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//===- LoopVersioning.cpp - Utility to version a loop ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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/Analysis/LoopAccessAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/IR/Dominators.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
using namespace llvm;
LoopVersioning::LoopVersioning(const LoopAccessInfo &LAI, Loop *L, LoopInfo *LI,
DominatorTree *DT, ScalarEvolution *SE,
bool UseLAIChecks)
: VersionedLoop(L), NonVersionedLoop(nullptr), LAI(LAI), LI(LI), DT(DT),
SE(SE) {
assert(L->getExitBlock() && "No single exit block");
assert(L->getLoopPreheader() && "No preheader");
if (UseLAIChecks) {
setAliasChecks(LAI.getRuntimePointerChecking()->getChecks());
setSCEVChecks(LAI.PSE.getUnionPredicate());
}
}
void LoopVersioning::setAliasChecks(
const SmallVector<RuntimePointerChecking::PointerCheck, 4> Checks) {
AliasChecks = std::move(Checks);
}
void LoopVersioning::setSCEVChecks(SCEVUnionPredicate Check) {
Preds = std::move(Check);
}
void LoopVersioning::versionLoop(
const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
Instruction *FirstCheckInst;
Instruction *MemRuntimeCheck;
Value *SCEVRuntimeCheck;
Value *RuntimeCheck = nullptr;
// Add the memcheck in the original preheader (this is empty initially).
BasicBlock *RuntimeCheckBB = VersionedLoop->getLoopPreheader();
std::tie(FirstCheckInst, MemRuntimeCheck) =
LAI.addRuntimeChecks(RuntimeCheckBB->getTerminator(), AliasChecks);
assert(MemRuntimeCheck && "called even though needsAnyChecking = false");
const SCEVUnionPredicate &Pred = LAI.PSE.getUnionPredicate();
SCEVExpander Exp(*SE, RuntimeCheckBB->getModule()->getDataLayout(),
"scev.check");
SCEVRuntimeCheck =
Exp.expandCodeForPredicate(&Pred, RuntimeCheckBB->getTerminator());
auto *CI = dyn_cast<ConstantInt>(SCEVRuntimeCheck);
// Discard the SCEV runtime check if it is always true.
if (CI && CI->isZero())
SCEVRuntimeCheck = nullptr;
if (MemRuntimeCheck && SCEVRuntimeCheck) {
RuntimeCheck = BinaryOperator::Create(Instruction::Or, MemRuntimeCheck,
SCEVRuntimeCheck, "ldist.safe");
if (auto *I = dyn_cast<Instruction>(RuntimeCheck))
I->insertBefore(RuntimeCheckBB->getTerminator());
} 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() +
".lver.check");
// Create empty preheader for the loop (and after cloning for the
// non-versioned loop).
BasicBlock *PH =
SplitBlock(RuntimeCheckBB, RuntimeCheckBB->getTerminator(), DT, LI);
PH->setName(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();
BranchInst::Create(NonVersionedLoop->getLoopPreheader(),
VersionedLoop->getLoopPreheader(), RuntimeCheck, OrigTerm);
OrigTerm->eraseFromParent();
// 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.
addPHINodes(DefsUsedOutside);
}
void LoopVersioning::addPHINodes(
const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
BasicBlock *PHIBlock = VersionedLoop->getExitBlock();
assert(PHIBlock && "No single successor to loop exit block");
for (auto *Inst : DefsUsedOutside) {
auto *NonVersionedLoopInst = cast<Instruction>(VMap[Inst]);
PHINode *PN;
// First 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) {
assert(PN->getNumOperands() == 1 &&
"Exit block should only have on predecessor");
if (PN->getIncomingValue(0) == Inst)
break;
}
// If not create it.
if (!PN) {
PN = PHINode::Create(Inst->getType(), 2, Inst->getName() + ".lver",
&PHIBlock->front());
for (auto *User : Inst->users())
if (!VersionedLoop->contains(cast<Instruction>(User)->getParent()))
User->replaceUsesOfWith(Inst, PN);
PN->addIncoming(Inst, VersionedLoop->getExitingBlock());
}
// Add the new incoming value from the non-versioned loop.
PN->addIncoming(NonVersionedLoopInst, NonVersionedLoop->getExitingBlock());
}
}
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