination {
ScalarEvolution &SE;
BranchProbabilityInfo *BPI;
DominatorTree &DT;
LoopInfo &LI;
using GetBFIFunc =
llvm::Optional<llvm::function_ref<llvm::BlockFrequencyInfo &()> >;
GetBFIFunc GetBFI;
// Returns true if it is profitable to do a transform basing on estimation of
// number of iterations.
bool isProfitableToTransform(const Loop &L, LoopStructure &LS);
public:
InductiveRangeCheckElimination(ScalarEvolution &SE,
BranchProbabilityInfo *BPI, DominatorTree &DT,
LoopInfo &LI, GetBFIFunc GetBFI = None)
: SE(SE), BPI(BPI), DT(DT), LI(LI), GetBFI(GetBFI) {}
bool run(Loop *L, function_ref<void(Loop *, bool)> LPMAddNewLoop);
};
class IRCELegacyPass : public FunctionPass {
public:
static char ID;
IRCELegacyPass() : FunctionPass(ID) {
initializeIRCELegacyPassPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<BranchProbabilityInfoWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addPreserved<LoopInfoWrapperPass>();
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addPreserved<ScalarEvolutionWrapperPass>();
}
bool runOnFunction(Function &F) override;
};
} // end anonymous namespace
char IRCELegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(IRCELegacyPass, "irce",
"Inductive range check elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(BranchProbabilityInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_END(IRCELegacyPass, "irce", "Inductive range check elimination",
false, false)
/// Parse a single ICmp instruction, `ICI`, into a range check. If `ICI` cannot
/// be interpreted as a range check, return false and set `Index` and `Length`
/// to `nullptr`. Otherwise set `Index` to the value being range checked, and
/// set `Length` to the upper limit `Index` is being range checked.
bool
InductiveRangeCheck::parseRangeCheckICmp(Loop *L, ICmpInst *ICI,
ScalarEvolution &SE, Value *&Index,
Value *&Length, bool &IsSigned) {
auto IsLoopInvariant = [&SE, L](Value *V) {
return SE.isLoopInvariant(SE.getSCEV(V), L);
};
ICmpInst::Predicate Pred = ICI->getPredicate();
Value *LHS = ICI->getOperand(0);
Value *RHS = ICI->getOperand(1);
switch (Pred) {
default:
return false;
case ICmpInst::ICMP_SLE:
std::swap(LHS, RHS);
LLVM_FALLTHROUGH;
case ICmpInst::ICMP_SGE:
IsSigned = true;
if (match(RHS, m_ConstantInt<0>())) {
Index = LHS;
return true; // Lower.
}
return false;
case ICmpInst::ICMP_SLT:
std::swap(LHS, RHS);
LLVM_FALLTHROUGH;
case ICmpInst::ICMP_SGT:
IsSigned = true;
if (match(RHS, m_ConstantInt<-1>())) {
Index = LHS;
return true; // Lower.
}
if (IsLoopInvariant(LHS)) {
Index = RHS;
Length = LHS;
return true; // Upper.
}
return false;
case ICmpInst::ICMP_ULT:
std::swap(LHS, RHS);
LLVM_FALLTHROUGH;
case ICmpInst::ICMP_UGT:
IsSigned = false;
if (IsLoopInvariant(LHS)) {
Index = RHS;
Length = LHS;
return true; // Both lower and upper.
}
return false;
}
llvm_unreachable("default clause returns!");
}
void InductiveRangeCheck::extractRangeChecksFromCond(
Loop *L, ScalarEvolution &SE, Use &ConditionUse,
SmallVectorImpl<InductiveRangeCheck> &Checks,
SmallPtrSetImpl<Value *> &Visited) {
Value *Condition = ConditionUse.get();
if (!Visited.insert(Condition).second)
return;
// TODO: Do the same for OR, XOR, NOT etc?
if (match(Condition, m_And(m_Value(), m_Value()))) {
extractRangeChecksFromCond(L, SE, cast<User>(Condition)->getOperandUse(0),
Checks, Visited);
extractRangeChecksFromCond(L, SE, cast<User>(Condition)->getOperandUse(1),
Checks, Visited);
return;
}
ICmpInst *ICI = dyn_cast<ICmpInst>(Condition);
if (!ICI)
return;
Value *Length = nullptr, *Index;
bool IsSigned;
if (!parseRangeCheckICmp(L, ICI, SE, Index, Length, IsSigned))
return;
const auto *IndexAddRec = dyn_cast<SCEVAddRecExpr>(SE.getSCEV(Index));
bool IsAffineIndex =
IndexAddRec && (IndexAddRec->getLoop() == L) && IndexAddRec->isAffine();
if (!IsAffineIndex)
return;
const SCEV *End = nullptr;
// We strengthen "0 <= I" to "0 <= I < INT_SMAX" and "I < L" to "0 <= I < L".
// We can potentially do much better here.
if (Length)
End = SE.getSCEV(Length);
else {
// So far we can only reach this point for Signed range check. This may
// change in future. In this case we will need to pick Unsigned max for the
// unsigned range check.
unsigned BitWidth = cast<IntegerType>(IndexAddRec->getType())->getBitWidth();
const SCEV *SIntMax = SE.getConstant(APInt::getSignedMaxValue(BitWidth));
End = SIntMax;
}
InductiveRangeCheck IRC;
IRC.End = End;
IRC.Begin = IndexAddRec->getStart();
IRC.Step = IndexAddRec->getStepRecurrence(SE);
IRC.CheckUse = &ConditionUse;
Checks.push_back(IRC);
}
void InductiveRangeCheck::extractRangeChecksFromBranch(
BranchInst *BI, Loop *L, ScalarEvolution &SE, BranchProbabilityInfo *BPI,
SmallVectorImpl<InductiveRangeCheck> &Checks) {
if (BI->isUnconditional() || BI->getParent() == L->getLoopLatch())
return;
BranchProbability LikelyTaken(15, 16);
if (!SkipProfitabilityChecks && BPI &&
BPI->getEdgeProbability(BI->getParent(), (unsigned)0) < LikelyTaken)
return;
SmallPtrSet<Value *, 8> Visited;
InductiveRangeCheck::extractRangeChecksFromCond(L, SE, BI->getOperandUse(0),
Checks, Visited);
}
// Add metadata to the loop L to disable loop optimizations. Callers need to
// confirm that optimizing loop L is not beneficial.
static void DisableAllLoopOptsOnLoop(Loop &L) {
// We do not care about any existing loopID related metadata for L, since we
// are setting all loop metadata to false.
LLVMContext &Context = L.getHeader()->getContext();
// Reserve first location for self reference to the LoopID metadata node.
MDNode *Dummy = MDNode::get(Context, {});
MDNode *DisableUnroll = MDNode::get(
Context, {MDString::get(Context, "llvm.loop.unroll.disable")});
Metadata *FalseVal =
ConstantAsMetadata::get(ConstantInt::get(Type::getInt1Ty(Context), 0));
MDNode *DisableVectorize = MDNode::get(
Context,
{MDString::get(Context, "llvm.loop.vectorize.enable"), FalseVal});
MDNode *DisableLICMVersioning = MDNode::get(
Context, {MDString::get(Context, "llvm.loop.licm_versioning.disable")});
MDNode *DisableDistribution= MDNode::get(
Context,
{MDString::get(Context, "llvm.loop.distribute.enable"), FalseVal});
MDNode *NewLoopID =
MDNode::get(Context, {Dummy, DisableUnroll, DisableVectorize,
DisableLICMVersioning, DisableDistribution});
// Set operand 0 to refer to the loop id itself.
NewLoopID->replaceOperandWith(0, NewLoopID);
L.setLoopID(NewLoopID);
}
namespace {
// Keeps track of the structure of a loop. This is similar to llvm::Loop,
// except that it is more lightweight and can track the state of a loop through
// changing and potentially invalid IR. This structure also formalizes the
// kinds of loops we can deal with -- ones that have a single latch that is also
// an exiting block *and* have a canonical induction variable.
struct LoopStructure {
const char *Tag = "";
BasicBlock *Header = nullptr;
BasicBlock *Latch = nullptr;
// `Latch's terminator instruction is `LatchBr', and it's `LatchBrExitIdx'th
// successor is `LatchExit', the exit block of the loop.
BranchInst *LatchBr = nullptr;
BasicBlock *LatchExit = nullptr;
unsigned LatchBrExitIdx = std::numeric_limits<unsigned>::max();
// The loop represented by this instance of LoopStructure is semantically
// equivalent to:
//
// intN_ty inc = IndVarIncreasing ? 1 : -1;
// pred_ty predicate = IndVarIncreasing ? ICMP_SLT : ICMP_SGT;
//
// for (intN_ty iv = IndVarStart; predicate(iv, LoopExitAt); iv = IndVarBase)
// ... body ...
Value *IndVarBase = nullptr;
Value *IndVarStart = nullptr;
Value *IndVarStep = nullptr;
Value *LoopExitAt = nullptr;
bool IndVarIncreasing = false;
bool IsSignedPredicate = true;
LoopStructure() = default;
template <typename M> LoopStructure map(M Map) const {
LoopStructure Result;
Result.Tag = Tag;
Result.Header = cast<BasicBlock>(Map(Header));
Result.Latch = cast<BasicBlock>(Map(Latch));
Result.LatchBr = cast<BranchInst>(Map(LatchBr));
Result.LatchExit = cast<BasicBlock>(Map(LatchExit));
Result.LatchBrExitIdx = LatchBrExitIdx;
Result.IndVarBase = Map(IndVarBase);
Result.IndVarStart = Map(IndVarStart);
Result.IndVarStep = Map(IndVarStep);
Result.LoopExitAt = Map(LoopExitAt);
Result.IndVarIncreasing = IndVarIncreasing;
Result.IsSignedPredicate = IsSignedPredicate;
return Result;
}
static Optional<LoopStructure> parseLoopStructure(ScalarEvolution &, Loop &,
const char *&);
};
/// This class is used to constrain loops to run within a given iteration space.
/// The algorithm this class implements is given a Loop and a range [Begin,
/// End). The algorithm then tries to break out a "main loop" out of the loop
/// it is given in a way that the "main loop" runs with the induction variable
/// in a subset of [Begin, End). The algorithm emits appropriate pre and post
/// loops to run any remaining iterations. The pre loop runs any iterations in
/// which the induction variable is < Begin, and the post loop runs any
/// iterations in which the induction variable is >= End.
class LoopConstrainer {
// The representation of a clone of the original loop we started out with.
struct ClonedLoop {
// The cloned blocks
std::vector<BasicBlock *> Blocks;
// `Map` maps values in the clonee into values in the cloned version
ValueToValueMapTy Map;
// An instance of `LoopStructure` for the cloned loop
LoopStructure Structure;
};
// Result of rewriting the range of a loop. See changeIterationSpaceEnd for
// more details on what these fields mean.
struct RewrittenRangeInfo {
BasicBlock *PseudoExit = nullptr;
BasicBlock *ExitSelector = nullptr;
std::vector<PHINode *> PHIValuesAtPseudoExit;
PHINode *IndVarEnd = nullptr;
RewrittenRangeInfo() = default;
};
// Calculated subranges we restrict the iteration space of the main loop to.
// See the implementation of `calculateSubRanges' for more details on how
// these fields are computed. `LowLimit` is None if there is no restriction
// on low end of the restricted iteration space of the main loop. `HighLimit`
// is None if there is no restriction on high end of the restricted iteration
// space of the main loop.
struct SubRanges {
Optional<const SCEV *> LowLimit;
Optional<const SCEV *> HighLimit;
};
// Compute a safe set of limits for the main loop to run in -- effectively the
// intersection of `Range' and the iteration space of the original loop.
// Return None if unable to compute the set of subranges.
Optional<SubRanges> calculateSubRanges(bool IsSignedPredicate) const;
// Clone `OriginalLoop' and return the result in CLResult. The IR after
// running `cloneLoop' is well formed except for the PHI nodes in CLResult --
// the PHI nodes say that there is an incoming edge from `OriginalPreheader`
// but there is no such edge.
void cloneLoop(ClonedLoop &CLResult, const char *Tag) const;
// Create the appropriate loop structure needed to describe a cloned copy of
// `Original`. The clone is described by `VM`.
Loop *createClonedLoopStructure(Loop *Original, Loop *Parent,
ValueToValueMapTy &VM, bool IsSubloop);
// Rewrite the iteration space of the loop denoted by (LS, Preheader). The
// iteration space of the rewritten loop ends at ExitLoopAt. The start of the
// iteration space is not changed. `ExitLoopAt' is assumed to be slt
// `OriginalHeaderCount'.
//
// If there are iterations left to execute, control is made to jump to
// `ContinuationBlock', otherwise they take the normal loop exit. The
// returned `RewrittenRangeInfo' object is populated as follows:
//
// .PseudoExit is a basic block that unconditionally branches to
// `ContinuationBlock'.
//
// .ExitSelector is a basic block that decides, on exit from the loop,
// whether to branch to the "true" exit or to `PseudoExit'.
//
// .PHIValuesAtPseudoExit are PHINodes in `PseudoExit' that compute the value
// for each PHINode in the loop header on taking the pseudo exit.
//
// After changeIterationSpaceEnd, `Preheader' is no longer a legitimate
// preheader because it is made to branch to the loop header only
// conditionally.
RewrittenRangeInfo
changeIterationSpaceEnd(const LoopStructure &LS, BasicBlock *Preheader,
Value *ExitLoopAt,
BasicBlock *ContinuationBlock) const;
// The loop denoted by `LS' has `OldPreheader' as its preheader. This
// function creates a new preheader for `LS' and returns it.
BasicBlock *createPreheader(const LoopStructure &LS, BasicBlock *OldPreheader,
const char *Tag) const;
// `ContinuationBlockAndPreheader' was the continuation block for some call to
// `changeIterationSpaceEnd' and is the preheader to the loop denoted by `LS'.
// This function rewrites the PHI nodes in `LS.Header' to start with the
// correct value.
void rewriteIncomingValuesForPHIs(
LoopStructure &LS, BasicBlock *ContinuationBlockAndPreheader,
const LoopConstrainer::RewrittenRangeInfo &RRI) const;
// Even though we do not preserve any passes at this time, we at least need to
// keep the parent loop structure consistent. The `LPPassManager' seems to
// verify this after running a loop pass. This function adds the list of
// blocks denoted by BBs to this loops parent loop if required.
void addToParentLoopIfNeeded(ArrayRef<BasicBlock *> BBs);
// Some global state.
Function &F;
LLVMContext &Ctx;
ScalarEvolution &SE;
DominatorTree &DT;
LoopInfo &LI;
function_ref<void(Loop *, bool)> LPMAddNewLoop;
// Information about the original loop we started out with.
Loop &OriginalLoop;
const SCEV *LatchTakenCount = nullptr;
BasicBlock *OriginalPreheader = nullptr;
// The preheader of the main loop. This may or may not be different from
// `OriginalPreheader'.
BasicBlock *MainLoopPreheader = nullptr;
// The range we need to run the main loop in.
InductiveRangeCheck::Range Range;
// The structure of the main loop (see comment at the beginning of this class
// for a definition)
LoopStructure MainLoopStructure;
public:
LoopConstrainer(Loop &L, LoopInfo &LI,
function_ref<void(Loop *, bool)> LPMAddNewLoop,
const LoopStructure &LS, ScalarEvolution &SE,
DominatorTree &DT, InductiveRangeCheck::Range R)
: F(*L.getHeader()->getParent()), Ctx(L.getHeader()->getContext()),
SE(SE), DT(DT), LI(LI), LPMAddNewLoop(LPMAddNewLoop), OriginalLoop(L),
Range(R), MainLoopStructure(LS) {}
// Entry point for the algorithm. Returns true on success.
bool run();
};
} // end anonymous namespace
/// Given a loop with an deccreasing induction variable, is it possible to
/// safely calculate the bounds of a new loop using the given Predicate.
static bool isSafeDecreasingBound(const SCEV *Start,
const SCEV *BoundSCEV, const SCEV *Step,
ICmpInst::Predicate Pred,
unsigned LatchBrExitIdx,
Loop *L, ScalarEvolution &SE) {
if (Pred != ICmpInst::ICMP_SLT && Pred != ICmpInst::ICMP_SGT &&
Pred != ICmpInst::ICMP_ULT && Pred != ICmpInst::ICMP_UGT)
return false;
if (!SE.isAvailableAtLoopEntry(BoundSCEV, L))
return false;
assert(SE.isKnownNegative(Step) && "expecting negative step");
LLVM_DEBUG(dbgs() << "irce: isSafeDecreasingBound with:\n");
LLVM_DEBUG(dbgs() << "irce: Start: " << *Start << "\n");
LLVM_DEBUG(dbgs() << "irce: Step: " << *Step << "\n");
LLVM_DEBUG(dbgs() << "irce: BoundSCEV: " << *BoundSCEV << "\n");
LLVM_DEBUG(dbgs() << "irce: Pred: " << ICmpInst::getPredicateName(Pred)
<< "\n");
LLVM_DEBUG(dbgs() << "irce: LatchExitBrIdx: " << LatchBrExitIdx << "\n");
bool IsSigned = ICmpInst::isSigned(Pred);
// The predicate that we need to check that the induction variable lies
// within bounds.
ICmpInst::Predicate BoundPred =
IsSigned ? CmpInst::ICMP_SGT : CmpInst::ICMP_UGT;
if (LatchBrExitIdx == 1)
return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, BoundSCEV);
assert(LatchBrExitIdx == 0 &&
"LatchBrExitIdx should be either 0 or 1");
const SCEV *StepPlusOne = SE.getAddExpr(Step, SE.getOne(Step->getType()));
unsigned BitWidth = cast<IntegerType>(BoundSCEV->getType())->getBitWidth();
APInt Min = IsSigned ? APInt::getSignedMinValue(BitWidth) :
APInt::getMinValue(BitWidth);
const SCEV *Limit = SE.getMinusSCEV(SE.getConstant(Min), StepPlusOne);
const SCEV *MinusOne =
SE.getMinusSCEV(BoundSCEV, SE.getOne(BoundSCEV->getType()));
return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, MinusOne) &&
SE.isLoopEntryGuardedByCond(L, BoundPred, BoundSCEV, Limit);
}
/// Given a loop with an increasing induction variable, is it possible to
/// safely calculate the bounds of a new loop using the given Predicate.
static bool isSafeIncreasingBound(const SCEV *Start,
const SCEV *BoundSCEV, const SCEV *Step,
ICmpInst::Predicate Pred,
unsigned LatchBrExitIdx,
Loop *L, ScalarEvolution &SE) {
if (Pred != ICmpInst::ICMP_SLT && Pred != ICmpInst::ICMP_SGT &&
Pred != ICmpInst::ICMP_ULT && Pred != ICmpInst::ICMP_UGT)
return false;
if (!SE.isAvailableAtLoopEntry(BoundSCEV, L))
return false;
LLVM_DEBUG(dbgs() << "irce: isSafeIncreasingBound with:\n");
LLVM_DEBUG(dbgs() << "irce: Start: " << *Start << "\n");
LLVM_DEBUG(dbgs() << "irce: Step: " << *Step << "\n");
LLVM_DEBUG(dbgs() << "irce: BoundSCEV: " << *BoundSCEV << "\n");
LLVM_DEBUG(dbgs() << "irce: Pred: " << ICmpInst::getPredicateName(Pred)
<< "\n");
LLVM_DEBUG(dbgs() << "irce: LatchExitBrIdx: " << LatchBrExitIdx << "\n");
bool IsSigned = ICmpInst::isSigned(Pred);
// The predicate that we need to check that the induction variable lies
// within bounds.
ICmpInst::Predicate BoundPred =
IsSigned ? CmpInst::ICMP_SLT : CmpInst::ICMP_ULT;
if (LatchBrExitIdx == 1)
return SE.isLoopEntryGuardedByCond(L, BoundPred, Start, BoundSCEV);
assert(LatchBrExitIdx == 0 && "LatchBrExitIdx should be 0 or 1");
const SCEV *StepMinusOne =
SE.getMinusSCEV(Step, SE.getOne(Step->getType()));
unsigned BitWidth = cast<IntegerType>(BoundSCEV->getType())->getBitWidth();
APInt Max = IsSigned ? APInt::getSignedMaxValue(BitWidth) :
APInt::getMaxValue(BitWidth);
const SCEV *Limit = SE.getMinusSCEV(SE.getConstant(Max), StepMinusOne);
return (SE.isLoopEntryGuardedByCond(L, BoundPred, Start,
SE.getAddExpr(BoundSCEV, Step)) &&
SE.isLoopEntryGuardedByCond(L, BoundPred, BoundSCEV, Limit));
}
Optional<LoopStructure>
LoopStructure::parseLoopStructure(ScalarEvolution &SE, Loop &L,
const char *&FailureReason) {
if (!L.isLoopSimplifyForm()) {
FailureReason = "loop not in LoopSimplify form";
return None;
}
BasicBlock *Latch = L.getLoopLatch();
assert(Latch && "Simplified loops only have one latch!");
if (Latch->getTerminator()->getMetadata(ClonedLoopTag)) {
FailureReason = "loop has already been cloned";
return None;
}
if (!L.isLoopExiting(Latch)) {
FailureReason = "no loop latch";
return None;
}
BasicBlock *Header = L.getHeader();
BasicBlock *Preheader = L.getLoopPreheader();
if (!Preheader) {
FailureReason = "no preheader";
return None;
}
BranchInst *LatchBr = dyn_cast<BranchInst>(Latch->getTerminator());
if (!LatchBr || LatchBr->isUnconditional()) {
FailureReason = "latch terminator not conditional branch";
return None;
}
unsigned LatchBrExitIdx = LatchBr->getSuccessor(0) == Header ? 1 : 0;
ICmpInst *ICI = dyn_cast<ICmpInst>(LatchBr->getCondition());
if (!ICI || !isa<IntegerType>(ICI->getOperand(0)->getType())) {
FailureReason = "latch terminator branch not conditional on integral icmp";
return None;
}
const SCEV *LatchCount = SE.getExitCount(&L, Latch);
if (isa<SCEVCouldNotCompute>(LatchCount)) {
FailureReason = "could not compute latch count";
return None;
}
ICmpInst::Predicate Pred = ICI->getPredicate();
Value *LeftValue = ICI->getOperand(0);
const SCEV *LeftSCEV = SE.getSCEV(LeftValue);
IntegerType *IndVarTy = cast<IntegerType>(LeftValue->getType());
Value *RightValue = ICI->getOperand(1);
const SCEV *RightSCEV = SE.getSCEV(RightValue);
// We canonicalize `ICI` such that `LeftSCEV` is an add recurrence.
if (!isa<SCEVAddRecExpr>(LeftSCEV)) {
if (isa<SCEVAddRecExpr>(RightSCEV)) {
std::swap(LeftSCEV, RightSCEV);
std::swap(LeftValue, RightValue);
Pred = ICmpInst::getSwappedPredicate(Pred);
} else {
FailureReason = "no add recurrences in the icmp";
return None;
}
}
auto HasNoSignedWrap = [&](const SCEVAddRecExpr *AR) {
if (AR->getNoWrapFlags(SCEV::FlagNSW))
return true;
IntegerType *Ty = cast<IntegerType>(AR->getType());
IntegerType *WideTy =
IntegerType::get(Ty->getContext(), Ty->getBitWidth() * 2);
const SCEVAddRecExpr *ExtendAfterOp =
dyn_cast<SCEVAddRecExpr>(SE.getSignExtendExpr(AR, WideTy));
if (ExtendAfterOp) {
const SCEV *ExtendedStart = SE.getSignExtendExpr(AR->getStart(), WideTy);
const SCEV *ExtendedStep =
SE.getSignExtendExpr(AR->getStepRecurrence(SE), WideTy);
bool NoSignedWrap = ExtendAfterOp->getStart() == ExtendedStart &&
ExtendAfterOp->getStepRecurrence(SE) == ExtendedStep;
if (NoSignedWrap)
return true;
}
// We may have proved this when computing the sign extension above.
return AR->getNoWrapFlags(SCEV::FlagNSW) != SCEV::FlagAnyWrap;
};
// `ICI` is interpreted as taking the backedge if the *next* value of the
// induction variable satisfies some constraint.
const SCEVAddRecExpr *IndVarBase = cast<SCEVAddRecExpr>(LeftSCEV);
if (!IndVarBase->isAffine()) {
FailureReason = "LHS in icmp not induction variable";
return None;
}
const SCEV* StepRec = IndVarBase->getStepRecurrence(SE);
if (!isa<SCEVConstant>(StepRec)) {
FailureReason = "LHS in icmp not induction variable";
return None;
}
ConstantInt *StepCI = cast<SCEVConstant>(StepRec)->getValue();
if (ICI->isEquality() && !HasNoSignedWrap(IndVarBase)) {
FailureReason = "LHS in icmp needs nsw for equality predicates";
return None;
}
assert(!StepCI->isZero() && "Zero step?");
bool IsIncreasing = !StepCI->isNegative();
bool IsSignedPredicate;
const SCEV *StartNext = IndVarBase->getStart();
const SCEV *Addend = SE.getNegativeSCEV(IndVarBase->getStepRecurrence(SE));
const SCEV *IndVarStart = SE.getAddExpr(StartNext, Addend);
const SCEV *Step = SE.getSCEV(StepCI);
const SCEV *FixedRightSCEV = nullptr;
// If RightValue resides within loop (but still being loop invariant),
// regenerate it as preheader.
if (auto *I = dyn_cast<Instruction>(RightValue))
if (L.contains(I->getParent()))
FixedRightSCEV = RightSCEV;
if (IsIncreasing) {
bool DecreasedRightValueByOne = false;
if (StepCI->isOne()) {
// Try to turn eq/ne predicates to those we can work with.
if (Pred == ICmpInst::ICMP_NE && LatchBrExitIdx == 1)
// while (++i != len) { while (++i < len) {
// ... ---> ...
// } }
// If both parts are known non-negative, it is profitable to use
// unsigned comparison in increasing loop. This allows us to make the
// comparison check against "RightSCEV + 1" more optimistic.
if (isKnownNonNegativeInLoop(IndVarStart, &L, SE) &&
isKnownNonNegativeInLoop(RightSCEV, &L, SE))
Pred = ICmpInst::ICMP_ULT;
else
Pred = ICmpInst::ICMP_SLT;
else if (Pred == ICmpInst::ICMP_EQ && LatchBrExitIdx == 0) {
// while (true) { while (true) {
// if (++i == len) ---> if (++i > len - 1)
// break; break;
// ... ...
// } }
if (IndVarBase->getNoWrapFlags(SCEV::FlagNUW) &&
cannotBeMinInLoop(RightSCEV, &L, SE, /*Signed*/false)) {
Pred = ICmpInst::ICMP_UGT;
RightSCEV = SE.getMinusSCEV(RightSCEV,
SE.getOne(RightSCEV->getType()));
DecreasedRightValueByOne = true;
} else if (cannotBeMinInLoop(RightSCEV, &L, SE, /*Signed*/true)) {
Pred = ICmpInst::ICMP_SGT;
RightSCEV = SE.getMinusSCEV(RightSCEV,
SE.getOne(RightSCEV->getType()));
DecreasedRightValueByOne = true;
}
}
}
bool LTPred = (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_ULT);
bool GTPred = (Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_UGT);
bool FoundExpectedPred =
(LTPred && LatchBrExitIdx == 1) || (GTPred && LatchBrExitIdx == 0);
if (!FoundExpectedPred) {
FailureReason = "expected icmp slt semantically, found something else";
return None;
}
IsSignedPredicate = ICmpInst::isSigned(Pred);
if (!IsSignedPredicate && !AllowUnsignedLatchCondition) {
FailureReason = "unsigned latch conditions are explicitly prohibited";
return None;
}
if (!isSafeIncreasingBound(IndVarStart, RightSCEV, Step, Pred,
LatchBrExitIdx, &L, SE)) {
FailureReason = "Unsafe loop bounds";
return None;
}
if (LatchBrExitIdx == 0) {
// We need to increase the right value unless we have already decreased
// it virtually when we replaced EQ with SGT.
if (!DecreasedRightValueByOne)
FixedRightSCEV =
SE.getAddExpr(RightSCEV, SE.getOne(RightSCEV->getType()));
} else {
assert(!DecreasedRightValueByOne &&
"Right value can be decreased only for LatchBrExitIdx == 0!");
}
} else {
bool IncreasedRightValueByOne = false;
if (StepCI->isMinusOne()) {
// Try to turn eq/ne predicates to those we can work with.
if (Pred == ICmpInst::ICMP_NE && LatchBrExitIdx == 1)
// while (--i != len) { while (--i > len) {
// ... ---> ...
// } }
// We intentionally don't turn the predicate into UGT even if we know
// that both operands are non-negative, because it will only pessimize
// our check against "RightSCEV - 1".
Pred = ICmpInst::ICMP_SGT;
else if (Pred == ICmpInst::ICMP_EQ && LatchBrExitIdx == 0) {
// while (true) { while (true) {
// if (--i == len) ---> if (--i < len + 1)
// break; break;
// ... ...
// } }
if (IndVarBase->getNoWrapFlags(SCEV::FlagNUW) &&
cannotBeMaxInLoop(RightSCEV, &L, SE, /* Signed */ false)) {
Pred = ICmpInst::ICMP_ULT;
RightSCEV = SE.getAddExpr(RightSCEV, SE.getOne(RightSCEV->getType()));
IncreasedRightValueByOne = true;
} else if (cannotBeMaxInLoop(RightSCEV, &L, SE, /* Signed */ true)) {
Pred = ICmpInst::ICMP_SLT;
RightSCEV = SE.getAddExpr(RightSCEV, SE.getOne(RightSCEV->getType()));
IncreasedRightValueByOne = true;
}
}
}
bool LTPred = (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_ULT);
bool GTPred = (Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_UGT);
bool FoundExpectedPred =
(GTPred && LatchBrExitIdx == 1) || (LTPred && LatchBrExitIdx == 0);
if (!FoundExpectedPred) {
FailureReason = "expected icmp sgt semantically, found something else";
return None;
}
IsSignedPredicate =
Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SGT;
if (!IsSignedPredicate && !AllowUnsignedLatchCondition) {
FailureReason = "unsigned latch conditions are explicitly prohibited";
return None;
}
if (!isSafeDecreasingBound(IndVarStart, RightSCEV, Step, Pred,
LatchBrExitIdx, &L, SE)) {
FailureReason = "Unsafe bounds";
return None;
}
if (LatchBrExitIdx == 0) {
// We need to decrease the right value unless we have already increased
// it virtually when we replaced EQ with SLT.
if (!IncreasedRightValueByOne)
FixedRightSCEV =
SE.getMinusSCEV(RightSCEV, SE.getOne(RightSCEV->getType()));
} else {
assert(!IncreasedRightValueByOne &&
"Right value can be increased only for LatchBrExitIdx == 0!");
}
}
BasicBlock *LatchExit = LatchBr->getSuccessor(LatchBrExitIdx);
assert(SE.getLoopDisposition(LatchCount, &L) ==
ScalarEvolution::LoopInvariant &&
"loop variant exit count doesn't make sense!");
assert(!L.contains(LatchExit) && "expected an exit block!");
const DataLayout &DL = Preheader->getModule()->getDataLayout();
SCEVExpander Expander(SE, DL, "irce");
Instruction *Ins = Preheader->getTerminator();
if (FixedRightSCEV)
RightValue =
Expander.expandCodeFor(FixedRightSCEV, FixedRightSCEV->getType(), Ins);
Value *IndVarStartV = Expander.expandCodeFor(IndVarStart, IndVarTy, Ins);
IndVarStartV->setName("indvar.start");
LoopStructure Result;
Result.Tag = "main";
Result.Header = Header;
Result.Latch = Latch;
Result.LatchBr = LatchBr;
Result.LatchExit = LatchExit;
Result.LatchBrExitIdx = LatchBrExitIdx;
Result.IndVarStart = IndVarStartV;
Result.IndVarStep = StepCI;
Result.IndVarBase = LeftValue;
Result.IndVarIncreasing = IsIncreasing;
Result.LoopExitAt = RightValue;
Result.IsSignedPredicate = IsSignedPredicate;
FailureReason = nullptr;
return Result;
}
/// If the type of \p S matches with \p Ty, return \p S. Otherwise, return
/// signed or unsigned extension of \p S to type \p Ty.
static const SCEV *NoopOrExtend(const SCEV *S, Type *Ty, ScalarEvolution &SE,
bool Signed) {
return Signed ? SE.getNoopOrSignExtend(S, Ty) : SE.getNoopOrZeroExtend(S, Ty);
}
Optional<LoopConstrainer::SubRanges>
LoopConstrainer::calculateSubRanges(bool IsSignedPredicate) const {
IntegerType *Ty = cast<IntegerType>(LatchTakenCount->getType());
auto *RTy = cast<IntegerType>(Range.getType());
// We only support wide range checks and narrow latches.
if (!AllowNarrowLatchCondition && RTy != Ty)
return None;
if (RTy->getBitWidth() < Ty->getBitWidth())
return None;
LoopConstrainer::SubRanges Result;
// I think we can be more aggressive here and make this nuw / nsw if the
// addition that feeds into the icmp for the latch's terminating branch is nuw
// / nsw. In any case, a wrapping 2's complement addition is safe.
const SCEV *Start = NoopOrExtend(SE.getSCEV(MainLoopStructure.IndVarStart),
RTy, SE, IsSignedPredicate);
const SCEV *End = NoopOrExtend(SE.getSCEV(MainLoopStructure.LoopExitAt), RTy,
SE, IsSignedPredicate);
bool Increasing = MainLoopStructure.IndVarIncreasing;
// We compute `Smallest` and `Greatest` such that [Smallest, Greatest), or
// [Smallest, GreatestSeen] is the range of values the induction variable
// takes.
const SCEV *Smallest = nullptr, *Greatest = nullptr, *GreatestSeen = nullptr;
const SCEV *One = SE.getOne(RTy);
if (Increasing) {
Smallest = Start;
Greatest = End;
// No overflow, because the range [Smallest, GreatestSeen] is not empty.
GreatestSeen = SE.getMinusSCEV(End, One);
} else {
// These two computations may sign-overflow. Here is why that is okay:
//
// We know that the induction variable does not sign-overflow on any
// iteration except the last one, and it starts at `Start` and ends at
// `End`, decrementing by one every time.
//
// * if `Smallest` sign-overflows we know `End` is `INT_SMAX`. Since the
// induction variable is decreasing we know that that the smallest value
// the loop body is actually executed with is `INT_SMIN` == `Smallest`.
//
// * if `Greatest` sign-overflows, we know it can only be `INT_SMIN`. In
// that case, `Clamp` will always return `Smallest` and
// [`Result.LowLimit`, `Result.HighLimit`) = [`Smallest`, `Smallest`)
// will be an empty range. Returning an empty range is always safe.
Smallest = SE.getAddExpr(End, One);
Greatest = SE.getAddExpr(Start, One);
GreatestSeen = Start;
}
auto Clamp = [this, Smallest, Greatest, IsSignedPredicate](const SCEV *S) {
return IsSignedPredicate
? SE.getSMaxExpr(Smallest, SE.getSMinExpr(Greatest, S))
: SE.getUMaxExpr(Smallest, SE.getUMinExpr(Greatest, S));
};
// In some cases we can prove that we don't need a pre or post loop.
ICmpInst::Predicate PredLE =
IsSignedPredicate ? ICmpInst::ICMP_SLE : ICmpInst::ICMP_ULE;
ICmpInst::Predicate PredLT =
IsSignedPredicate ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
bool ProvablyNoPreloop =
SE.isKnownPredicate(PredLE, Range.getBegin(), Smallest);
if (!ProvablyNoPreloop)
Result.LowLimit = Clamp(Range.getBegin());
bool ProvablyNoPostLoop =
SE.isKnownPredicate(PredLT, GreatestSeen, Range.getEnd());
if (!ProvablyNoPostLoop)
Result.HighLimit = Clamp(Range.getEnd());
return Result;
}
void LoopConstrainer::cloneLoop(LoopConstrainer::ClonedLoop &Result,
const char *Tag) const {
for (BasicBlock *BB : OriginalLoop.getBlocks()) {
BasicBlock *Clone = CloneBasicBlock(BB, Result.Map, Twine(".") + Tag, &F);
Result.Blocks.push_back(Clone);
Result.Map[BB] = Clone;
}
auto GetClonedValue = [&Result](Value *V) {
assert(V && "null values not in domain!");
auto It = Result.Map.find(V);
if (It == Result.Map.end())
return V;
return static_cast<Value *>(It->second);
};
auto *ClonedLatch =
cast<BasicBlock>(GetClonedValue(OriginalLoop.getLoopLatch()));
ClonedLatch->getTerminator()->setMetadata(ClonedLoopTag,
MDNode::get(Ctx, {}));
Result.Structure = MainLoopStructure.map(GetClonedValue);
Result.Structure.Tag = Tag;
for (unsigned i = 0, e = Result.Blocks.size(); i != e; ++i) {
BasicBlock *ClonedBB = Result.Blocks[i];
BasicBlock *OriginalBB = OriginalLoop.getBlocks()[i];
assert(Result.Map[OriginalBB] == ClonedBB && "invariant!");
for (Instruction &I : *ClonedBB)
RemapInstruction(&I, Result.Map,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
// Exit blocks will now have one more predecessor and their PHI nodes need
// to be edited to reflect that. No phi nodes need to be introduced because
// the loop is in LCSSA.
for (auto *SBB : successors(OriginalBB)) {
if (OriginalLoop.contains(SBB))
continue; // not an exit block
for (PHINode &PN : SBB->phis()) {
Value *OldIncoming = PN.getIncomingValueForBlock(OriginalBB);
PN.addIncoming(GetClonedValue(OldIncoming), ClonedBB);
}
}
}
}
LoopConstrainer::RewrittenRangeInfo LoopConstrainer::changeIterationSpaceEnd(
const LoopStructure &LS, BasicBlock *Preheader, Value *ExitSubloopAt,
BasicBlock *ContinuationBlock) const {
// We start with a loop with a single latch:
//
// +--------------------+
// | |
// | preheader |
// | |
// +--------+-----------+
// | ----------------\
// | / |
// +--------v----v------+ |
// | | |
// | header | |
// | | |
// +--------------------+ |
// |
// ..... |
// |
// +--------------------+ |
// | | |
// | latch >----------/
// | |
// +-------v------------+
// |
// |
// | +--------------------+
// | | |
// +---> original exit |
// | |
// +--------------------+
//
// We change the control flow to look like
//
//
// +--------------------+
// | |
// | preheader >-------------------------+
// | | |
// +--------v-----------+ |
// | /-------------+ |
// | / | |
// +--------v--v--------+ | |
// | | | |
// | header | | +--------+ |
// | | | | | |
// +--------------------+ | | +-----v-----v-----------+
// | | | |
// | | | .pseudo.exit |
// | | | |
// | | +-----------v-----------+
// | | |
// ..... | | |
// | | +--------v-------------+
// +--------------------+ | | | |
// | | | | | ContinuationBlock |
// | latch >------+ | | |
// | | | +----------------------+
// +---------v----------+ |
// | |
// | |
// | +---------------^-----+
// | | |
// +-----> .exit.selector |
// | |
// +----------v----------+
// |
// +--------------------+ |
// | | |
// | original exit <----+
// | |
// +--------------------+
RewrittenRangeInfo RRI;
BasicBlock *BBInsertLocation = LS.Latch->getNextNode();
RRI.ExitSelector = BasicBlock::Create(Ctx, Twine(LS.Tag) + ".exit.selector",
&F, BBInsertLocation);
RRI.PseudoExit = BasicBlock::Create(Ctx, Twine(LS.Tag) + ".pseudo.exit", &F,
BBInsertLocation);
BranchInst *PreheaderJump = cast<BranchInst>(Preheader->getTerminator());
bool Increasing = LS.IndVarIncreasing;
bool IsSignedPredicate = LS.IsSignedPredicate;
IRBuilder<> B(PreheaderJump);
auto *RangeTy = Range.getBegin()->getType();
auto NoopOrExt = [&](Value *V) {
if (V->getType() == RangeTy)
return V;
return IsSignedPredicate ? B.CreateSExt(V, RangeTy, "wide." + V->getName())
: B.CreateZExt(V, RangeTy, "wide." + V->getName());
};
// EnterLoopCond - is it okay to start executing this `LS'?
Value *EnterLoopCond = nullptr;
auto Pred =
Increasing
? (IsSignedPredicate ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT)
: (IsSignedPredicate ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT);
Value *IndVarStart = NoopOrExt(LS.IndVarStart);
EnterLoopCond = B.CreateICmp(Pred, IndVarStart, ExitSubloopAt);
B.CreateCondBr(EnterLoopCond, LS.Header, RRI.PseudoExit);
PreheaderJump->eraseFromParent();
LS.LatchBr->setSuccessor(LS.LatchBrExitIdx, RRI.ExitSelector);
B.SetInsertPoint(LS.LatchBr);
Value *IndVarBase = NoopOrExt(LS.IndVarBase);
Value *TakeBackedgeLoopCond = B.CreateICmp(Pred, IndVarBase, ExitSubloopAt);
Value *CondForBranch = LS.LatchBrExitIdx == 1
? TakeBackedgeLoopCond
: B.CreateNot(TakeBackedgeLoopCond);
LS.LatchBr->setCondition(CondForBranch);
B.SetInsertPoint(RRI.ExitSelector);
// IterationsLeft - are there any more iterations left, given the original
// upper bound on the induction variable? If not, we branch to the "real"
// exit.
Value *LoopExitAt = NoopOrExt(LS.LoopExitAt);
Value *IterationsLeft = B.CreateICmp(Pred, IndVarBase, LoopExitAt);
B.CreateCondBr(IterationsLeft, RRI.PseudoExit, LS.LatchExit);
BranchInst *BranchToContinuation =
BranchInst::Create(ContinuationBlock, RRI.PseudoExit);
// We emit PHI nodes into `RRI.PseudoExit' that compute the "latest" value of
// each of the PHI nodes in the loop header. This feeds into the initial
// value of the same PHI nodes if/when we continue execution.
for (PHINode &PN : LS.Header->phis()) {
PHINode *NewPHI = PHINode::Create(PN.getType(), 2, PN.getName() + ".copy",
BranchToContinuation);
NewPHI->addIncoming(PN.getIncomingValueForBlock(Preheader), Preheader);
NewPHI->addIncoming(PN.getIncomingValueForBlock(LS.Latch),
RRI.ExitSelector);
RRI.PHIValuesAtPseudoExit.push_back(NewPHI);
}
RRI.IndVarEnd = PHINode::Create(IndVarBase->getType(), 2, "indvar.end",
BranchToContinuation);
RRI.IndVarEnd->addIncoming(IndVarStart, Preheader);
RRI.IndVarEnd->addIncoming(IndVarBase, RRI.ExitSelector);
// The latch exit now has a branch from `RRI.ExitSelector' instead of
// `LS.Latch'. The PHI nodes need to be updated to reflect that.
LS.LatchExit->replacePhiUsesWith(LS.Latch, RRI.ExitSelector);
return RRI;
}
void LoopConstrainer::rewriteIncomingValuesForPHIs(
LoopStructure &LS, BasicBlock *ContinuationBlock,
const LoopConstrainer::RewrittenRangeInfo &RRI) const {
unsigned PHIIndex = 0;
for (PHINode &PN : LS.Header->phis())
PN.setIncomingValueForBlock(ContinuationBlock,
RRI.PHIValuesAtPseudoExit[PHIIndex++]);
LS.IndVarStart = RRI.IndVarEnd;
}
BasicBlock *LoopConstrainer::createPreheader(const LoopStructure &LS,
BasicBlock *OldPreheader,
const char *Tag) const {
BasicBlock *Preheader = BasicBlock::Create(Ctx, Tag, &F, LS.Header);
BranchInst::Create(LS.Header, Preheader);
LS.Header->replacePhiUsesWith(OldPreheader, Preheader);
return Preheader;
}
void LoopConstrainer::addToParentLoopIfNeeded(ArrayRef<BasicBlock *> BBs) {
Loop *ParentLoop = OriginalLoop.getParentLoop();
if (!ParentLoop)
return;
for (BasicBlock *BB : BBs)
ParentLoop->addBasicBlockToLoop(BB, LI);
}
Loop *LoopConstrainer::createClonedLoopStructure(Loop *Original, Loop *Parent,
ValueToValueMapTy &VM,
bool IsSubloop) {
Loop &New = *LI.AllocateLoop();
if (Parent)
Parent->addChildLoop(&New);
else
LI.addTopLevelLoop(&New);
LPMAddNewLoop(&New, IsSubloop);
// Add all of the blocks in Original to the new loop.
for (auto *BB : Original->blocks())
if (LI.getLoopFor(BB) == Original)
New.addBasicBlockToLoop(cast<BasicBlock>(VM[BB]), LI);
// Add all of the subloops to the new loop.
for (Loop *SubLoop : *Original)
createClonedLoopStructure(SubLoop, &New, VM, /* IsSubloop */ true);
return &New;
}
bool LoopConstrainer::run() {
BasicBlock *Preheader = nullptr;
LatchTakenCount = SE.getExitCount(&OriginalLoop, MainLoopStructure.Latch);
Preheader = OriginalLoop.getLoopPreheader();
assert(!isa<SCEVCouldNotCompute>(LatchTakenCount) && Preheader != nullptr &&
"preconditions!");
OriginalPreheader = Preheader;
MainLoopPreheader = Preheader;
bool IsSignedPredicate = MainLoopStructure.IsSignedPredicate;
Optional<SubRanges> MaybeSR = calculateSubRanges(IsSignedPredicate);
if (!MaybeSR.hasValue()) {
LLVM_DEBUG(dbgs() << "irce: could not compute subranges\n");
return false;
}
SubRanges SR = MaybeSR.getValue();
bool Increasing = MainLoopStructure.IndVarIncreasing;
IntegerType *IVTy =
cast<IntegerType>(Range.getBegin()->getType());
SCEVExpander Expander(SE, F.getParent()->getDataLayout(), "irce");
Instruction *InsertPt = OriginalPreheader->getTerminator();
// It would have been better to make `PreLoop' and `PostLoop'
// `Optional<ClonedLoop>'s, but `ValueToValueMapTy' does not have a copy
// constructor.
ClonedLoop PreLoop, PostLoop;
bool NeedsPreLoop =
Increasing ? SR.LowLimit.hasValue() : SR.HighLimit.hasValue();
bool NeedsPostLoop =
Increasing ? SR.HighLimit.hasValue() : SR.LowLimit.hasValue();
Value *ExitPreLoopAt = nullptr;
Value *ExitMainLoopAt = nullptr;
const SCEVConstant *MinusOneS =
cast<SCEVConstant>(SE.getConstant(IVTy, -1, true /* isSigned */));
if (NeedsPreLoop) {
const SCEV *ExitPreLoopAtSCEV = nullptr;
if (Increasing)
ExitPreLoopAtSCEV = *SR.LowLimit;
else if (cannotBeMinInLoop(*SR.HighLimit, &OriginalLoop, SE,
IsSignedPredicate))
ExitPreLoopAtSCEV = SE.getAddExpr(*SR.HighLimit, MinusOneS);
else {
LLVM_DEBUG(dbgs() << "irce: could not prove no-overflow when computing "
<< "preloop exit limit. HighLimit = "
<< *(*SR.HighLimit) << "\n");
return false;
}
if (!isSafeToExpandAt(ExitPreLoopAtSCEV, InsertPt, SE)) {
LLVM_DEBUG(dbgs() << "irce: could not prove that it is safe to expand the"
<< " preloop exit limit " << *ExitPreLoopAtSCEV
<< " at block " << InsertPt->getParent()->getName()
<< "\n");
return false;
}
ExitPreLoopAt = Expander.expandCodeFor(ExitPreLoopAtSCEV, IVTy, InsertPt);
ExitPreLoopAt->setName("exit.preloop.at");
}
if (NeedsPostLoop) {
const SCEV *ExitMainLoopAtSCEV = nullptr;
if (Increasing)
ExitMainLoopAtSCEV = *SR.HighLimit;
else if (cannotBeMinInLoop(*SR.LowLimit, &OriginalLoop, SE,
IsSignedPredicate))
ExitMainLoopAtSCEV = SE.getAddExpr(*SR.LowLimit, MinusOneS);
else {
LLVM_DEBUG(dbgs() << "irce: could not prove no-overflow when computing "
<< "mainloop exit limit. LowLimit = "
<< *(*SR.LowLimit) << "\n");
return false;
}
if (!isSafeToExpandAt(ExitMainLoopAtSCEV, InsertPt, SE)) {
LLVM_DEBUG(dbgs() << "irce: could not prove that it is safe to expand the"
<< " main loop exit limit " << *ExitMainLoopAtSCEV
<< " at block " << InsertPt->getParent()->getName()
<< "\n");
return false;
}
ExitMainLoopAt = Expander.expandCodeFor(ExitMainLoopAtSCEV, IVTy, InsertPt);
ExitMainLoopAt->setName("exit.mainloop.at");
}
// We clone these ahead of time so that we don't have to deal with changing
// and temporarily invalid IR as we transform the loops.
if (NeedsPreLoop)
cloneLoop(PreLoop, "preloop");
if (NeedsPostLoop)
cloneLoop(PostLoop, "postloop");
RewrittenRangeInfo PreLoopRRI;
if (NeedsPreLoop) {
Preheader->getTerminator()->replaceUsesOfWith(MainLoopStructure.Header,
PreLoop.Structure.Header);
MainLoopPreheader =
createPreheader(MainLoopStructure, Preheader, "mainloop");
PreLoopRRI = changeIterationSpaceEnd(PreLoop.Structure, Preheader,
ExitPreLoopAt, MainLoopPreheader);
rewriteIncomingValuesForPHIs(MainLoopStructure, MainLoopPreheader,
PreLoopRRI);
}
BasicBlock *PostLoopPreheader = nullptr;
RewrittenRangeInfo PostLoopRRI;
if (NeedsPostLoop) {
PostLoopPreheader =
createPreheader(PostLoop.Structure, Preheader, "postloop");
PostLoopRRI = changeIterationSpaceEnd(MainLoopStructure, MainLoopPreheader,
ExitMainLoopAt, PostLoopPreheader);
rewriteIncomingValuesForPHIs(PostLoop.Structure, PostLoopPreheader,
PostLoopRRI);
}
BasicBlock *NewMainLoopPreheader =
MainLoopPreheader != Preheader ? MainLoopPreheader : nullptr;
BasicBlock *NewBlocks[] = {PostLoopPreheader, PreLoopRRI.PseudoExit,
PreLoopRRI.ExitSelector, PostLoopRRI.PseudoExit,
PostLoopRRI.ExitSelector, NewMainLoopPreheader};
// Some of the above may be nullptr, filter them out before passing to
// addToParentLoopIfNeeded.
auto NewBlocksEnd =
std::remove(std::begin(NewBlocks), std::end(NewBlocks), nullptr);
addToParentLoopIfNeeded(makeArrayRef(std::begin(NewBlocks), NewBlocksEnd));
DT.recalculate(F);
// We need to first add all the pre and post loop blocks into the loop
// structures (as part of createClonedLoopStructure), and then update the
// LCSSA form and LoopSimplifyForm. This is necessary for correctly updating
// LI when LoopSimplifyForm is generated.
Loop *PreL = nullptr, *PostL = nullptr;
if (!PreLoop.Blocks.empty()) {
PreL = createClonedLoopStructure(&OriginalLoop,
OriginalLoop.getParentLoop(), PreLoop.Map,
/* IsSubLoop */ false);
}
if (!PostLoop.Blocks.empty()) {
PostL =
createClonedLoopStructure(&OriginalLoop, OriginalLoop.getParentLoop(),
PostLoop.Map, /* IsSubLoop */ false);
}
// This function canonicalizes the loop into Loop-Simplify and LCSSA forms.
auto CanonicalizeLoop = [&] (Loop *L, bool IsOriginalLoop) {
formLCSSARecursively(*L, DT, &LI, &SE);
simplifyLoop(L, &DT, &LI, &SE, nullptr, nullptr, true);
// Pre/post loops are slow paths, we do not need to perform any loop
// optimizations on them.
if (!IsOriginalLoop)
DisableAllLoopOptsOnLoop(*L);
};
if (PreL)
CanonicalizeLoop(PreL, false);
if (PostL)
CanonicalizeLoop(PostL, false);
CanonicalizeLoop(&OriginalLoop, true);
return true;
}
/// Computes and returns a range of values for the induction variable (IndVar)
/// in which the range check can be safely elided. If it cannot compute such a
/// range, returns None.
Optional<InductiveRangeCheck::Range>
InductiveRangeCheck::computeSafeIterationSpace(
ScalarEvolution &SE, const SCEVAddRecExpr *IndVar,
bool IsLatchSigned) const {
// We can deal when types of latch check and range checks don't match in case
// if latch check is more narrow.
auto *IVType = cast<IntegerType>(IndVar->getType());
auto *RCType = cast<IntegerType>(getBegin()->getType());
if (IVType->getBitWidth() > RCType->getBitWidth())
return None;
// IndVar is of the form "A + B * I" (where "I" is the canonical induction
// variable, that may or may not exist as a real llvm::Value in the loop) and
// this inductive range check is a range check on the "C + D * I" ("C" is
// getBegin() and "D" is getStep()). We rewrite the value being range
// checked to "M + N * IndVar" where "N" = "D * B^(-1)" and "M" = "C - NA".
//
// The actual inequalities we solve are of the form
//
// 0 <= M + 1 * IndVar < L given L >= 0 (i.e. N == 1)
//
// Here L stands for upper limit of the safe iteration space.
// The inequality is satisfied by (0 - M) <= IndVar < (L - M). To avoid
// overflows when calculating (0 - M) and (L - M) we, depending on type of
// IV's iteration space, limit the calculations by borders of the iteration
// space. For example, if IndVar is unsigned, (0 - M) overflows for any M > 0.
// If we figured out that "anything greater than (-M) is safe", we strengthen
// this to "everything greater than 0 is safe", assuming that values between
// -M and 0 just do not exist in unsigned iteration space, and we don't want
// to deal with overflown values.
if (!IndVar->isAffine())
return None;
const SCEV *A = NoopOrExtend(IndVar->getStart(), RCType, SE, IsLatchSigned);
const SCEVConstant *B = dyn_cast<SCEVConstant>(
NoopOrExtend(IndVar->getStepRecurrence(SE), RCType, SE, IsLatchSigned));
if (!B)
return None;
assert(!B->isZero() && "Recurrence with zero step?");
const SCEV *C = getBegin();
const SCEVConstant *D = dyn_cast<SCEVConstant>(getStep());
if (D != B)
return None;
assert(!D->getValue()->isZero() && "Recurrence with zero step?");
unsigned BitWidth = RCType->getBitWidth();
const SCEV *SIntMax = SE.getConstant(APInt::getSignedMaxValue(BitWidth));
// Subtract Y from X so that it does not go through border of the IV
// iteration space. Mathematically, it is equivalent to:
//
// ClampedSubtract(X, Y) = min(max(X - Y, INT_MIN), INT_MAX). [1]
//
// In [1], 'X - Y' is a mathematical subtraction (result is not bounded to
// any width of bit grid). But after we take min/max, the result is
// guaranteed to be within [INT_MIN, INT_MAX].
//
// In [1], INT_MAX and INT_MIN are respectively signed and unsigned max/min
// values, depending on type of latch condition that defines IV iteration
// space.
auto ClampedSubtract = [&](const SCEV *X, const SCEV *Y) {
// FIXME: The current implementation assumes that X is in [0, SINT_MAX].
// This is required to ensure that SINT_MAX - X does not overflow signed and
// that X - Y does not overflow unsigned if Y is negative. Can we lift this
// restriction and make it work for negative X either?
if (IsLatchSigned) {
// X is a number from signed range, Y is interpreted as signed.
// Even if Y is SINT_MAX, (X - Y) does not reach SINT_MIN. So the only
// thing we should care about is that we didn't cross SINT_MAX.
// So, if Y is positive, we subtract Y safely.
// Rule 1: Y > 0 ---> Y.
// If 0 <= -Y <= (SINT_MAX - X), we subtract Y safely.
// Rule 2: Y >=s (X - SINT_MAX) ---> Y.
// If 0 <= (SINT_MAX - X) < -Y, we can only subtract (X - SINT_MAX).
// Rule 3: Y <s (X - SINT_MAX) ---> (X - SINT_MAX).
// It gives us smax(Y, X - SINT_MAX) to subtract in all cases.
const SCEV *XMinusSIntMax = SE.getMinusSCEV(X, SIntMax);
return SE.getMinusSCEV(X, SE.getSMaxExpr(Y, XMinusSIntMax),
SCEV::FlagNSW);
} else
// X is a number from unsigned range, Y is interpreted as signed.
// Even if Y is SINT_MIN, (X - Y) does not reach UINT_MAX. So the only
// thing we should care about is that we didn't cross zero.
// So, if Y is negative, we subtract Y safely.
// Rule 1: Y <s 0 ---> Y.
// If 0 <= Y <= X, we subtract Y safely.
// Rule 2: Y <=s X ---> Y.
// If 0 <= X < Y, we should stop at 0 and can only subtract X.
// Rule 3: Y >s X ---> X.
// It gives us smin(X, Y) to subtract in all cases.
return SE.getMinusSCEV(X, SE.getSMinExpr(X, Y), SCEV::FlagNUW);
};
const SCEV *M = SE.getMinusSCEV(C, A);
const SCEV *Zero = SE.getZero(M->getType());
// This function returns SCEV equal to 1 if X is non-negative 0 otherwise.
auto SCEVCheckNonNegative = [&](const SCEV *X) {
const Loop *L = IndVar->getLoop();
const SCEV *One = SE.getOne(X->getType());
// Can we trivially prove that X is a non-negative or negative value?
if (isKnownNonNegativeInLoop(X, L, SE))
return One;
else if (isKnownNegativeInLoop(X, L, SE))
return Zero;
// If not, we will have to figure it out during the execution.
// Function smax(smin(X, 0), -1) + 1 equals to 1 if X >= 0 and 0 if X < 0.
const SCEV *NegOne = SE.getNegativeSCEV(One);
return SE.getAddExpr(SE.getSMaxExpr(SE.getSMinExpr(X, Zero), NegOne), One);
};
// FIXME: Current implementation of ClampedSubtract implicitly assumes that
// X is non-negative (in sense of a signed value). We need to re-implement
// this function in a way that it will correctly handle negative X as well.
// We use it twice: for X = 0 everything is fine, but for X = getEnd() we can
// end up with a negative X and produce wrong results. So currently we ensure
// that if getEnd() is negative then both ends of the safe range are zero.
// Note that this may pessimize elimination of unsigned range checks against
// negative values.
const SCEV *REnd = getEnd();
const SCEV *EndIsNonNegative = SCEVCheckNonNegative(REnd);
const SCEV *Begin = SE.getMulExpr(ClampedSubtract(Zero, M), EndIsNonNegative);
const SCEV *End = SE.getMulExpr(ClampedSubtract(REnd, M), EndIsNonNegative);
return InductiveRangeCheck::Range(Begin, End);
}
static Optional<InductiveRangeCheck::Range>
IntersectSignedRange(ScalarEvolution &SE,
const Optional<InductiveRangeCheck::Range> &R1,
const InductiveRangeCheck::Range &R2) {
if (R2.isEmpty(SE, /* IsSigned */ true))
return None;
if (!R1.hasValue())
return R2;
auto &R1Value = R1.getValue();
// We never return empty ranges from this function, and R1 is supposed to be
// a result of intersection. Thus, R1 is never empty.
assert(!R1Value.isEmpty(SE, /* IsSigned */ true) &&
"We should never have empty R1!");
// TODO: we could widen the smaller range and have this work; but for now we
// bail out to keep things simple.
if (R1Value.getType() != R2.getType())
return None;
const SCEV *NewBegin = SE.getSMaxExpr(R1Value.getBegin(), R2.getBegin());
const SCEV *NewEnd = SE.getSMinExpr(R1Value.getEnd(), R2.getEnd());
// If the resulting range is empty, just return None.
auto Ret = InductiveRangeCheck::Range(NewBegin, NewEnd);
if (Ret.isEmpty(SE, /* IsSigned */ true))
return None;
return Ret;
}
static Optional<InductiveRangeCheck::Range>
IntersectUnsignedRange(ScalarEvolution &SE,
const Optional<InductiveRangeCheck::Range> &R1,
const InductiveRangeCheck::Range &R2) {
if (R2.isEmpty(SE, /* IsSigned */ false))
return None;
if (!R1.hasValue())
return R2;
auto &R1Value = R1.getValue();
// We never return empty ranges from this function, and R1 is supposed to be
// a result of intersection. Thus, R1 is never empty.
assert(!R1Value.isEmpty(SE, /* IsSigned */ false) &&
"We should never have empty R1!");
// TODO: we could widen the smaller range and have this work; but for now we
// bail out to keep things simple.
if (R1Value.getType() != R2.getType())
return None;
const SCEV *NewBegin = SE.getUMaxExpr(R1Value.getBegin(), R2.getBegin());
const SCEV *NewEnd = SE.getUMinExpr(R1Value.getEnd(), R2.getEnd());
// If the resulting range is empty, just return None.
auto Ret = InductiveRangeCheck::Range(NewBegin, NewEnd);
if (Ret.isEmpty(SE, /* IsSigned */ false))
return None;
return Ret;
}
PreservedAnalyses IRCEPass::run(Function &F, FunctionAnalysisManager &AM) {
auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &BPI = AM.getResult<BranchProbabilityAnalysis>(F);
LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
// Get BFI analysis result on demand. Please note that modification of
// CFG invalidates this analysis and we should handle it.
auto getBFI = [&F, &AM ]()->BlockFrequencyInfo & {
return AM.getResult<BlockFrequencyAnalysis>(F);
};
InductiveRangeCheckElimination IRCE(SE, &BPI, DT, LI, { getBFI });
bool Changed = false;
{
bool CFGChanged = false;
for (const auto &L : LI) {
CFGChanged |= simplifyLoop(L, &DT, &LI, &SE, nullptr, nullptr,
/*PreserveLCSSA=*/false);
Changed |= formLCSSARecursively(*L, DT, &LI, &SE);
}
Changed |= CFGChanged;
if (CFGChanged && !SkipProfitabilityChecks)
AM.invalidate<BlockFrequencyAnalysis>(F);
}
SmallPriorityWorklist<Loop *, 4> Worklist;
appendLoopsToWorklist(LI, Worklist);
auto LPMAddNewLoop = [&Worklist](Loop *NL, bool IsSubloop) {
if (!IsSubloop)
appendLoopsToWorklist(*NL, Worklist);
};
while (!Worklist.empty()) {
Loop *L = Worklist.pop_back_val();
if (IRCE.run(L, LPMAddNewLoop)) {
Changed = true;
if (!SkipProfitabilityChecks)
AM.invalidate<BlockFrequencyAnalysis>(F);
}
}
if (!Changed)
return PreservedAnalyses::all();
return getLoopPassPreservedAnalyses();
}
bool IRCELegacyPass::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
BranchProbabilityInfo &BPI =
getAnalysis<BranchProbabilityInfoWrapperPass>().getBPI();
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
InductiveRangeCheckElimination IRCE(SE, &BPI, DT, LI);
bool Changed = false;
for (const auto &L : LI) {
Changed |= simplifyLoop(L, &DT, &LI, &SE, nullptr, nullptr,
/*PreserveLCSSA=*/false);
Changed |= formLCSSARecursively(*L, DT, &LI, &SE);
}
SmallPriorityWorklist<Loop *, 4> Worklist;
appendLoopsToWorklist(LI, Worklist);
auto LPMAddNewLoop = [&](Loop *NL, bool IsSubloop) {
if (!IsSubloop)
appendLoopsToWorklist(*NL, Worklist);
};
while (!Worklist.empty()) {
Loop *L = Worklist.pop_back_val();
Changed |= IRCE.run(L, LPMAddNewLoop);
}
return Changed;
}
bool
InductiveRangeCheckElimination::isProfitableToTransform(const Loop &L,
LoopStructure &LS) {
if (SkipProfitabilityChecks)
return true;
if (GetBFI.hasValue()) {
BlockFrequencyInfo &BFI = (*GetBFI)();
uint64_t hFreq = BFI.getBlockFreq(LS.Header).getFrequency();
uint64_t phFreq = BFI.getBlockFreq(L.getLoopPreheader()).getFrequency();
if (phFreq != 0 && hFreq != 0 && (hFreq / phFreq < MinRuntimeIterations)) {
LLVM_DEBUG(dbgs() << "irce: could not prove profitability: "
<< "the estimated number of iterations basing on "
"frequency info is " << (hFreq / phFreq) << "\n";);
return false;
}
return true;
}
if (!BPI)
return true;
BranchProbability ExitProbability =
BPI->getEdgeProbability(LS.Latch, LS.LatchBrExitIdx);
if (ExitProbability > BranchProbability(1, MinRuntimeIterations)) {
LLVM_DEBUG(dbgs() << "irce: could not prove profitability: "
<< "the exit probability is too big " << ExitProbability
<< "\n";);
return false;
}
return true;
}
bool InductiveRangeCheckElimination::run(
Loop *L, function_ref<void(Loop *, bool)> LPMAddNewLoop) {
if (L->getBlocks().size() >= LoopSizeCutoff) {
LLVM_DEBUG(dbgs() << "irce: giving up constraining loop, too large\n");
return false;
}
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader) {
LLVM_DEBUG(dbgs() << "irce: loop has no preheader, leaving\n");
return false;
}
LLVMContext &Context = Preheader->getContext();
SmallVector<InductiveRangeCheck, 16> RangeChecks;
for (auto BBI : L->getBlocks())
if (BranchInst *TBI = dyn_cast<BranchInst>(BBI->getTerminator()))
InductiveRangeCheck::extractRangeChecksFromBranch(TBI, L, SE, BPI,
RangeChecks);
if (RangeChecks.empty())
return false;
auto PrintRecognizedRangeChecks = [&](raw_ostream &OS) {
OS << "irce: looking at loop "; L->print(OS);
OS << "irce: loop has " << RangeChecks.size()
<< " inductive range checks: \n";
for (InductiveRangeCheck &IRC : RangeChecks)
IRC.print(OS);
};
LLVM_DEBUG(PrintRecognizedRangeChecks(dbgs()));
if (PrintRangeChecks)
PrintRecognizedRangeChecks(errs());
const char *FailureReason = nullptr;
Optional<LoopStructure> MaybeLoopStructure =
LoopStructure::parseLoopStructure(SE, *L, FailureReason);
if (!MaybeLoopStructure.hasValue()) {
LLVM_DEBUG(dbgs() << "irce: could not parse loop structure: "
<< FailureReason << "\n";);
return false;
}
LoopStructure LS = MaybeLoopStructure.getValue();
if (!isProfitableToTransform(*L, LS))
return false;
const SCEVAddRecExpr *IndVar =
cast<SCEVAddRecExpr>(SE.getMinusSCEV(SE.getSCEV(LS.IndVarBase), SE.getSCEV(LS.IndVarStep)));
Optional<InductiveRangeCheck::Range> SafeIterRange;
Instruction *ExprInsertPt = Preheader->getTerminator();
SmallVector<InductiveRangeCheck, 4> RangeChecksToEliminate;
// Basing on the type of latch predicate, we interpret the IV iteration range
// as signed or unsigned range. We use different min/max functions (signed or
// unsigned) when intersecting this range with safe iteration ranges implied
// by range checks.
auto IntersectRange =
LS.IsSignedPredicate ? IntersectSignedRange : IntersectUnsignedRange;
IRBuilder<> B(ExprInsertPt);
for (InductiveRangeCheck &IRC : RangeChecks) {
auto Result = IRC.computeSafeIterationSpace(SE, IndVar,
LS.IsSignedPredicate);
if (Result.hasValue()) {
auto MaybeSafeIterRange =
IntersectRange(SE, SafeIterRange, Result.getValue());
if (MaybeSafeIterRange.hasValue()) {
assert(
!MaybeSafeIterRange.getValue().isEmpty(SE, LS.IsSignedPredicate) &&
"We should never return empty ranges!");
RangeChecksToEliminate.push_back(IRC);
SafeIterRange = MaybeSafeIterRange.getValue();
}
}
}
if (!SafeIterRange.hasValue())
return false;
LoopConstrainer LC(*L, LI, LPMAddNewLoop, LS, SE, DT,
SafeIterRange.getValue());
bool Changed = LC.run();
if (Changed) {
auto PrintConstrainedLoopInfo = [L]() {
dbgs() << "irce: in function ";
dbgs() << L->getHeader()->getParent()->getName() << ": ";
dbgs() << "constrained ";
L->print(dbgs());
};
LLVM_DEBUG(PrintConstrainedLoopInfo());
if (PrintChangedLoops)
PrintConstrainedLoopInfo();
// Optimize away the now-redundant range checks.
for (InductiveRangeCheck &IRC : RangeChecksToEliminate) {
ConstantInt *FoldedRangeCheck = IRC.getPassingDirection()
? ConstantInt::getTrue(Context)
: ConstantInt::getFalse(Context);
IRC.getCheckUse()->set(FoldedRangeCheck);
}
}
return Changed;
}
Pass *llvm::createInductiveRangeCheckEliminationPass() {
return new IRCELegacyPass();
}
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