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//===- GuardWidening.cpp - ---- Guard widening ----------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements the guard widening pass. The semantics of the
// @llvm.experimental.guard intrinsic lets LLVM transform it so that it fails
// more often that it did before the transform. This optimization is called
// "widening" and can be used hoist and common runtime checks in situations like
// these:
// %cmp0 = 7 u< Length
// call @llvm.experimental.guard(i1 %cmp0) [ "deopt"(...) ]
// call @unknown_side_effects()
// %cmp1 = 9 u< Length
// call @llvm.experimental.guard(i1 %cmp1) [ "deopt"(...) ]
// ...
// =>
// %cmp0 = 9 u< Length
// call @llvm.experimental.guard(i1 %cmp0) [ "deopt"(...) ]
// call @unknown_side_effects()
// ...
// If %cmp0 is false, @llvm.experimental.guard will "deoptimize" back to a
// generic implementation of the same function, which will have the correct
// semantics from that point onward. It is always _legal_ to deoptimize (so
// replacing %cmp0 with false is "correct"), though it may not always be
// profitable to do so.
// NB! This pass is a work in progress. It hasn't been tuned to be "production
// ready" yet. It is known to have quadriatic running time and will not scale
// to large numbers of guards
#include "llvm/Transforms/Scalar/GuardWidening.h"
#include <functional>
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
using namespace llvm;
#define DEBUG_TYPE "guard-widening"
STATISTIC(GuardsEliminated, "Number of eliminated guards");
STATISTIC(CondBranchEliminated, "Number of eliminated conditional branches");
static cl::opt<bool> WidenFrequentBranches(
"guard-widening-widen-frequent-branches", cl::Hidden,
cl::desc("Widen conditions of explicit branches into dominating guards in "
"case if their taken frequency exceeds threshold set by "
"guard-widening-frequent-branch-threshold option"),
static cl::opt<unsigned> FrequentBranchThreshold(
"guard-widening-frequent-branch-threshold", cl::Hidden,
cl::desc("When WidenFrequentBranches is set to true, this option is used "
"to determine which branches are frequently taken. The criteria "
"that a branch is taken more often than "
"((FrequentBranchThreshold - 1) / FrequentBranchThreshold), then "
"it is considered frequently taken"),
static cl::opt<bool>
WidenBranchGuards("guard-widening-widen-branch-guards", cl::Hidden,
cl::desc("Whether or not we should widen guards "
"expressed as branches by widenable conditions"),
namespace {
// Get the condition of \p I. It can either be a guard or a conditional branch.
static Value *getCondition(Instruction *I) {
if (IntrinsicInst *GI = dyn_cast<IntrinsicInst>(I)) {
assert(GI->getIntrinsicID() == Intrinsic::experimental_guard &&
"Bad guard intrinsic?");
return GI->getArgOperand(0);
if (isGuardAsWidenableBranch(I)) {
auto *Cond = cast<BranchInst>(I)->getCondition();
return cast<BinaryOperator>(Cond)->getOperand(0);
return cast<BranchInst>(I)->getCondition();
// Set the condition for \p I to \p NewCond. \p I can either be a guard or a
// conditional branch.
static void setCondition(Instruction *I, Value *NewCond) {
if (IntrinsicInst *GI = dyn_cast<IntrinsicInst>(I)) {
assert(GI->getIntrinsicID() == Intrinsic::experimental_guard &&
"Bad guard intrinsic?");
GI->setArgOperand(0, NewCond);
// Eliminates the guard instruction properly.
static void eliminateGuard(Instruction *GuardInst) {
class GuardWideningImpl {
DominatorTree &DT;
PostDominatorTree *PDT;
LoopInfo &LI;
BranchProbabilityInfo *BPI;
/// Together, these describe the region of interest. This might be all of
/// the blocks within a function, or only a given loop's blocks and preheader.
DomTreeNode *Root;
std::function<bool(BasicBlock*)> BlockFilter;
/// The set of guards and conditional branches whose conditions have been
/// widened into dominating guards.
SmallVector<Instruction *, 16> EliminatedGuardsAndBranches;
/// The set of guards which have been widened to include conditions to other
/// guards.
DenseSet<Instruction *> WidenedGuards;
/// Try to eliminate instruction \p Instr by widening it into an earlier
/// dominating guard. \p DFSI is the DFS iterator on the dominator tree that
/// is currently visiting the block containing \p Guard, and \p GuardsPerBlock
/// maps BasicBlocks to the set of guards seen in that block.
bool eliminateInstrViaWidening(
Instruction *Instr, const df_iterator<DomTreeNode *> &DFSI,
const DenseMap<BasicBlock *, SmallVector<Instruction *, 8>> &
GuardsPerBlock, bool InvertCondition = false);
/// Used to keep track of which widening potential is more effective.
enum WideningScore {
/// Don't widen.
/// Widening is performance neutral as far as the cycles spent in check
/// conditions goes (but can still help, e.g., code layout, having less
/// deopt state).
/// Widening is profitable.
/// Widening is very profitable. Not significantly different from \c
/// WS_Positive, except by the order.
static StringRef scoreTypeToString(WideningScore WS);
/// Compute the score for widening the condition in \p DominatedInstr
/// into \p DominatingGuard. If \p InvertCond is set, then we widen the
/// inverted condition of the dominating guard.
WideningScore computeWideningScore(Instruction *DominatedInstr,
Instruction *DominatingGuard,
bool InvertCond);
/// Helper to check if \p V can be hoisted to \p InsertPos.
bool isAvailableAt(const Value *V, const Instruction *InsertPos) const {
SmallPtrSet<const Instruction *, 8> Visited;
return isAvailableAt(V, InsertPos, Visited);
bool isAvailableAt(const Value *V, const Instruction *InsertPos,
SmallPtrSetImpl<const Instruction *> &Visited) const;
/// Helper to hoist \p V to \p InsertPos. Guaranteed to succeed if \c
/// isAvailableAt returned true.
void makeAvailableAt(Value *V, Instruction *InsertPos) const;
/// Common helper used by \c widenGuard and \c isWideningCondProfitable. Try
/// to generate an expression computing the logical AND of \p Cond0 and (\p
/// Cond1 XOR \p InvertCondition).
/// Return true if the expression computing the AND is only as
/// expensive as computing one of the two. If \p InsertPt is true then
/// actually generate the resulting expression, make it available at \p
/// InsertPt and return it in \p Result (else no change to the IR is made).
bool widenCondCommon(Value *Cond0, Value *Cond1, Instruction *InsertPt,
Value *&Result, bool InvertCondition);
/// Represents a range check of the form \c Base + \c Offset u< \c Length,
/// with the constraint that \c Length is not negative. \c CheckInst is the
/// pre-existing instruction in the IR that computes the result of this range
/// check.
class RangeCheck {
const Value *Base;
const ConstantInt *Offset;
const Value *Length;
ICmpInst *CheckInst;
explicit RangeCheck(const Value *Base, const ConstantInt *Offset,
const Value *Length, ICmpInst *CheckInst)
: Base(Base), Offset(Offset), Length(Length), CheckInst(CheckInst) {}
void setBase(const Value *NewBase) { Base = NewBase; }
void setOffset(const ConstantInt *NewOffset) { Offset = NewOffset; }
const Value *getBase() const { return Base; }
const ConstantInt *getOffset() const { return Offset; }
const APInt &getOffsetValue() const { return getOffset()->getValue(); }
const Value *getLength() const { return Length; };
ICmpInst *getCheckInst() const { return CheckInst; }
void print(raw_ostream &OS, bool PrintTypes = false) {
OS << "Base: ";
Base->printAsOperand(OS, PrintTypes);
OS << " Offset: ";
Offset->printAsOperand(OS, PrintTypes);
OS << " Length: ";
Length->printAsOperand(OS, PrintTypes);
LLVM_DUMP_METHOD void dump() {
dbgs() << "\n";
/// Parse \p CheckCond into a conjunction (logical-and) of range checks; and
/// append them to \p Checks. Returns true on success, may clobber \c Checks
/// on failure.
bool parseRangeChecks(Value *CheckCond, SmallVectorImpl<RangeCheck> &Checks) {
SmallPtrSet<const Value *, 8> Visited;
return parseRangeChecks(CheckCond, Checks, Visited);
bool parseRangeChecks(Value *CheckCond, SmallVectorImpl<RangeCheck> &Checks,
SmallPtrSetImpl<const Value *> &Visited);
/// Combine the checks in \p Checks into a smaller set of checks and append
/// them into \p CombinedChecks. Return true on success (i.e. all of checks
/// in \p Checks were combined into \p CombinedChecks). Clobbers \p Checks
/// and \p CombinedChecks on success and on failure.
bool combineRangeChecks(SmallVectorImpl<RangeCheck> &Checks,
SmallVectorImpl<RangeCheck> &CombinedChecks) const;
/// Can we compute the logical AND of \p Cond0 and \p Cond1 for the price of
/// computing only one of the two expressions?
bool isWideningCondProfitable(Value *Cond0, Value *Cond1, bool InvertCond) {
Value *ResultUnused;
return widenCondCommon(Cond0, Cond1, /*InsertPt=*/nullptr, ResultUnused,
/// If \p InvertCondition is false, Widen \p ToWiden to fail if
/// \p NewCondition is false, otherwise make it fail if \p NewCondition is
/// true (in addition to whatever it is already checking).
void widenGuard(Instruction *ToWiden, Value *NewCondition,
bool InvertCondition) {
Value *Result;
widenCondCommon(getCondition(ToWiden), NewCondition, ToWiden, Result,
Value *WidenableCondition = nullptr;
if (isGuardAsWidenableBranch(ToWiden)) {
auto *Cond = cast<BranchInst>(ToWiden)->getCondition();
WidenableCondition = cast<BinaryOperator>(Cond)->getOperand(1);
if (WidenableCondition)
Result = BinaryOperator::CreateAnd(Result, WidenableCondition,
"guard.chk", ToWiden);
setCondition(ToWiden, Result);
explicit GuardWideningImpl(DominatorTree &DT, PostDominatorTree *PDT,
LoopInfo &LI, BranchProbabilityInfo *BPI,
DomTreeNode *Root,
std::function<bool(BasicBlock*)> BlockFilter)
: DT(DT), PDT(PDT), LI(LI), BPI(BPI), Root(Root), BlockFilter(BlockFilter)
/// The entry point for this pass.
bool run();
static bool isSupportedGuardInstruction(const Instruction *Insn) {
if (isGuard(Insn))
return true;
if (WidenBranchGuards && isGuardAsWidenableBranch(Insn))
return true;
return false;
bool GuardWideningImpl::run() {
DenseMap<BasicBlock *, SmallVector<Instruction *, 8>> GuardsInBlock;
bool Changed = false;
Optional<BranchProbability> LikelyTaken = None;
if (WidenFrequentBranches && BPI) {
unsigned Threshold = FrequentBranchThreshold;
assert(Threshold > 0 && "Zero threshold makes no sense!");
LikelyTaken = BranchProbability(Threshold - 1, Threshold);
for (auto DFI = df_begin(Root), DFE = df_end(Root);
DFI != DFE; ++DFI) {
auto *BB = (*DFI)->getBlock();
if (!BlockFilter(BB))
auto &CurrentList = GuardsInBlock[BB];
for (auto &I : *BB)
if (isSupportedGuardInstruction(&I))
for (auto *II : CurrentList)
Changed |= eliminateInstrViaWidening(II, DFI, GuardsInBlock);
if (WidenFrequentBranches && BPI)
if (auto *BI = dyn_cast<BranchInst>(BB->getTerminator()))
if (BI->isConditional()) {
// If one of branches of a conditional is likely taken, try to
// eliminate it.
if (BPI->getEdgeProbability(BB, 0U) >= *LikelyTaken)
Changed |= eliminateInstrViaWidening(BI, DFI, GuardsInBlock);
else if (BPI->getEdgeProbability(BB, 1U) >= *LikelyTaken)
Changed |= eliminateInstrViaWidening(BI, DFI, GuardsInBlock,
assert(EliminatedGuardsAndBranches.empty() || Changed);
for (auto *I : EliminatedGuardsAndBranches)
if (!WidenedGuards.count(I)) {
assert(isa<ConstantInt>(getCondition(I)) && "Should be!");
if (isSupportedGuardInstruction(I))
else {
assert(isa<BranchInst>(I) &&
"Eliminated something other than guard or branch?");
return Changed;
bool GuardWideningImpl::eliminateInstrViaWidening(
Instruction *Instr, const df_iterator<DomTreeNode *> &DFSI,
const DenseMap<BasicBlock *, SmallVector<Instruction *, 8>> &
GuardsInBlock, bool InvertCondition) {
// Ignore trivial true or false conditions. These instructions will be
// trivially eliminated by any cleanup pass. Do not erase them because other
// guards can possibly be widened into them.
if (isa<ConstantInt>(getCondition(Instr)))
return false;
Instruction *BestSoFar = nullptr;
auto BestScoreSoFar = WS_IllegalOrNegative;
// In the set of dominating guards, find the one we can merge GuardInst with
// for the most profit.
for (unsigned i = 0, e = DFSI.getPathLength(); i != e; ++i) {
auto *CurBB = DFSI.getPath(i)->getBlock();
if (!BlockFilter(CurBB))
assert(GuardsInBlock.count(CurBB) && "Must have been populated by now!");
const auto &GuardsInCurBB = GuardsInBlock.find(CurBB)->second;
auto I = GuardsInCurBB.begin();
auto E = Instr->getParent() == CurBB
? std::find(GuardsInCurBB.begin(), GuardsInCurBB.end(), Instr)
: GuardsInCurBB.end();
#ifndef NDEBUG
unsigned Index = 0;
for (auto &I : *CurBB) {
if (Index == GuardsInCurBB.size())
if (GuardsInCurBB[Index] == &I)
assert(Index == GuardsInCurBB.size() &&
"Guards expected to be in order!");
assert((i == (e - 1)) == (Instr->getParent() == CurBB) && "Bad DFS?");
for (auto *Candidate : make_range(I, E)) {
auto Score = computeWideningScore(Instr, Candidate, InvertCondition);
LLVM_DEBUG(dbgs() << "Score between " << *getCondition(Instr)
<< " and " << *getCondition(Candidate) << " is "
<< scoreTypeToString(Score) << "\n");
if (Score > BestScoreSoFar) {
BestScoreSoFar = Score;
BestSoFar = Candidate;
if (BestScoreSoFar == WS_IllegalOrNegative) {
LLVM_DEBUG(dbgs() << "Did not eliminate guard " << *Instr << "\n");
return false;
assert(BestSoFar != Instr && "Should have never visited same guard!");
assert(DT.dominates(BestSoFar, Instr) && "Should be!");
LLVM_DEBUG(dbgs() << "Widening " << *Instr << " into " << *BestSoFar
<< " with score " << scoreTypeToString(BestScoreSoFar)
<< "\n");
widenGuard(BestSoFar, getCondition(Instr), InvertCondition);
auto NewGuardCondition = InvertCondition
? ConstantInt::getFalse(Instr->getContext())
: ConstantInt::getTrue(Instr->getContext());
setCondition(Instr, NewGuardCondition);
return true;
GuardWideningImpl::computeWideningScore(Instruction *DominatedInstr,
Instruction *DominatingGuard,
bool InvertCond) {
Loop *DominatedInstrLoop = LI.getLoopFor(DominatedInstr->getParent());
Loop *DominatingGuardLoop = LI.getLoopFor(DominatingGuard->getParent());
bool HoistingOutOfLoop = false;
if (DominatingGuardLoop != DominatedInstrLoop) {
// Be conservative and don't widen into a sibling loop. TODO: If the
// sibling is colder, we should consider allowing this.
if (DominatingGuardLoop &&
return WS_IllegalOrNegative;
HoistingOutOfLoop = true;
if (!isAvailableAt(getCondition(DominatedInstr), DominatingGuard))
return WS_IllegalOrNegative;
// If the guard was conditional executed, it may never be reached
// dynamically. There are two potential downsides to hoisting it out of the
// conditionally executed region: 1) we may spuriously deopt without need and
// 2) we have the extra cost of computing the guard condition in the common
// case. At the moment, we really only consider the second in our heuristic
// here. TODO: evaluate cost model for spurious deopt
// NOTE: As written, this also lets us hoist right over another guard which
// is essentially just another spelling for control flow.
if (isWideningCondProfitable(getCondition(DominatedInstr),
getCondition(DominatingGuard), InvertCond))
return HoistingOutOfLoop ? WS_VeryPositive : WS_Positive;
if (HoistingOutOfLoop)
return WS_Positive;
// Returns true if we might be hoisting above explicit control flow. Note
// that this completely ignores implicit control flow (guards, calls which
// throw, etc...). That choice appears arbitrary.
auto MaybeHoistingOutOfIf = [&]() {
auto *DominatingBlock = DominatingGuard->getParent();
auto *DominatedBlock = DominatedInstr->getParent();
if (isGuardAsWidenableBranch(DominatingGuard))
DominatingBlock = cast<BranchInst>(DominatingGuard)->getSuccessor(0);
// Same Block?
if (DominatedBlock == DominatingBlock)
return false;
// Obvious successor (common loop header/preheader case)
if (DominatedBlock == DominatingBlock->getUniqueSuccessor())
return false;
// TODO: diamond, triangle cases
if (!PDT) return true;
return !PDT->dominates(DominatedBlock, DominatingBlock);
return MaybeHoistingOutOfIf() ? WS_IllegalOrNegative : WS_Neutral;
bool GuardWideningImpl::isAvailableAt(
const Value *V, const Instruction *Loc,
SmallPtrSetImpl<const Instruction *> &Visited) const {
auto *Inst = dyn_cast<Instruction>(V);
if (!Inst || DT.dominates(Inst, Loc) || Visited.count(Inst))
return true;
if (!isSafeToSpeculativelyExecute(Inst, Loc, &DT) ||
return false;
// We only want to go _up_ the dominance chain when recursing.
assert(!isa<PHINode>(Loc) &&
"PHIs should return false for isSafeToSpeculativelyExecute");
assert(DT.isReachableFromEntry(Inst->getParent()) &&
"We did a DFS from the block entry!");
return all_of(Inst->operands(),
[&](Value *Op) { return isAvailableAt(Op, Loc, Visited); });
void GuardWideningImpl::makeAvailableAt(Value *V, Instruction *Loc) const {
auto *Inst = dyn_cast<Instruction>(V);
if (!Inst || DT.dominates(Inst, Loc))
assert(isSafeToSpeculativelyExecute(Inst, Loc, &DT) &&
!Inst->mayReadFromMemory() && "Should've checked with isAvailableAt!");
for (Value *Op : Inst->operands())
makeAvailableAt(Op, Loc);
bool GuardWideningImpl::widenCondCommon(Value *Cond0, Value *Cond1,
Instruction *InsertPt, Value *&Result,
bool InvertCondition) {
using namespace llvm::PatternMatch;
// L >u C0 && L >u C1 -> L >u max(C0, C1)
ConstantInt *RHS0, *RHS1;
Value *LHS;
ICmpInst::Predicate Pred0, Pred1;
if (match(Cond0, m_ICmp(Pred0, m_Value(LHS), m_ConstantInt(RHS0))) &&
match(Cond1, m_ICmp(Pred1, m_Specific(LHS), m_ConstantInt(RHS1)))) {
if (InvertCondition)
Pred1 = ICmpInst::getInversePredicate(Pred1);
ConstantRange CR0 =
ConstantRange::makeExactICmpRegion(Pred0, RHS0->getValue());
ConstantRange CR1 =
ConstantRange::makeExactICmpRegion(Pred1, RHS1->getValue());
// SubsetIntersect is a subset of the actual mathematical intersection of
// CR0 and CR1, while SupersetIntersect is a superset of the actual
// mathematical intersection. If these two ConstantRanges are equal, then
// we know we were able to represent the actual mathematical intersection
// of CR0 and CR1, and can use the same to generate an icmp instruction.
// Given what we're doing here and the semantics of guards, it would
// actually be correct to just use SubsetIntersect, but that may be too
// aggressive in cases we care about.
auto SubsetIntersect = CR0.inverse().unionWith(CR1.inverse()).inverse();
auto SupersetIntersect = CR0.intersectWith(CR1);
CmpInst::Predicate Pred;
if (SubsetIntersect == SupersetIntersect &&
SubsetIntersect.getEquivalentICmp(Pred, NewRHSAP)) {
if (InsertPt) {
ConstantInt *NewRHS = ConstantInt::get(Cond0->getContext(), NewRHSAP);
Result = new ICmpInst(InsertPt, Pred, LHS, NewRHS, "wide.chk");
return true;
SmallVector<GuardWideningImpl::RangeCheck, 4> Checks, CombinedChecks;
// TODO: Support InvertCondition case?
if (!InvertCondition &&
parseRangeChecks(Cond0, Checks) && parseRangeChecks(Cond1, Checks) &&
combineRangeChecks(Checks, CombinedChecks)) {
if (InsertPt) {
Result = nullptr;
for (auto &RC : CombinedChecks) {
makeAvailableAt(RC.getCheckInst(), InsertPt);
if (Result)
Result = BinaryOperator::CreateAnd(RC.getCheckInst(), Result, "",
Result = RC.getCheckInst();
return true;
// Base case -- just logical-and the two conditions together.
if (InsertPt) {
makeAvailableAt(Cond0, InsertPt);
makeAvailableAt(Cond1, InsertPt);
if (InvertCondition)
Cond1 = BinaryOperator::CreateNot(Cond1, "inverted", InsertPt);
Result = BinaryOperator::CreateAnd(Cond0, Cond1, "wide.chk", InsertPt);
// We were not able to compute Cond0 AND Cond1 for the price of one.
return false;
bool GuardWideningImpl::parseRangeChecks(
Value *CheckCond, SmallVectorImpl<GuardWideningImpl::RangeCheck> &Checks,
SmallPtrSetImpl<const Value *> &Visited) {
if (!Visited.insert(CheckCond).second)
return true;
using namespace llvm::PatternMatch;
Value *AndLHS, *AndRHS;
if (match(CheckCond, m_And(m_Value(AndLHS), m_Value(AndRHS))))
return parseRangeChecks(AndLHS, Checks) &&
parseRangeChecks(AndRHS, Checks);
auto *IC = dyn_cast<ICmpInst>(CheckCond);
if (!IC || !IC->getOperand(0)->getType()->isIntegerTy() ||
(IC->getPredicate() != ICmpInst::ICMP_ULT &&
IC->getPredicate() != ICmpInst::ICMP_UGT))
return false;
const Value *CmpLHS = IC->getOperand(0), *CmpRHS = IC->getOperand(1);
if (IC->getPredicate() == ICmpInst::ICMP_UGT)
std::swap(CmpLHS, CmpRHS);
auto &DL = IC->getModule()->getDataLayout();
GuardWideningImpl::RangeCheck Check(
CmpLHS, cast<ConstantInt>(ConstantInt::getNullValue(CmpRHS->getType())),
CmpRHS, IC);
if (!isKnownNonNegative(Check.getLength(), DL))
return false;
// What we have in \c Check now is a correct interpretation of \p CheckCond.
// Try to see if we can move some constant offsets into the \c Offset field.
bool Changed;
auto &Ctx = CheckCond->getContext();
do {
Value *OpLHS;
ConstantInt *OpRHS;
Changed = false;
#ifndef NDEBUG
auto *BaseInst = dyn_cast<Instruction>(Check.getBase());
assert((!BaseInst || DT.isReachableFromEntry(BaseInst->getParent())) &&
"Unreachable instruction?");
if (match(Check.getBase(), m_Add(m_Value(OpLHS), m_ConstantInt(OpRHS)))) {
APInt NewOffset = Check.getOffsetValue() + OpRHS->getValue();
Check.setOffset(ConstantInt::get(Ctx, NewOffset));
Changed = true;
} else if (match(Check.getBase(),
m_Or(m_Value(OpLHS), m_ConstantInt(OpRHS)))) {
KnownBits Known = computeKnownBits(OpLHS, DL);
if ((OpRHS->getValue() & Known.Zero) == OpRHS->getValue()) {
APInt NewOffset = Check.getOffsetValue() + OpRHS->getValue();
Check.setOffset(ConstantInt::get(Ctx, NewOffset));
Changed = true;
} while (Changed);
return true;
bool GuardWideningImpl::combineRangeChecks(
SmallVectorImpl<GuardWideningImpl::RangeCheck> &Checks,
SmallVectorImpl<GuardWideningImpl::RangeCheck> &RangeChecksOut) const {
unsigned OldCount = Checks.size();
while (!Checks.empty()) {
// Pick all of the range checks with a specific base and length, and try to
// merge them.
const Value *CurrentBase = Checks.front().getBase();
const Value *CurrentLength = Checks.front().getLength();
SmallVector<GuardWideningImpl::RangeCheck, 3> CurrentChecks;
auto IsCurrentCheck = [&](GuardWideningImpl::RangeCheck &RC) {
return RC.getBase() == CurrentBase && RC.getLength() == CurrentLength;
copy_if(Checks, std::back_inserter(CurrentChecks), IsCurrentCheck);
Checks.erase(remove_if(Checks, IsCurrentCheck), Checks.end());
assert(CurrentChecks.size() != 0 && "We know we have at least one!");
if (CurrentChecks.size() < 3) {
RangeChecksOut.insert(RangeChecksOut.end(), CurrentChecks.begin(),
// CurrentChecks.size() will typically be 3 here, but so far there has been
// no need to hard-code that fact.
llvm::sort(CurrentChecks, [&](const GuardWideningImpl::RangeCheck &LHS,
const GuardWideningImpl::RangeCheck &RHS) {
return LHS.getOffsetValue().slt(RHS.getOffsetValue());
// Note: std::sort should not invalidate the ChecksStart iterator.
const ConstantInt *MinOffset = CurrentChecks.front().getOffset();
const ConstantInt *MaxOffset = CurrentChecks.back().getOffset();
unsigned BitWidth = MaxOffset->getValue().getBitWidth();
if ((MaxOffset->getValue() - MinOffset->getValue())
return false;
APInt MaxDiff = MaxOffset->getValue() - MinOffset->getValue();
const APInt &HighOffset = MaxOffset->getValue();
auto OffsetOK = [&](const GuardWideningImpl::RangeCheck &RC) {
return (HighOffset - RC.getOffsetValue()).ult(MaxDiff);
if (MaxDiff.isMinValue() ||
!std::all_of(std::next(CurrentChecks.begin()), CurrentChecks.end(),
return false;
// We have a series of f+1 checks as:
// I+k_0 u< L ... Chk_0
// I+k_1 u< L ... Chk_1
// ...
// I+k_f u< L ... Chk_f
// with forall i in [0,f]: k_f-k_i u< k_f-k_0 ... Precond_0
// k_f-k_0 u< INT_MIN+k_f ... Precond_1
// k_f != k_0 ... Precond_2
// Claim:
// Chk_0 AND Chk_f implies all the other checks
// Informal proof sketch:
// We will show that the integer range [I+k_0,I+k_f] does not unsigned-wrap
// (i.e. going from I+k_0 to I+k_f does not cross the -1,0 boundary) and
// thus I+k_f is the greatest unsigned value in that range.
// This combined with Ckh_(f+1) shows that everything in that range is u< L.
// Via Precond_0 we know that all of the indices in Chk_0 through Chk_(f+1)
// lie in [I+k_0,I+k_f], this proving our claim.
// To see that [I+k_0,I+k_f] is not a wrapping range, note that there are
// two possibilities: I+k_0 u< I+k_f or I+k_0 >u I+k_f (they can't be equal
// since k_0 != k_f). In the former case, [I+k_0,I+k_f] is not a wrapping
// range by definition, and the latter case is impossible:
// 0-----I+k_f---I+k_0----L---INT_MAX,INT_MIN------------------(-1)
// xxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
// For Chk_0 to succeed, we'd have to have k_f-k_0 (the range highlighted
// with 'x' above) to be at least >u INT_MIN.
assert(RangeChecksOut.size() <= OldCount && "We pessimized!");
return RangeChecksOut.size() != OldCount;
#ifndef NDEBUG
StringRef GuardWideningImpl::scoreTypeToString(WideningScore WS) {
switch (WS) {
case WS_IllegalOrNegative:
return "IllegalOrNegative";
case WS_Neutral:
return "Neutral";
case WS_Positive:
return "Positive";
case WS_VeryPositive:
return "VeryPositive";
llvm_unreachable("Fully covered switch above!");
PreservedAnalyses GuardWideningPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &LI = AM.getResult<LoopAnalysis>(F);
auto &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
BranchProbabilityInfo *BPI = nullptr;
if (WidenFrequentBranches)
BPI = AM.getCachedResult<BranchProbabilityAnalysis>(F);
if (!GuardWideningImpl(DT, &PDT, LI, BPI, DT.getRootNode(),
[](BasicBlock*) { return true; } ).run())
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
PreservedAnalyses GuardWideningPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &U) {
const auto &FAM =
AM.getResult<FunctionAnalysisManagerLoopProxy>(L, AR).getManager();
Function &F = *L.getHeader()->getParent();
BranchProbabilityInfo *BPI = nullptr;
if (WidenFrequentBranches)
BPI = FAM.getCachedResult<BranchProbabilityAnalysis>(F);
BasicBlock *RootBB = L.getLoopPredecessor();
if (!RootBB)
RootBB = L.getHeader();
auto BlockFilter = [&](BasicBlock *BB) {
return BB == RootBB || L.contains(BB);
if (!GuardWideningImpl(AR.DT, nullptr, AR.LI, BPI,
return PreservedAnalyses::all();
return getLoopPassPreservedAnalyses();
namespace {
struct GuardWideningLegacyPass : public FunctionPass {
static char ID;
GuardWideningLegacyPass() : FunctionPass(ID) {
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
BranchProbabilityInfo *BPI = nullptr;
if (WidenFrequentBranches)
BPI = &getAnalysis<BranchProbabilityInfoWrapperPass>().getBPI();
return GuardWideningImpl(DT, &PDT, LI, BPI, DT.getRootNode(),
[](BasicBlock*) { return true; } ).run();
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (WidenFrequentBranches)
/// Same as above, but restricted to a single loop at a time. Can be
/// scheduled with other loop passes w/o breaking out of LPM
struct LoopGuardWideningLegacyPass : public LoopPass {
static char ID;
LoopGuardWideningLegacyPass() : LoopPass(ID) {
bool runOnLoop(Loop *L, LPPassManager &LPM) override {
if (skipLoop(L))
return false;
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>();
auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr;
BasicBlock *RootBB = L->getLoopPredecessor();
if (!RootBB)
RootBB = L->getHeader();
auto BlockFilter = [&](BasicBlock *BB) {
return BB == RootBB || L->contains(BB);
BranchProbabilityInfo *BPI = nullptr;
if (WidenFrequentBranches)
BPI = &getAnalysis<BranchProbabilityInfoWrapperPass>().getBPI();
return GuardWideningImpl(DT, PDT, LI, BPI,
DT.getNode(RootBB), BlockFilter).run();
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (WidenFrequentBranches)
char GuardWideningLegacyPass::ID = 0;
char LoopGuardWideningLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(GuardWideningLegacyPass, "guard-widening", "Widen guards",
false, false)
if (WidenFrequentBranches)
INITIALIZE_PASS_END(GuardWideningLegacyPass, "guard-widening", "Widen guards",
false, false)
INITIALIZE_PASS_BEGIN(LoopGuardWideningLegacyPass, "loop-guard-widening",
"Widen guards (within a single loop, as a loop pass)",
false, false)
if (WidenFrequentBranches)
INITIALIZE_PASS_END(LoopGuardWideningLegacyPass, "loop-guard-widening",
"Widen guards (within a single loop, as a loop pass)",
false, false)
FunctionPass *llvm::createGuardWideningPass() {
return new GuardWideningLegacyPass();
Pass *llvm::createLoopGuardWideningPass() {
return new LoopGuardWideningLegacyPass();
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