1 //===- LoopVersioning.cpp - Utility to version a loop ---------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
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.
13 //
14 //===----------------------------------------------------------------------===//
15 
16 #include "llvm/Analysis/LoopAccessAnalysis.h"
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/IR/Dominators.h"
19 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
20 #include "llvm/Transforms/Utils/Cloning.h"
21 #include "llvm/Transforms/Utils/LoopVersioning.h"
22 
23 using namespace llvm;
24 
25 LoopVersioning::LoopVersioning(
26     SmallVector<RuntimePointerChecking::PointerCheck, 4> Checks,
27     const LoopAccessInfo &LAI, Loop *L, LoopInfo *LI, DominatorTree *DT,
28     const SmallVector<int, 8> *PtrToPartition)
29     : VersionedLoop(L), NonVersionedLoop(nullptr),
30       PtrToPartition(PtrToPartition), Checks(Checks), LAI(LAI), LI(LI), DT(DT) {
31   assert(L->getExitBlock() && "No single exit block");
32   assert(L->getLoopPreheader() && "No preheader");
33 }
34 
35 bool LoopVersioning::needsRuntimeChecks() const {
36   return LAI.getRuntimePointerChecking()->needsAnyChecking(PtrToPartition);
37 }
38 
39 void LoopVersioning::versionLoop(Pass *P) {
40   Instruction *FirstCheckInst;
41   Instruction *MemRuntimeCheck;
42   // Add the memcheck in the original preheader (this is empty initially).
43   BasicBlock *MemCheckBB = VersionedLoop->getLoopPreheader();
44   std::tie(FirstCheckInst, MemRuntimeCheck) =
45       LAI.addRuntimeCheck(MemCheckBB->getTerminator(), Checks);
46   assert(MemRuntimeCheck && "called even though needsAnyChecking = false");
47 
48   // Rename the block to make the IR more readable.
49   MemCheckBB->setName(VersionedLoop->getHeader()->getName() + ".lver.memcheck");
50 
51   // Create empty preheader for the loop (and after cloning for the
52   // non-versioned loop).
53   BasicBlock *PH = SplitBlock(MemCheckBB, MemCheckBB->getTerminator(), DT, LI);
54   PH->setName(VersionedLoop->getHeader()->getName() + ".ph");
55 
56   // Clone the loop including the preheader.
57   //
58   // FIXME: This does not currently preserve SimplifyLoop because the exit
59   // block is a join between the two loops.
60   SmallVector<BasicBlock *, 8> NonVersionedLoopBlocks;
61   NonVersionedLoop =
62       cloneLoopWithPreheader(PH, MemCheckBB, VersionedLoop, VMap, ".lver.orig",
63                              LI, DT, NonVersionedLoopBlocks);
64   remapInstructionsInBlocks(NonVersionedLoopBlocks, VMap);
65 
66   // Insert the conditional branch based on the result of the memchecks.
67   Instruction *OrigTerm = MemCheckBB->getTerminator();
68   BranchInst::Create(NonVersionedLoop->getLoopPreheader(),
69                      VersionedLoop->getLoopPreheader(), MemRuntimeCheck,
70                      OrigTerm);
71   OrigTerm->eraseFromParent();
72 
73   // The loops merge in the original exit block.  This is now dominated by the
74   // memchecking block.
75   DT->changeImmediateDominator(VersionedLoop->getExitBlock(), MemCheckBB);
76 }
77 
78 void LoopVersioning::addPHINodes(
79     const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
80   BasicBlock *PHIBlock = VersionedLoop->getExitBlock();
81   assert(PHIBlock && "No single successor to loop exit block");
82 
83   for (auto *Inst : DefsUsedOutside) {
84     auto *NonVersionedLoopInst = cast<Instruction>(VMap[Inst]);
85     PHINode *PN;
86 
87     // First see if we have a single-operand PHI with the value defined by the
88     // original loop.
89     for (auto I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(I)); ++I) {
90       assert(PN->getNumOperands() == 1 &&
91              "Exit block should only have on predecessor");
92       if (PN->getIncomingValue(0) == Inst)
93         break;
94     }
95     // If not create it.
96     if (!PN) {
97       PN = PHINode::Create(Inst->getType(), 2, Inst->getName() + ".lver",
98                            PHIBlock->begin());
99       for (auto *User : Inst->users())
100         if (!VersionedLoop->contains(cast<Instruction>(User)->getParent()))
101           User->replaceUsesOfWith(Inst, PN);
102       PN->addIncoming(Inst, VersionedLoop->getExitingBlock());
103     }
104     // Add the new incoming value from the non-versioned loop.
105     PN->addIncoming(NonVersionedLoopInst, NonVersionedLoop->getExitingBlock());
106   }
107 }
108