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(std::move(Checks)), LAI(LAI),
31       LI(LI), DT(DT) {
32   assert(L->getExitBlock() && "No single exit block");
33   assert(L->getLoopPreheader() && "No preheader");
34 }
35 
36 bool LoopVersioning::needsRuntimeChecks() const {
37   return LAI.getRuntimePointerChecking()->needsAnyChecking(PtrToPartition);
38 }
39 
40 void LoopVersioning::versionLoop(Pass *P) {
41   Instruction *FirstCheckInst;
42   Instruction *MemRuntimeCheck;
43   // Add the memcheck in the original preheader (this is empty initially).
44   BasicBlock *MemCheckBB = VersionedLoop->getLoopPreheader();
45   std::tie(FirstCheckInst, MemRuntimeCheck) =
46       LAI.addRuntimeCheck(MemCheckBB->getTerminator(), Checks);
47   assert(MemRuntimeCheck && "called even though needsAnyChecking = false");
48 
49   // Rename the block to make the IR more readable.
50   MemCheckBB->setName(VersionedLoop->getHeader()->getName() + ".lver.memcheck");
51 
52   // Create empty preheader for the loop (and after cloning for the
53   // non-versioned loop).
54   BasicBlock *PH = SplitBlock(MemCheckBB, MemCheckBB->getTerminator(), DT, LI);
55   PH->setName(VersionedLoop->getHeader()->getName() + ".ph");
56 
57   // Clone the loop including the preheader.
58   //
59   // FIXME: This does not currently preserve SimplifyLoop because the exit
60   // block is a join between the two loops.
61   SmallVector<BasicBlock *, 8> NonVersionedLoopBlocks;
62   NonVersionedLoop =
63       cloneLoopWithPreheader(PH, MemCheckBB, VersionedLoop, VMap, ".lver.orig",
64                              LI, DT, NonVersionedLoopBlocks);
65   remapInstructionsInBlocks(NonVersionedLoopBlocks, VMap);
66 
67   // Insert the conditional branch based on the result of the memchecks.
68   Instruction *OrigTerm = MemCheckBB->getTerminator();
69   BranchInst::Create(NonVersionedLoop->getLoopPreheader(),
70                      VersionedLoop->getLoopPreheader(), MemRuntimeCheck,
71                      OrigTerm);
72   OrigTerm->eraseFromParent();
73 
74   // The loops merge in the original exit block.  This is now dominated by the
75   // memchecking block.
76   DT->changeImmediateDominator(VersionedLoop->getExitBlock(), MemCheckBB);
77 }
78 
79 void LoopVersioning::addPHINodes(
80     const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
81   BasicBlock *PHIBlock = VersionedLoop->getExitBlock();
82   assert(PHIBlock && "No single successor to loop exit block");
83 
84   for (auto *Inst : DefsUsedOutside) {
85     auto *NonVersionedLoopInst = cast<Instruction>(VMap[Inst]);
86     PHINode *PN;
87 
88     // First see if we have a single-operand PHI with the value defined by the
89     // original loop.
90     for (auto I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(I)); ++I) {
91       assert(PN->getNumOperands() == 1 &&
92              "Exit block should only have on predecessor");
93       if (PN->getIncomingValue(0) == Inst)
94         break;
95     }
96     // If not create it.
97     if (!PN) {
98       PN = PHINode::Create(Inst->getType(), 2, Inst->getName() + ".lver",
99                            PHIBlock->begin());
100       for (auto *User : Inst->users())
101         if (!VersionedLoop->contains(cast<Instruction>(User)->getParent()))
102           User->replaceUsesOfWith(Inst, PN);
103       PN->addIncoming(Inst, VersionedLoop->getExitingBlock());
104     }
105     // Add the new incoming value from the non-versioned loop.
106     PN->addIncoming(NonVersionedLoopInst, NonVersionedLoop->getExitingBlock());
107   }
108 }
109