1 //===- LoopVersioning.cpp - Utility to version a loop ---------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines a utility class to perform loop versioning. The versioned
11 // loop speculates that otherwise may-aliasing memory accesses don't overlap and
12 // emits checks to prove this.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Transforms/Utils/LoopVersioning.h"
18 #include "llvm/Analysis/LoopAccessAnalysis.h"
19 #include "llvm/Analysis/LoopInfo.h"
20 #include "llvm/Analysis/ScalarEvolutionExpander.h"
21 #include "llvm/IR/Dominators.h"
22 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
23 #include "llvm/Transforms/Utils/Cloning.h"
27 LoopVersioning::LoopVersioning(const LoopAccessInfo &LAI, Loop *L, LoopInfo *LI,
28 DominatorTree *DT, ScalarEvolution *SE,
30 : VersionedLoop(L), NonVersionedLoop(nullptr), LAI(LAI), LI(LI), DT(DT),
32 assert(L->getExitBlock() && "No single exit block");
33 assert(L->getLoopPreheader() && "No preheader");
35 setAliasChecks(LAI.getRuntimePointerChecking()->getChecks());
36 setSCEVChecks(LAI.Preds);
40 void LoopVersioning::setAliasChecks(
41 const SmallVector<RuntimePointerChecking::PointerCheck, 4> Checks) {
42 AliasChecks = std::move(Checks);
45 void LoopVersioning::setSCEVChecks(SCEVUnionPredicate Check) {
46 Preds = std::move(Check);
49 void LoopVersioning::versionLoop(
50 const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
51 Instruction *FirstCheckInst;
52 Instruction *MemRuntimeCheck;
53 Value *SCEVRuntimeCheck;
54 Value *RuntimeCheck = nullptr;
56 // Add the memcheck in the original preheader (this is empty initially).
57 BasicBlock *RuntimeCheckBB = VersionedLoop->getLoopPreheader();
58 std::tie(FirstCheckInst, MemRuntimeCheck) =
59 LAI.addRuntimeChecks(RuntimeCheckBB->getTerminator(), AliasChecks);
60 assert(MemRuntimeCheck && "called even though needsAnyChecking = false");
62 const SCEVUnionPredicate &Pred = LAI.Preds;
63 SCEVExpander Exp(*SE, RuntimeCheckBB->getModule()->getDataLayout(),
66 Exp.expandCodeForPredicate(&Pred, RuntimeCheckBB->getTerminator());
67 auto *CI = dyn_cast<ConstantInt>(SCEVRuntimeCheck);
69 // Discard the SCEV runtime check if it is always true.
70 if (CI && CI->isZero())
71 SCEVRuntimeCheck = nullptr;
73 if (MemRuntimeCheck && SCEVRuntimeCheck) {
74 RuntimeCheck = BinaryOperator::Create(Instruction::Or, MemRuntimeCheck,
75 SCEVRuntimeCheck, "ldist.safe");
76 if (auto *I = dyn_cast<Instruction>(RuntimeCheck))
77 I->insertBefore(RuntimeCheckBB->getTerminator());
79 RuntimeCheck = MemRuntimeCheck ? MemRuntimeCheck : SCEVRuntimeCheck;
81 assert(RuntimeCheck && "called even though we don't need "
82 "any runtime checks");
84 // Rename the block to make the IR more readable.
85 RuntimeCheckBB->setName(VersionedLoop->getHeader()->getName() +
88 // Create empty preheader for the loop (and after cloning for the
89 // non-versioned loop).
91 SplitBlock(RuntimeCheckBB, RuntimeCheckBB->getTerminator(), DT, LI);
92 PH->setName(VersionedLoop->getHeader()->getName() + ".ph");
94 // Clone the loop including the preheader.
96 // FIXME: This does not currently preserve SimplifyLoop because the exit
97 // block is a join between the two loops.
98 SmallVector<BasicBlock *, 8> NonVersionedLoopBlocks;
100 cloneLoopWithPreheader(PH, RuntimeCheckBB, VersionedLoop, VMap,
101 ".lver.orig", LI, DT, NonVersionedLoopBlocks);
102 remapInstructionsInBlocks(NonVersionedLoopBlocks, VMap);
104 // Insert the conditional branch based on the result of the memchecks.
105 Instruction *OrigTerm = RuntimeCheckBB->getTerminator();
106 BranchInst::Create(NonVersionedLoop->getLoopPreheader(),
107 VersionedLoop->getLoopPreheader(), RuntimeCheck, OrigTerm);
108 OrigTerm->eraseFromParent();
110 // The loops merge in the original exit block. This is now dominated by the
111 // memchecking block.
112 DT->changeImmediateDominator(VersionedLoop->getExitBlock(), RuntimeCheckBB);
114 // Adds the necessary PHI nodes for the versioned loops based on the
115 // loop-defined values used outside of the loop.
116 addPHINodes(DefsUsedOutside);
119 void LoopVersioning::addPHINodes(
120 const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
121 BasicBlock *PHIBlock = VersionedLoop->getExitBlock();
122 assert(PHIBlock && "No single successor to loop exit block");
124 for (auto *Inst : DefsUsedOutside) {
125 auto *NonVersionedLoopInst = cast<Instruction>(VMap[Inst]);
128 // First see if we have a single-operand PHI with the value defined by the
130 for (auto I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(I)); ++I) {
131 assert(PN->getNumOperands() == 1 &&
132 "Exit block should only have on predecessor");
133 if (PN->getIncomingValue(0) == Inst)
138 PN = PHINode::Create(Inst->getType(), 2, Inst->getName() + ".lver",
140 for (auto *User : Inst->users())
141 if (!VersionedLoop->contains(cast<Instruction>(User)->getParent()))
142 User->replaceUsesOfWith(Inst, PN);
143 PN->addIncoming(Inst, VersionedLoop->getExitingBlock());
145 // Add the new incoming value from the non-versioned loop.
146 PN->addIncoming(NonVersionedLoopInst, NonVersionedLoop->getExitingBlock());