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     : VersionedLoop(L), NonVersionedLoop(nullptr), Checks(std::move(Checks)),
29       LAI(LAI), LI(LI), DT(DT) {
30   assert(L->getExitBlock() && "No single exit block");
31   assert(L->getLoopPreheader() && "No preheader");
32 }
33 
34 LoopVersioning::LoopVersioning(const LoopAccessInfo &LAInfo, Loop *L,
35                                LoopInfo *LI, DominatorTree *DT)
36     : VersionedLoop(L), NonVersionedLoop(nullptr),
37       Checks(LAInfo.getRuntimePointerChecking()->getChecks()), LAI(LAInfo),
38       LI(LI), DT(DT) {
39   assert(L->getExitBlock() && "No single exit block");
40   assert(L->getLoopPreheader() && "No preheader");
41 }
42 
43 void LoopVersioning::versionLoop(
44     const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
45   Instruction *FirstCheckInst;
46   Instruction *MemRuntimeCheck;
47   // Add the memcheck in the original preheader (this is empty initially).
48   BasicBlock *MemCheckBB = VersionedLoop->getLoopPreheader();
49   std::tie(FirstCheckInst, MemRuntimeCheck) =
50       LAI.addRuntimeChecks(MemCheckBB->getTerminator(), Checks);
51   assert(MemRuntimeCheck && "called even though needsAnyChecking = false");
52 
53   // Rename the block to make the IR more readable.
54   MemCheckBB->setName(VersionedLoop->getHeader()->getName() + ".lver.memcheck");
55 
56   // Create empty preheader for the loop (and after cloning for the
57   // non-versioned loop).
58   BasicBlock *PH = SplitBlock(MemCheckBB, MemCheckBB->getTerminator(), DT, LI);
59   PH->setName(VersionedLoop->getHeader()->getName() + ".ph");
60 
61   // Clone the loop including the preheader.
62   //
63   // FIXME: This does not currently preserve SimplifyLoop because the exit
64   // block is a join between the two loops.
65   SmallVector<BasicBlock *, 8> NonVersionedLoopBlocks;
66   NonVersionedLoop =
67       cloneLoopWithPreheader(PH, MemCheckBB, VersionedLoop, VMap, ".lver.orig",
68                              LI, DT, NonVersionedLoopBlocks);
69   remapInstructionsInBlocks(NonVersionedLoopBlocks, VMap);
70 
71   // Insert the conditional branch based on the result of the memchecks.
72   Instruction *OrigTerm = MemCheckBB->getTerminator();
73   BranchInst::Create(NonVersionedLoop->getLoopPreheader(),
74                      VersionedLoop->getLoopPreheader(), MemRuntimeCheck,
75                      OrigTerm);
76   OrigTerm->eraseFromParent();
77 
78   // The loops merge in the original exit block.  This is now dominated by the
79   // memchecking block.
80   DT->changeImmediateDominator(VersionedLoop->getExitBlock(), MemCheckBB);
81 
82   // Adds the necessary PHI nodes for the versioned loops based on the
83   // loop-defined values used outside of the loop.
84   addPHINodes(DefsUsedOutside);
85 }
86 
87 void LoopVersioning::addPHINodes(
88     const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
89   BasicBlock *PHIBlock = VersionedLoop->getExitBlock();
90   assert(PHIBlock && "No single successor to loop exit block");
91 
92   for (auto *Inst : DefsUsedOutside) {
93     auto *NonVersionedLoopInst = cast<Instruction>(VMap[Inst]);
94     PHINode *PN;
95 
96     // First see if we have a single-operand PHI with the value defined by the
97     // original loop.
98     for (auto I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(I)); ++I) {
99       assert(PN->getNumOperands() == 1 &&
100              "Exit block should only have on predecessor");
101       if (PN->getIncomingValue(0) == Inst)
102         break;
103     }
104     // If not create it.
105     if (!PN) {
106       PN = PHINode::Create(Inst->getType(), 2, Inst->getName() + ".lver",
107                            PHIBlock->begin());
108       for (auto *User : Inst->users())
109         if (!VersionedLoop->contains(cast<Instruction>(User)->getParent()))
110           User->replaceUsesOfWith(Inst, PN);
111       PN->addIncoming(Inst, VersionedLoop->getExitingBlock());
112     }
113     // Add the new incoming value from the non-versioned loop.
114     PN->addIncoming(NonVersionedLoopInst, NonVersionedLoop->getExitingBlock());
115   }
116 }
117