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1 //===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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 implements dead code elimination and basic block merging, along
11 // with a collection of other peephole control flow optimizations.  For example:
12 //
13 //   * Removes basic blocks with no predecessors.
14 //   * Merges a basic block into its predecessor if there is only one and the
15 //     predecessor only has one successor.
16 //   * Eliminates PHI nodes for basic blocks with a single predecessor.
17 //   * Eliminates a basic block that only contains an unconditional branch.
18 //   * Changes invoke instructions to nounwind functions to be calls.
19 //   * Change things like "if (x) if (y)" into "if (x&y)".
20 //   * etc..
21 //
22 //===----------------------------------------------------------------------===//
23 
24 #include "llvm/Transforms/Scalar/SimplifyCFG.h"
25 #include "llvm/ADT/SmallPtrSet.h"
26 #include "llvm/ADT/SmallVector.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/Analysis/GlobalsModRef.h"
29 #include "llvm/Analysis/AssumptionCache.h"
30 #include "llvm/Analysis/TargetTransformInfo.h"
31 #include "llvm/IR/Attributes.h"
32 #include "llvm/IR/CFG.h"
33 #include "llvm/IR/Constants.h"
34 #include "llvm/IR/DataLayout.h"
35 #include "llvm/IR/Instructions.h"
36 #include "llvm/IR/IntrinsicInst.h"
37 #include "llvm/IR/Module.h"
38 #include "llvm/Pass.h"
39 #include "llvm/Support/CommandLine.h"
40 #include "llvm/Transforms/Utils/Local.h"
41 #include "llvm/Transforms/Scalar.h"
42 using namespace llvm;
43 
44 #define DEBUG_TYPE "simplifycfg"
45 
46 static cl::opt<unsigned>
47 UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
48    cl::desc("Control the number of bonus instructions (default = 1)"));
49 
50 STATISTIC(NumSimpl, "Number of blocks simplified");
51 
52 /// If we have more than one empty (other than phi node) return blocks,
53 /// merge them together to promote recursive block merging.
mergeEmptyReturnBlocks(Function & F)54 static bool mergeEmptyReturnBlocks(Function &F) {
55   bool Changed = false;
56 
57   BasicBlock *RetBlock = nullptr;
58 
59   // Scan all the blocks in the function, looking for empty return blocks.
60   for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
61     BasicBlock &BB = *BBI++;
62 
63     // Only look at return blocks.
64     ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
65     if (!Ret) continue;
66 
67     // Only look at the block if it is empty or the only other thing in it is a
68     // single PHI node that is the operand to the return.
69     if (Ret != &BB.front()) {
70       // Check for something else in the block.
71       BasicBlock::iterator I(Ret);
72       --I;
73       // Skip over debug info.
74       while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
75         --I;
76       if (!isa<DbgInfoIntrinsic>(I) &&
77           (!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 ||
78            Ret->getOperand(0) != &*I))
79         continue;
80     }
81 
82     // If this is the first returning block, remember it and keep going.
83     if (!RetBlock) {
84       RetBlock = &BB;
85       continue;
86     }
87 
88     // Otherwise, we found a duplicate return block.  Merge the two.
89     Changed = true;
90 
91     // Case when there is no input to the return or when the returned values
92     // agree is trivial.  Note that they can't agree if there are phis in the
93     // blocks.
94     if (Ret->getNumOperands() == 0 ||
95         Ret->getOperand(0) ==
96           cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
97       BB.replaceAllUsesWith(RetBlock);
98       BB.eraseFromParent();
99       continue;
100     }
101 
102     // If the canonical return block has no PHI node, create one now.
103     PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
104     if (!RetBlockPHI) {
105       Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
106       pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
107       RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
108                                     std::distance(PB, PE), "merge",
109                                     &RetBlock->front());
110 
111       for (pred_iterator PI = PB; PI != PE; ++PI)
112         RetBlockPHI->addIncoming(InVal, *PI);
113       RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
114     }
115 
116     // Turn BB into a block that just unconditionally branches to the return
117     // block.  This handles the case when the two return blocks have a common
118     // predecessor but that return different things.
119     RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
120     BB.getTerminator()->eraseFromParent();
121     BranchInst::Create(RetBlock, &BB);
122   }
123 
124   return Changed;
125 }
126 
127 /// Call SimplifyCFG on all the blocks in the function,
128 /// iterating until no more changes are made.
iterativelySimplifyCFG(Function & F,const TargetTransformInfo & TTI,AssumptionCache * AC,unsigned BonusInstThreshold)129 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
130                                    AssumptionCache *AC,
131                                    unsigned BonusInstThreshold) {
132   bool Changed = false;
133   bool LocalChange = true;
134   while (LocalChange) {
135     LocalChange = false;
136 
137     // Loop over all of the basic blocks and remove them if they are unneeded.
138     for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
139       if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC)) {
140         LocalChange = true;
141         ++NumSimpl;
142       }
143     }
144     Changed |= LocalChange;
145   }
146   return Changed;
147 }
148 
simplifyFunctionCFG(Function & F,const TargetTransformInfo & TTI,AssumptionCache * AC,int BonusInstThreshold)149 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
150                                 AssumptionCache *AC, int BonusInstThreshold) {
151   bool EverChanged = removeUnreachableBlocks(F);
152   EverChanged |= mergeEmptyReturnBlocks(F);
153   EverChanged |= iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
154 
155   // If neither pass changed anything, we're done.
156   if (!EverChanged) return false;
157 
158   // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
159   // removeUnreachableBlocks is needed to nuke them, which means we should
160   // iterate between the two optimizations.  We structure the code like this to
161   // avoid rerunning iterativelySimplifyCFG if the second pass of
162   // removeUnreachableBlocks doesn't do anything.
163   if (!removeUnreachableBlocks(F))
164     return true;
165 
166   do {
167     EverChanged = iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
168     EverChanged |= removeUnreachableBlocks(F);
169   } while (EverChanged);
170 
171   return true;
172 }
173 
SimplifyCFGPass()174 SimplifyCFGPass::SimplifyCFGPass()
175     : BonusInstThreshold(UserBonusInstThreshold) {}
176 
SimplifyCFGPass(int BonusInstThreshold)177 SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold)
178     : BonusInstThreshold(BonusInstThreshold) {}
179 
run(Function & F,AnalysisManager<Function> * AM)180 PreservedAnalyses SimplifyCFGPass::run(Function &F,
181                                        AnalysisManager<Function> *AM) {
182   auto &TTI = AM->getResult<TargetIRAnalysis>(F);
183   auto &AC = AM->getResult<AssumptionAnalysis>(F);
184 
185   if (!simplifyFunctionCFG(F, TTI, &AC, BonusInstThreshold))
186     return PreservedAnalyses::none();
187 
188   return PreservedAnalyses::all();
189 }
190 
191 namespace {
192 struct CFGSimplifyPass : public FunctionPass {
193   static char ID; // Pass identification, replacement for typeid
194   unsigned BonusInstThreshold;
195   std::function<bool(const Function &)> PredicateFtor;
196 
CFGSimplifyPass__anon4facdab50111::CFGSimplifyPass197   CFGSimplifyPass(int T = -1,
198                   std::function<bool(const Function &)> Ftor = nullptr)
199       : FunctionPass(ID), PredicateFtor(Ftor) {
200     BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
201     initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
202   }
runOnFunction__anon4facdab50111::CFGSimplifyPass203   bool runOnFunction(Function &F) override {
204     if (PredicateFtor && !PredicateFtor(F))
205       return false;
206 
207     if (skipOptnoneFunction(F))
208       return false;
209 
210     AssumptionCache *AC =
211         &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
212     const TargetTransformInfo &TTI =
213         getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
214     return simplifyFunctionCFG(F, TTI, AC, BonusInstThreshold);
215   }
216 
getAnalysisUsage__anon4facdab50111::CFGSimplifyPass217   void getAnalysisUsage(AnalysisUsage &AU) const override {
218     AU.addRequired<AssumptionCacheTracker>();
219     AU.addRequired<TargetTransformInfoWrapperPass>();
220     AU.addPreserved<GlobalsAAWrapperPass>();
221   }
222 };
223 }
224 
225 char CFGSimplifyPass::ID = 0;
226 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
227                       false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)228 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
229 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
230 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
231                     false)
232 
233 // Public interface to the CFGSimplification pass
234 FunctionPass *
235 llvm::createCFGSimplificationPass(int Threshold,
236                                   std::function<bool(const Function &)> Ftor) {
237   return new CFGSimplifyPass(Threshold, Ftor);
238 }
239 
240