1 //===- LoopExtractor.cpp - Extract each loop into a new function ----------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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/Transforms/IPO.h"
18 #include "llvm/iTerminators.h"
19 #include "llvm/Module.h"
20 #include "llvm/Pass.h"
21 #include "llvm/Analysis/Dominators.h"
22 #include "llvm/Analysis/LoopInfo.h"
23 #include "llvm/Transforms/Scalar.h"
24 #include "llvm/Transforms/Utils/FunctionUtils.h"
25 #include "Support/Statistic.h"
26 using namespace llvm;
27 
28 namespace {
29   Statistic<> NumExtracted("loop-extract", "Number of loops extracted");
30 
31   // FIXME: This is not a function pass, but the PassManager doesn't allow
32   // Module passes to require FunctionPasses, so we can't get loop info if we're
33   // not a function pass.
34   struct LoopExtractor : public FunctionPass {
35     unsigned NumLoops;
36 
37     LoopExtractor(unsigned numLoops = ~0) : NumLoops(numLoops) {}
38 
39     virtual bool runOnFunction(Function &F);
40 
41     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
42       AU.addRequiredID(BreakCriticalEdgesID);
43       AU.addRequiredID(LoopSimplifyID);
44       AU.addRequired<DominatorSet>();
45       AU.addRequired<LoopInfo>();
46     }
47   };
48 
49   RegisterOpt<LoopExtractor>
50   X("loop-extract", "Extract loops into new functions");
51 
52   /// SingleLoopExtractor - For bugpoint.
53   struct SingleLoopExtractor : public LoopExtractor {
54     SingleLoopExtractor() : LoopExtractor(1) {}
55   };
56 
57   RegisterOpt<SingleLoopExtractor>
58   Y("loop-extract-single", "Extract at most one loop into a new function");
59 } // End anonymous namespace
60 
61 bool LoopExtractor::runOnFunction(Function &F) {
62   LoopInfo &LI = getAnalysis<LoopInfo>();
63 
64   // If this function has no loops, there is nothing to do.
65   if (LI.begin() == LI.end())
66     return false;
67 
68   DominatorSet &DS = getAnalysis<DominatorSet>();
69 
70   // If there is more than one top-level loop in this function, extract all of
71   // the loops.
72   bool Changed = false;
73   if (LI.end()-LI.begin() > 1) {
74     for (LoopInfo::iterator i = LI.begin(), e = LI.end(); i != e; ++i) {
75       if (NumLoops == 0) return Changed;
76       --NumLoops;
77       Changed |= ExtractLoop(DS, *i) != 0;
78       ++NumExtracted;
79     }
80   } else {
81     // Otherwise there is exactly one top-level loop.  If this function is more
82     // than a minimal wrapper around the loop, extract the loop.
83     Loop *TLL = *LI.begin();
84     bool ShouldExtractLoop = false;
85 
86     // Extract the loop if the entry block doesn't branch to the loop header.
87     TerminatorInst *EntryTI = F.getEntryBlock().getTerminator();
88     if (!isa<BranchInst>(EntryTI) ||
89         !cast<BranchInst>(EntryTI)->isUnconditional() ||
90         EntryTI->getSuccessor(0) != TLL->getHeader())
91       ShouldExtractLoop = true;
92     else {
93       // Check to see if any exits from the loop are more than just return
94       // blocks.
95       std::vector<BasicBlock*> ExitBlocks;
96       TLL->getExitBlocks(ExitBlocks);
97       for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
98         if (!isa<ReturnInst>(ExitBlocks[i]->getTerminator())) {
99           ShouldExtractLoop = true;
100           break;
101         }
102     }
103 
104     if (ShouldExtractLoop) {
105       if (NumLoops == 0) return Changed;
106       --NumLoops;
107       Changed |= ExtractLoop(DS, TLL) != 0;
108       ++NumExtracted;
109     } else {
110       // Okay, this function is a minimal container around the specified loop.
111       // If we extract the loop, we will continue to just keep extracting it
112       // infinitely... so don't extract it.  However, if the loop contains any
113       // subloops, extract them.
114       for (Loop::iterator i = TLL->begin(), e = TLL->end(); i != e; ++i) {
115         if (NumLoops == 0) return Changed;
116         --NumLoops;
117         Changed |= ExtractLoop(DS, *i) != 0;
118         ++NumExtracted;
119       }
120     }
121   }
122 
123   return Changed;
124 }
125 
126 // createSingleLoopExtractorPass - This pass extracts one natural loop from the
127 // program into a function if it can.  This is used by bugpoint.
128 //
129 Pass *llvm::createSingleLoopExtractorPass() {
130   return new SingleLoopExtractor();
131 }
132