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//===- LoopVersioning.cpp - Utility to version a loop ---------------------===//
// 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 defines a utility class to perform loop versioning. The versioned
// loop speculates that otherwise may-aliasing memory accesses don't overlap and
// emits checks to prove this.
#include "llvm/Transforms/Utils/LoopVersioning.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/InstSimplifyFolder.h"
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
using namespace llvm;
#define DEBUG_TYPE "loop-versioning"
static cl::opt<bool>
AnnotateNoAlias("loop-version-annotate-no-alias", cl::init(true),
cl::desc("Add no-alias annotation for instructions that "
"are disambiguated by memchecks"));
LoopVersioning::LoopVersioning(const LoopAccessInfo &LAI,
ArrayRef<RuntimePointerCheck> Checks, Loop *L,
LoopInfo *LI, DominatorTree *DT,
ScalarEvolution *SE)
: VersionedLoop(L), AliasChecks(Checks.begin(), Checks.end()),
Preds(LAI.getPSE().getPredicate()), LAI(LAI), LI(LI), DT(DT),
SE(SE) {
void LoopVersioning::versionLoop(
const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
assert(VersionedLoop->getUniqueExitBlock() && "No single exit block");
assert(VersionedLoop->isLoopSimplifyForm() &&
"Loop is not in loop-simplify form");
Value *MemRuntimeCheck;
Value *SCEVRuntimeCheck;
Value *RuntimeCheck = nullptr;
// Add the memcheck in the original preheader (this is empty initially).
BasicBlock *RuntimeCheckBB = VersionedLoop->getLoopPreheader();
const auto &RtPtrChecking = *LAI.getRuntimePointerChecking();
SCEVExpander Exp2(*RtPtrChecking.getSE(),
MemRuntimeCheck = addRuntimeChecks(RuntimeCheckBB->getTerminator(),
VersionedLoop, AliasChecks, Exp2);
SCEVExpander Exp(*SE, RuntimeCheckBB->getModule()->getDataLayout(),
SCEVRuntimeCheck =
Exp.expandCodeForPredicate(&Preds, RuntimeCheckBB->getTerminator());
IRBuilder<InstSimplifyFolder> Builder(
if (MemRuntimeCheck && SCEVRuntimeCheck) {
RuntimeCheck =
Builder.CreateOr(MemRuntimeCheck, SCEVRuntimeCheck, "");
} else
RuntimeCheck = MemRuntimeCheck ? MemRuntimeCheck : SCEVRuntimeCheck;
assert(RuntimeCheck && "called even though we don't need "
"any runtime checks");
// Rename the block to make the IR more readable.
RuntimeCheckBB->setName(VersionedLoop->getHeader()->getName() +
// Create empty preheader for the loop (and after cloning for the
// non-versioned loop).
BasicBlock *PH =
SplitBlock(RuntimeCheckBB, RuntimeCheckBB->getTerminator(), DT, LI,
nullptr, VersionedLoop->getHeader()->getName() + ".ph");
// Clone the loop including the preheader.
// FIXME: This does not currently preserve SimplifyLoop because the exit
// block is a join between the two loops.
SmallVector<BasicBlock *, 8> NonVersionedLoopBlocks;
NonVersionedLoop =
cloneLoopWithPreheader(PH, RuntimeCheckBB, VersionedLoop, VMap,
".lver.orig", LI, DT, NonVersionedLoopBlocks);
remapInstructionsInBlocks(NonVersionedLoopBlocks, VMap);
// Insert the conditional branch based on the result of the memchecks.
Instruction *OrigTerm = RuntimeCheckBB->getTerminator();
Builder.CreateCondBr(RuntimeCheck, NonVersionedLoop->getLoopPreheader(),
// The loops merge in the original exit block. This is now dominated by the
// memchecking block.
DT->changeImmediateDominator(VersionedLoop->getExitBlock(), RuntimeCheckBB);
// Adds the necessary PHI nodes for the versioned loops based on the
// loop-defined values used outside of the loop.
formDedicatedExitBlocks(NonVersionedLoop, DT, LI, nullptr, true);
formDedicatedExitBlocks(VersionedLoop, DT, LI, nullptr, true);
assert(NonVersionedLoop->isLoopSimplifyForm() &&
VersionedLoop->isLoopSimplifyForm() &&
"The versioned loops should be in simplify form.");
void LoopVersioning::addPHINodes(
const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
BasicBlock *PHIBlock = VersionedLoop->getExitBlock();
assert(PHIBlock && "No single successor to loop exit block");
PHINode *PN;
// First add a single-operand PHI for each DefsUsedOutside if one does not
// exists yet.
for (auto *Inst : DefsUsedOutside) {
// See if we have a single-operand PHI with the value defined by the
// original loop.
for (auto I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(I)); ++I) {
if (PN->getIncomingValue(0) == Inst) {
// If not create it.
if (!PN) {
PN = PHINode::Create(Inst->getType(), 2, Inst->getName() + ".lver");
SmallVector<User*, 8> UsersToUpdate;
for (User *U : Inst->users())
if (!VersionedLoop->contains(cast<Instruction>(U)->getParent()))
for (User *U : UsersToUpdate)
U->replaceUsesOfWith(Inst, PN);
PN->addIncoming(Inst, VersionedLoop->getExitingBlock());
// Then for each PHI add the operand for the edge from the cloned loop.
for (auto I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(I)); ++I) {
assert(PN->getNumOperands() == 1 &&
"Exit block should only have on predecessor");
// If the definition was cloned used that otherwise use the same value.
Value *ClonedValue = PN->getIncomingValue(0);
auto Mapped = VMap.find(ClonedValue);
if (Mapped != VMap.end())
ClonedValue = Mapped->second;
PN->addIncoming(ClonedValue, NonVersionedLoop->getExitingBlock());
void LoopVersioning::prepareNoAliasMetadata() {
// We need to turn the no-alias relation between pointer checking groups into
// no-aliasing annotations between instructions.
// We accomplish this by mapping each pointer checking group (a set of
// pointers memchecked together) to an alias scope and then also mapping each
// group to the list of scopes it can't alias.
const RuntimePointerChecking *RtPtrChecking = LAI.getRuntimePointerChecking();
LLVMContext &Context = VersionedLoop->getHeader()->getContext();
// First allocate an aliasing scope for each pointer checking group.
// While traversing through the checking groups in the loop, also create a
// reverse map from pointers to the pointer checking group they were assigned
// to.
MDBuilder MDB(Context);
MDNode *Domain = MDB.createAnonymousAliasScopeDomain("LVerDomain");
for (const auto &Group : RtPtrChecking->CheckingGroups) {
GroupToScope[&Group] = MDB.createAnonymousAliasScope(Domain);
for (unsigned PtrIdx : Group.Members)
PtrToGroup[RtPtrChecking->getPointerInfo(PtrIdx).PointerValue] = &Group;
// Go through the checks and for each pointer group, collect the scopes for
// each non-aliasing pointer group.
DenseMap<const RuntimeCheckingPtrGroup *, SmallVector<Metadata *, 4>>
for (const auto &Check : AliasChecks)
// Finally, transform the above to actually map to scope list which is what
// the metadata uses.
for (const auto &Pair : GroupToNonAliasingScopes)
GroupToNonAliasingScopeList[Pair.first] = MDNode::get(Context, Pair.second);
void LoopVersioning::annotateLoopWithNoAlias() {
if (!AnnotateNoAlias)
// First prepare the maps.
// Add the scope and no-alias metadata to the instructions.
for (Instruction *I : LAI.getDepChecker().getMemoryInstructions()) {
void LoopVersioning::annotateInstWithNoAlias(Instruction *VersionedInst,
const Instruction *OrigInst) {
if (!AnnotateNoAlias)
LLVMContext &Context = VersionedLoop->getHeader()->getContext();
const Value *Ptr = isa<LoadInst>(OrigInst)
? cast<LoadInst>(OrigInst)->getPointerOperand()
: cast<StoreInst>(OrigInst)->getPointerOperand();
// Find the group for the pointer and then add the scope metadata.
auto Group = PtrToGroup.find(Ptr);
if (Group != PtrToGroup.end()) {
MDNode::get(Context, GroupToScope[Group->second])));
// Add the no-alias metadata.
auto NonAliasingScopeList = GroupToNonAliasingScopeList.find(Group->second);
if (NonAliasingScopeList != GroupToNonAliasingScopeList.end())
namespace {
bool runImpl(LoopInfo *LI, LoopAccessInfoManager &LAIs, DominatorTree *DT,
ScalarEvolution *SE) {
// Build up a worklist of inner-loops to version. This is necessary as the
// act of versioning a loop creates new loops and can invalidate iterators
// across the loops.
SmallVector<Loop *, 8> Worklist;
for (Loop *TopLevelLoop : *LI)
for (Loop *L : depth_first(TopLevelLoop))
// We only handle inner-most loops.
if (L->isInnermost())
// Now walk the identified inner loops.
bool Changed = false;
for (Loop *L : Worklist) {
if (!L->isLoopSimplifyForm() || !L->isRotatedForm() ||
const LoopAccessInfo &LAI = LAIs.getInfo(*L);
if (!LAI.hasConvergentOp() &&
(LAI.getNumRuntimePointerChecks() ||
!LAI.getPSE().getPredicate().isAlwaysTrue())) {
LoopVersioning LVer(LAI, LAI.getRuntimePointerChecking()->getChecks(), L,
LI, DT, SE);
Changed = true;
return Changed;
PreservedAnalyses LoopVersioningPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
auto &LI = AM.getResult<LoopAnalysis>(F);
LoopAccessInfoManager &LAIs = AM.getResult<LoopAccessAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
if (runImpl(&LI, LAIs, &DT, &SE))
return PreservedAnalyses::none();
return PreservedAnalyses::all();