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 #define DEBUG_TYPE "simplifycfg"
25 #include "llvm/Transforms/Scalar.h"
26 #include "llvm/ADT/SmallPtrSet.h"
27 #include "llvm/ADT/SmallVector.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/Analysis/TargetTransformInfo.h"
30 #include "llvm/IR/Attributes.h"
31 #include "llvm/IR/CFG.h"
32 #include "llvm/IR/Constants.h"
33 #include "llvm/IR/DataLayout.h"
34 #include "llvm/IR/Instructions.h"
35 #include "llvm/IR/IntrinsicInst.h"
36 #include "llvm/IR/Module.h"
37 #include "llvm/Pass.h"
38 #include "llvm/Transforms/Utils/Local.h"
39 using namespace llvm;
40 
41 STATISTIC(NumSimpl, "Number of blocks simplified");
42 
43 namespace {
44 struct CFGSimplifyPass : public FunctionPass {
45   static char ID; // Pass identification, replacement for typeid
46   CFGSimplifyPass() : FunctionPass(ID) {
47     initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
48   }
49   bool runOnFunction(Function &F) override;
50 
51   void getAnalysisUsage(AnalysisUsage &AU) const override {
52     AU.addRequired<TargetTransformInfo>();
53   }
54 };
55 }
56 
57 char CFGSimplifyPass::ID = 0;
58 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
59                       false)
60 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
61 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
62                     false)
63 
64 // Public interface to the CFGSimplification pass
65 FunctionPass *llvm::createCFGSimplificationPass() {
66   return new CFGSimplifyPass();
67 }
68 
69 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
70 /// node) return blocks, merge them together to promote recursive block merging.
71 static bool mergeEmptyReturnBlocks(Function &F) {
72   bool Changed = false;
73 
74   BasicBlock *RetBlock = 0;
75 
76   // Scan all the blocks in the function, looking for empty return blocks.
77   for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
78     BasicBlock &BB = *BBI++;
79 
80     // Only look at return blocks.
81     ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
82     if (Ret == 0) continue;
83 
84     // Only look at the block if it is empty or the only other thing in it is a
85     // single PHI node that is the operand to the return.
86     if (Ret != &BB.front()) {
87       // Check for something else in the block.
88       BasicBlock::iterator I = Ret;
89       --I;
90       // Skip over debug info.
91       while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
92         --I;
93       if (!isa<DbgInfoIntrinsic>(I) &&
94           (!isa<PHINode>(I) || I != BB.begin() ||
95            Ret->getNumOperands() == 0 ||
96            Ret->getOperand(0) != I))
97         continue;
98     }
99 
100     // If this is the first returning block, remember it and keep going.
101     if (RetBlock == 0) {
102       RetBlock = &BB;
103       continue;
104     }
105 
106     // Otherwise, we found a duplicate return block.  Merge the two.
107     Changed = true;
108 
109     // Case when there is no input to the return or when the returned values
110     // agree is trivial.  Note that they can't agree if there are phis in the
111     // blocks.
112     if (Ret->getNumOperands() == 0 ||
113         Ret->getOperand(0) ==
114           cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
115       BB.replaceAllUsesWith(RetBlock);
116       BB.eraseFromParent();
117       continue;
118     }
119 
120     // If the canonical return block has no PHI node, create one now.
121     PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
122     if (RetBlockPHI == 0) {
123       Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
124       pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
125       RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
126                                     std::distance(PB, PE), "merge",
127                                     &RetBlock->front());
128 
129       for (pred_iterator PI = PB; PI != PE; ++PI)
130         RetBlockPHI->addIncoming(InVal, *PI);
131       RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
132     }
133 
134     // Turn BB into a block that just unconditionally branches to the return
135     // block.  This handles the case when the two return blocks have a common
136     // predecessor but that return different things.
137     RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
138     BB.getTerminator()->eraseFromParent();
139     BranchInst::Create(RetBlock, &BB);
140   }
141 
142   return Changed;
143 }
144 
145 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
146 /// iterating until no more changes are made.
147 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
148                                    const DataLayout *DL) {
149   bool Changed = false;
150   bool LocalChange = true;
151   while (LocalChange) {
152     LocalChange = false;
153 
154     // Loop over all of the basic blocks and remove them if they are unneeded...
155     //
156     for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
157       if (SimplifyCFG(BBIt++, TTI, DL)) {
158         LocalChange = true;
159         ++NumSimpl;
160       }
161     }
162     Changed |= LocalChange;
163   }
164   return Changed;
165 }
166 
167 // It is possible that we may require multiple passes over the code to fully
168 // simplify the CFG.
169 //
170 bool CFGSimplifyPass::runOnFunction(Function &F) {
171   if (skipOptnoneFunction(F))
172     return false;
173 
174   const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
175   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
176   const DataLayout *DL = DLP ? &DLP->getDataLayout() : 0;
177   bool EverChanged = removeUnreachableBlocks(F);
178   EverChanged |= mergeEmptyReturnBlocks(F);
179   EverChanged |= iterativelySimplifyCFG(F, TTI, DL);
180 
181   // If neither pass changed anything, we're done.
182   if (!EverChanged) return false;
183 
184   // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
185   // removeUnreachableBlocks is needed to nuke them, which means we should
186   // iterate between the two optimizations.  We structure the code like this to
187   // avoid reruning iterativelySimplifyCFG if the second pass of
188   // removeUnreachableBlocks doesn't do anything.
189   if (!removeUnreachableBlocks(F))
190     return true;
191 
192   do {
193     EverChanged = iterativelySimplifyCFG(F, TTI, DL);
194     EverChanged |= removeUnreachableBlocks(F);
195   } while (EverChanged);
196 
197   return true;
198 }
199