1 //===- LoopExtractor.cpp - Extract each loop into a new function ----------===//
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 // A pass wrapper around the ExtractLoop() scalar transformation to extract each
11 // top-level loop into its own new function. If the loop is the ONLY loop in a
12 // given function, it is not touched. This is a pass most useful for debugging
13 // via bugpoint.
14 //
15 //===----------------------------------------------------------------------===//
16 
17 #include "llvm/ADT/Statistic.h"
18 #include "llvm/Analysis/LoopPass.h"
19 #include "llvm/IR/Dominators.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/Module.h"
22 #include "llvm/Pass.h"
23 #include "llvm/Support/CommandLine.h"
24 #include "llvm/Transforms/IPO.h"
25 #include "llvm/Transforms/Scalar.h"
26 #include "llvm/Transforms/Utils.h"
27 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
28 #include "llvm/Transforms/Utils/CodeExtractor.h"
29 #include <fstream>
30 #include <set>
31 using namespace llvm;
32 
33 #define DEBUG_TYPE "loop-extract"
34 
35 STATISTIC(NumExtracted, "Number of loops extracted");
36 
37 namespace {
38   struct LoopExtractor : public LoopPass {
39     static char ID; // Pass identification, replacement for typeid
40     unsigned NumLoops;
41 
42     explicit LoopExtractor(unsigned numLoops = ~0)
43       : LoopPass(ID), NumLoops(numLoops) {
44         initializeLoopExtractorPass(*PassRegistry::getPassRegistry());
45       }
46 
47     bool runOnLoop(Loop *L, LPPassManager &) override;
48 
49     void getAnalysisUsage(AnalysisUsage &AU) const override {
50       AU.addRequiredID(BreakCriticalEdgesID);
51       AU.addRequiredID(LoopSimplifyID);
52       AU.addRequired<DominatorTreeWrapperPass>();
53       AU.addRequired<LoopInfoWrapperPass>();
54     }
55   };
56 }
57 
58 char LoopExtractor::ID = 0;
59 INITIALIZE_PASS_BEGIN(LoopExtractor, "loop-extract",
60                       "Extract loops into new functions", false, false)
61 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
62 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
63 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
64 INITIALIZE_PASS_END(LoopExtractor, "loop-extract",
65                     "Extract loops into new functions", false, false)
66 
67 namespace {
68   /// SingleLoopExtractor - For bugpoint.
69   struct SingleLoopExtractor : public LoopExtractor {
70     static char ID; // Pass identification, replacement for typeid
71     SingleLoopExtractor() : LoopExtractor(1) {}
72   };
73 } // End anonymous namespace
74 
75 char SingleLoopExtractor::ID = 0;
76 INITIALIZE_PASS(SingleLoopExtractor, "loop-extract-single",
77                 "Extract at most one loop into a new function", false, false)
78 
79 // createLoopExtractorPass - This pass extracts all natural loops from the
80 // program into a function if it can.
81 //
82 Pass *llvm::createLoopExtractorPass() { return new LoopExtractor(); }
83 
84 bool LoopExtractor::runOnLoop(Loop *L, LPPassManager &LPM) {
85   if (skipLoop(L))
86     return false;
87 
88   // Only visit top-level loops.
89   if (L->getParentLoop())
90     return false;
91 
92   // If LoopSimplify form is not available, stay out of trouble.
93   if (!L->isLoopSimplifyForm())
94     return false;
95 
96   DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
97   LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
98   bool Changed = false;
99 
100   // If there is more than one top-level loop in this function, extract all of
101   // the loops. Otherwise there is exactly one top-level loop; in this case if
102   // this function is more than a minimal wrapper around the loop, extract
103   // the loop.
104   bool ShouldExtractLoop = false;
105 
106   // Extract the loop if the entry block doesn't branch to the loop header.
107   Instruction *EntryTI =
108       L->getHeader()->getParent()->getEntryBlock().getTerminator();
109   if (!isa<BranchInst>(EntryTI) ||
110       !cast<BranchInst>(EntryTI)->isUnconditional() ||
111       EntryTI->getSuccessor(0) != L->getHeader()) {
112     ShouldExtractLoop = true;
113   } else {
114     // Check to see if any exits from the loop are more than just return
115     // blocks.
116     SmallVector<BasicBlock*, 8> ExitBlocks;
117     L->getExitBlocks(ExitBlocks);
118     for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
119       if (!isa<ReturnInst>(ExitBlocks[i]->getTerminator())) {
120         ShouldExtractLoop = true;
121         break;
122       }
123   }
124 
125   if (ShouldExtractLoop) {
126     // We must omit EH pads. EH pads must accompany the invoke
127     // instruction. But this would result in a loop in the extracted
128     // function. An infinite cycle occurs when it tries to extract that loop as
129     // well.
130     SmallVector<BasicBlock*, 8> ExitBlocks;
131     L->getExitBlocks(ExitBlocks);
132     for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
133       if (ExitBlocks[i]->isEHPad()) {
134         ShouldExtractLoop = false;
135         break;
136       }
137   }
138 
139   if (ShouldExtractLoop) {
140     if (NumLoops == 0) return Changed;
141     --NumLoops;
142     CodeExtractor Extractor(DT, *L);
143     if (Extractor.extractCodeRegion() != nullptr) {
144       Changed = true;
145       // After extraction, the loop is replaced by a function call, so
146       // we shouldn't try to run any more loop passes on it.
147       LPM.markLoopAsDeleted(*L);
148       LI.erase(L);
149     }
150     ++NumExtracted;
151   }
152 
153   return Changed;
154 }
155 
156 // createSingleLoopExtractorPass - This pass extracts one natural loop from the
157 // program into a function if it can.  This is used by bugpoint.
158 //
159 Pass *llvm::createSingleLoopExtractorPass() {
160   return new SingleLoopExtractor();
161 }
162